TOMOYO Linux Cross Reference
Linux/kernel/cgroup/cgroup.c

   /*
    *  Generic process-grouping system.
    *
    *  Based originally on the cpuset system, extracted by Paul Menage
    *  Copyright (C) 2006 Google, Inc
    *
    *  Notifications support
    *  Copyright (C) 2009 Nokia Corporation
    *  Author: Kirill A. Shutemov
   *
   *  Copyright notices from the original cpuset code:
   *  --------------------------------------------------
   *  Copyright (C) 2003 BULL SA.
   *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
   *
   *  Portions derived from Patrick Mochel's sysfs code.
   *  sysfs is Copyright (c) 2001-3 Patrick Mochel
   *
   *  2003-10-10 Written by Simon Derr.
   *  2003-10-22 Updates by Stephen Hemminger.
   *  2004 May-July Rework by Paul Jackson.
   *  ---------------------------------------------------
   *
   *  This file is subject to the terms and conditions of the GNU General Public
   *  License.  See the file COPYING in the main directory of the Linux
   *  distribution for more details.
   */
  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include "cgroup-internal.h"
  
  #include <linux/bpf-cgroup.h>
  #include <linux/cred.h>
  #include <linux/errno.h>
  #include <linux/init_task.h>
  #include <linux/kernel.h>
  #include <linux/magic.h>
  #include <linux/mutex.h>
  #include <linux/mount.h>
  #include <linux/pagemap.h>
  #include <linux/proc_fs.h>
  #include <linux/rcupdate.h>
  #include <linux/sched.h>
  #include <linux/sched/task.h>
  #include <linux/slab.h>
  #include <linux/spinlock.h>
  #include <linux/percpu-rwsem.h>
  #include <linux/string.h>
  #include <linux/hashtable.h>
  #include <linux/idr.h>
  #include <linux/kthread.h>
  #include <linux/atomic.h>
  #include <linux/cpuset.h>
  #include <linux/proc_ns.h>
  #include <linux/nsproxy.h>
  #include <linux/file.h>
  #include <linux/fs_parser.h>
  #include <linux/sched/cputime.h>
  #include <linux/sched/deadline.h>
  #include <linux/psi.h>
  #include <net/sock.h>
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/cgroup.h>
  
  #define CGROUP_FILE_NAME_MAX            (MAX_CGROUP_TYPE_NAMELEN +      \
                                           MAX_CFTYPE_NAME + 2)
  /* let's not notify more than 100 times per second */
  #define CGROUP_FILE_NOTIFY_MIN_INTV     DIV_ROUND_UP(HZ, 100)
  
  /*
   * To avoid confusing the compiler (and generating warnings) with code
   * that attempts to access what would be a 0-element array (i.e. sized
   * to a potentially empty array when CGROUP_SUBSYS_COUNT == 0), this
   * constant expression can be added.
   */
  #define CGROUP_HAS_SUBSYS_CONFIG        (CGROUP_SUBSYS_COUNT > 0)
  
  /*
   * cgroup_mutex is the master lock.  Any modification to cgroup or its
   * hierarchy must be performed while holding it.
   *
   * css_set_lock protects task->cgroups pointer, the list of css_set
   * objects, and the chain of tasks off each css_set.
   *
   * These locks are exported if CONFIG_PROVE_RCU so that accessors in
   * cgroup.h can use them for lockdep annotations.
   */
  DEFINE_MUTEX(cgroup_mutex);
  DEFINE_SPINLOCK(css_set_lock);
  
  #ifdef CONFIG_PROVE_RCU
  EXPORT_SYMBOL_GPL(cgroup_mutex);
  EXPORT_SYMBOL_GPL(css_set_lock);
  #endif
  
  DEFINE_SPINLOCK(trace_cgroup_path_lock);
  char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
 static bool cgroup_debug __read_mostly;
 
 /*
  * Protects cgroup_idr and css_idr so that IDs can be released without
  * grabbing cgroup_mutex.
  */
 static DEFINE_SPINLOCK(cgroup_idr_lock);
 
 /*
  * Protects cgroup_file->kn for !self csses.  It synchronizes notifications
  * against file removal/re-creation across css hiding.
  */
 static DEFINE_SPINLOCK(cgroup_file_kn_lock);
 
 DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
 
 #define cgroup_assert_mutex_or_rcu_locked()                             \
         RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&                       \
                            !lockdep_is_held(&cgroup_mutex),             \
                            "cgroup_mutex or RCU read lock required");
 
 /*
  * cgroup destruction makes heavy use of work items and there can be a lot
  * of concurrent destructions.  Use a separate workqueue so that cgroup
  * destruction work items don't end up filling up max_active of system_wq
  * which may lead to deadlock.
  */
 static struct workqueue_struct *cgroup_destroy_wq;
 
 /* generate an array of cgroup subsystem pointers */
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
 struct cgroup_subsys *cgroup_subsys[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 /* array of cgroup subsystem names */
 #define SUBSYS(_x) [_x ## _cgrp_id] = #_x,
 static const char *cgroup_subsys_name[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 /* array of static_keys for cgroup_subsys_enabled() and cgroup_subsys_on_dfl() */
 #define SUBSYS(_x)                                                              \
         DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_enabled_key);                 \
         DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_on_dfl_key);                  \
         EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_enabled_key);                      \
         EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_on_dfl_key);
 #include <linux/cgroup_subsys.h>
 #undef SUBSYS
 
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_enabled_key,
 static struct static_key_true *cgroup_subsys_enabled_key[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_on_dfl_key,
 static struct static_key_true *cgroup_subsys_on_dfl_key[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 static DEFINE_PER_CPU(struct cgroup_rstat_cpu, cgrp_dfl_root_rstat_cpu);
 
 /* the default hierarchy */
 struct cgroup_root cgrp_dfl_root = { .cgrp.rstat_cpu = &cgrp_dfl_root_rstat_cpu };
 EXPORT_SYMBOL_GPL(cgrp_dfl_root);
 
 /*
  * The default hierarchy always exists but is hidden until mounted for the
  * first time.  This is for backward compatibility.
  */
 static bool cgrp_dfl_visible;
 
 /* some controllers are not supported in the default hierarchy */
 static u16 cgrp_dfl_inhibit_ss_mask;
 
 /* some controllers are implicitly enabled on the default hierarchy */
 static u16 cgrp_dfl_implicit_ss_mask;
 
 /* some controllers can be threaded on the default hierarchy */
 static u16 cgrp_dfl_threaded_ss_mask;
 
 /* The list of hierarchy roots */
 LIST_HEAD(cgroup_roots);
 static int cgroup_root_count;
 
 /* hierarchy ID allocation and mapping, protected by cgroup_mutex */
 static DEFINE_IDR(cgroup_hierarchy_idr);
 
 /*
  * Assign a monotonically increasing serial number to csses.  It guarantees
  * cgroups with bigger numbers are newer than those with smaller numbers.
  * Also, as csses are always appended to the parent's ->children list, it
  * guarantees that sibling csses are always sorted in the ascending serial
  * number order on the list.  Protected by cgroup_mutex.
  */
 static u64 css_serial_nr_next = 1;
 
 /*
  * These bitmasks identify subsystems with specific features to avoid
  * having to do iterative checks repeatedly.
  */
 static u16 have_fork_callback __read_mostly;
 static u16 have_exit_callback __read_mostly;
 static u16 have_release_callback __read_mostly;
 static u16 have_canfork_callback __read_mostly;
 
 static bool have_favordynmods __ro_after_init = IS_ENABLED(CONFIG_CGROUP_FAVOR_DYNMODS);
 
 /* cgroup namespace for init task */
 struct cgroup_namespace init_cgroup_ns = {
         .ns.count       = REFCOUNT_INIT(2),
         .user_ns        = &init_user_ns,
         .ns.ops         = &cgroupns_operations,
         .ns.inum        = PROC_CGROUP_INIT_INO,
         .root_cset      = &init_css_set,
 };
 
 static struct file_system_type cgroup2_fs_type;
 static struct cftype cgroup_base_files[];
 static struct cftype cgroup_psi_files[];
 
 /* cgroup optional features */
 enum cgroup_opt_features {
 #ifdef CONFIG_PSI
         OPT_FEATURE_PRESSURE,
 #endif
         OPT_FEATURE_COUNT
 };
 
 static const char *cgroup_opt_feature_names[OPT_FEATURE_COUNT] = {
 #ifdef CONFIG_PSI
         "pressure",
 #endif
 };
 
 static u16 cgroup_feature_disable_mask __read_mostly;
 
 static int cgroup_apply_control(struct cgroup *cgrp);
 static void cgroup_finalize_control(struct cgroup *cgrp, int ret);
 static void css_task_iter_skip(struct css_task_iter *it,
                                struct task_struct *task);
 static int cgroup_destroy_locked(struct cgroup *cgrp);
 static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
                                               struct cgroup_subsys *ss);
 static void css_release(struct percpu_ref *ref);
 static void kill_css(struct cgroup_subsys_state *css);
 static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                               struct cgroup *cgrp, struct cftype cfts[],
                               bool is_add);
 
 #ifdef CONFIG_DEBUG_CGROUP_REF
 #define CGROUP_REF_FN_ATTRS     noinline
 #define CGROUP_REF_EXPORT(fn)   EXPORT_SYMBOL_GPL(fn);
 #include <linux/cgroup_refcnt.h>
 #endif
 
 /**
  * cgroup_ssid_enabled - cgroup subsys enabled test by subsys ID
  * @ssid: subsys ID of interest
  *
  * cgroup_subsys_enabled() can only be used with literal subsys names which
  * is fine for individual subsystems but unsuitable for cgroup core.  This
  * is slower static_key_enabled() based test indexed by @ssid.
  */
 bool cgroup_ssid_enabled(int ssid)
 {
         if (!CGROUP_HAS_SUBSYS_CONFIG)
                 return false;
 
         return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
 }
 
 /**
  * cgroup_on_dfl - test whether a cgroup is on the default hierarchy
  * @cgrp: the cgroup of interest
  *
  * The default hierarchy is the v2 interface of cgroup and this function
  * can be used to test whether a cgroup is on the default hierarchy for
  * cases where a subsystem should behave differently depending on the
  * interface version.
  *
  * List of changed behaviors:
  *
  * - Mount options "noprefix", "xattr", "clone_children", "release_agent"
  *   and "name" are disallowed.
  *
  * - When mounting an existing superblock, mount options should match.
  *
  * - rename(2) is disallowed.
  *
  * - "tasks" is removed.  Everything should be at process granularity.  Use
  *   "cgroup.procs" instead.
  *
  * - "cgroup.procs" is not sorted.  pids will be unique unless they got
  *   recycled in-between reads.
  *
  * - "release_agent" and "notify_on_release" are removed.  Replacement
  *   notification mechanism will be implemented.
  *
  * - "cgroup.clone_children" is removed.
  *
  * - "cgroup.subtree_populated" is available.  Its value is 0 if the cgroup
  *   and its descendants contain no task; otherwise, 1.  The file also
  *   generates kernfs notification which can be monitored through poll and
  *   [di]notify when the value of the file changes.
  *
  * - cpuset: tasks will be kept in empty cpusets when hotplug happens and
  *   take masks of ancestors with non-empty cpus/mems, instead of being
  *   moved to an ancestor.
  *
  * - cpuset: a task can be moved into an empty cpuset, and again it takes
  *   masks of ancestors.
  *
  * - blkcg: blk-throttle becomes properly hierarchical.
  */
 bool cgroup_on_dfl(const struct cgroup *cgrp)
 {
         return cgrp->root == &cgrp_dfl_root;
 }
 
 /* IDR wrappers which synchronize using cgroup_idr_lock */
 static int cgroup_idr_alloc(struct idr *idr, void *ptr, int start, int end,
                             gfp_t gfp_mask)
 {
         int ret;
 
         idr_preload(gfp_mask);
         spin_lock_bh(&cgroup_idr_lock);
         ret = idr_alloc(idr, ptr, start, end, gfp_mask & ~__GFP_DIRECT_RECLAIM);
         spin_unlock_bh(&cgroup_idr_lock);
         idr_preload_end();
         return ret;
 }
 
 static void *cgroup_idr_replace(struct idr *idr, void *ptr, int id)
 {
         void *ret;
 
         spin_lock_bh(&cgroup_idr_lock);
         ret = idr_replace(idr, ptr, id);
         spin_unlock_bh(&cgroup_idr_lock);
         return ret;
 }
 
 static void cgroup_idr_remove(struct idr *idr, int id)
 {
         spin_lock_bh(&cgroup_idr_lock);
         idr_remove(idr, id);
         spin_unlock_bh(&cgroup_idr_lock);
 }
 
 static bool cgroup_has_tasks(struct cgroup *cgrp)
 {
         return cgrp->nr_populated_csets;
 }
 
 static bool cgroup_is_threaded(struct cgroup *cgrp)
 {
         return cgrp->dom_cgrp != cgrp;
 }
 
 /* can @cgrp host both domain and threaded children? */
 static bool cgroup_is_mixable(struct cgroup *cgrp)
 {
         /*
          * Root isn't under domain level resource control exempting it from
          * the no-internal-process constraint, so it can serve as a thread
          * root and a parent of resource domains at the same time.
          */
         return !cgroup_parent(cgrp);
 }
 
 /* can @cgrp become a thread root? Should always be true for a thread root */
 static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
 {
         /* mixables don't care */
         if (cgroup_is_mixable(cgrp))
                 return true;
 
         /* domain roots can't be nested under threaded */
         if (cgroup_is_threaded(cgrp))
                 return false;
 
         /* can only have either domain or threaded children */
         if (cgrp->nr_populated_domain_children)
                 return false;
 
         /* and no domain controllers can be enabled */
         if (cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
                 return false;
 
         return true;
 }
 
 /* is @cgrp root of a threaded subtree? */
 static bool cgroup_is_thread_root(struct cgroup *cgrp)
 {
         /* thread root should be a domain */
         if (cgroup_is_threaded(cgrp))
                 return false;
 
         /* a domain w/ threaded children is a thread root */
         if (cgrp->nr_threaded_children)
                 return true;
 
         /*
          * A domain which has tasks and explicit threaded controllers
          * enabled is a thread root.
          */
         if (cgroup_has_tasks(cgrp) &&
             (cgrp->subtree_control & cgrp_dfl_threaded_ss_mask))
                 return true;
 
         return false;
 }
 
 /* a domain which isn't connected to the root w/o brekage can't be used */
 static bool cgroup_is_valid_domain(struct cgroup *cgrp)
 {
         /* the cgroup itself can be a thread root */
         if (cgroup_is_threaded(cgrp))
                 return false;
 
         /* but the ancestors can't be unless mixable */
         while ((cgrp = cgroup_parent(cgrp))) {
                 if (!cgroup_is_mixable(cgrp) && cgroup_is_thread_root(cgrp))
                         return false;
                 if (cgroup_is_threaded(cgrp))
                         return false;
         }
 
         return true;
 }
 
 /* subsystems visibly enabled on a cgroup */
 static u16 cgroup_control(struct cgroup *cgrp)
 {
         struct cgroup *parent = cgroup_parent(cgrp);
         u16 root_ss_mask = cgrp->root->subsys_mask;
 
         if (parent) {
                 u16 ss_mask = parent->subtree_control;
 
                 /* threaded cgroups can only have threaded controllers */
                 if (cgroup_is_threaded(cgrp))
                         ss_mask &= cgrp_dfl_threaded_ss_mask;
                 return ss_mask;
         }
 
         if (cgroup_on_dfl(cgrp))
                 root_ss_mask &= ~(cgrp_dfl_inhibit_ss_mask |
                                   cgrp_dfl_implicit_ss_mask);
         return root_ss_mask;
 }
 
 /* subsystems enabled on a cgroup */
 static u16 cgroup_ss_mask(struct cgroup *cgrp)
 {
         struct cgroup *parent = cgroup_parent(cgrp);
 
         if (parent) {
                 u16 ss_mask = parent->subtree_ss_mask;
 
                 /* threaded cgroups can only have threaded controllers */
                 if (cgroup_is_threaded(cgrp))
                         ss_mask &= cgrp_dfl_threaded_ss_mask;
                 return ss_mask;
         }
 
         return cgrp->root->subsys_mask;
 }
 
 /**
  * cgroup_css - obtain a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest (%NULL returns @cgrp->self)
  *
  * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
  * function must be called either under cgroup_mutex or rcu_read_lock() and
  * the caller is responsible for pinning the returned css if it wants to
  * keep accessing it outside the said locks.  This function may return
  * %NULL if @cgrp doesn't have @subsys_id enabled.
  */
 static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
                                               struct cgroup_subsys *ss)
 {
         if (CGROUP_HAS_SUBSYS_CONFIG && ss)
                 return rcu_dereference_check(cgrp->subsys[ss->id],
                                         lockdep_is_held(&cgroup_mutex));
         else
                 return &cgrp->self;
 }
 
 /**
  * cgroup_e_css_by_mask - obtain a cgroup's effective css for the specified ss
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest (%NULL returns @cgrp->self)
  *
  * Similar to cgroup_css() but returns the effective css, which is defined
  * as the matching css of the nearest ancestor including self which has @ss
  * enabled.  If @ss is associated with the hierarchy @cgrp is on, this
  * function is guaranteed to return non-NULL css.
  */
 static struct cgroup_subsys_state *cgroup_e_css_by_mask(struct cgroup *cgrp,
                                                         struct cgroup_subsys *ss)
 {
         lockdep_assert_held(&cgroup_mutex);
 
         if (!ss)
                 return &cgrp->self;
 
         /*
          * This function is used while updating css associations and thus
          * can't test the csses directly.  Test ss_mask.
          */
         while (!(cgroup_ss_mask(cgrp) & (1 << ss->id))) {
                 cgrp = cgroup_parent(cgrp);
                 if (!cgrp)
                         return NULL;
         }
 
         return cgroup_css(cgrp, ss);
 }
 
 /**
  * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get the effective css of @cgrp for @ss.  The effective css is
  * defined as the matching css of the nearest ancestor including self which
  * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
  * the root css is returned, so this function always returns a valid css.
  *
  * The returned css is not guaranteed to be online, and therefore it is the
  * callers responsibility to try get a reference for it.
  */
 struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
                                          struct cgroup_subsys *ss)
 {
         struct cgroup_subsys_state *css;
 
         if (!CGROUP_HAS_SUBSYS_CONFIG)
                 return NULL;
 
         do {
                 css = cgroup_css(cgrp, ss);
 
                 if (css)
                         return css;
                 cgrp = cgroup_parent(cgrp);
         } while (cgrp);
 
         return init_css_set.subsys[ss->id];
 }
 
 /**
  * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get the effective css of @cgrp for @ss.  The effective css is
  * defined as the matching css of the nearest ancestor including self which
  * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
  * the root css is returned, so this function always returns a valid css.
  * The returned css must be put using css_put().
  */
 struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
                                              struct cgroup_subsys *ss)
 {
         struct cgroup_subsys_state *css;
 
         if (!CGROUP_HAS_SUBSYS_CONFIG)
                 return NULL;
 
         rcu_read_lock();
 
         do {
                 css = cgroup_css(cgrp, ss);
 
                 if (css && css_tryget_online(css))
                         goto out_unlock;
                 cgrp = cgroup_parent(cgrp);
         } while (cgrp);
 
         css = init_css_set.subsys[ss->id];
         css_get(css);
 out_unlock:
         rcu_read_unlock();
         return css;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_e_css);
 
 static void cgroup_get_live(struct cgroup *cgrp)
 {
         WARN_ON_ONCE(cgroup_is_dead(cgrp));
         cgroup_get(cgrp);
 }
 
 /**
  * __cgroup_task_count - count the number of tasks in a cgroup. The caller
  * is responsible for taking the css_set_lock.
  * @cgrp: the cgroup in question
  */
 int __cgroup_task_count(const struct cgroup *cgrp)
 {
         int count = 0;
         struct cgrp_cset_link *link;
 
         lockdep_assert_held(&css_set_lock);
 
         list_for_each_entry(link, &cgrp->cset_links, cset_link)
                 count += link->cset->nr_tasks;
 
         return count;
 }
 
 /**
  * cgroup_task_count - count the number of tasks in a cgroup.
  * @cgrp: the cgroup in question
  */
 int cgroup_task_count(const struct cgroup *cgrp)
 {
         int count;
 
         spin_lock_irq(&css_set_lock);
         count = __cgroup_task_count(cgrp);
         spin_unlock_irq(&css_set_lock);
 
         return count;
 }
 
 struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
         struct cgroup *cgrp = of->kn->parent->priv;
         struct cftype *cft = of_cft(of);
 
         /*
          * This is open and unprotected implementation of cgroup_css().
          * seq_css() is only called from a kernfs file operation which has
          * an active reference on the file.  Because all the subsystem
          * files are drained before a css is disassociated with a cgroup,
          * the matching css from the cgroup's subsys table is guaranteed to
          * be and stay valid until the enclosing operation is complete.
          */
         if (CGROUP_HAS_SUBSYS_CONFIG && cft->ss)
                 return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
         else
                 return &cgrp->self;
 }
 EXPORT_SYMBOL_GPL(of_css);
 
 /**
  * for_each_css - iterate all css's of a cgroup
  * @css: the iteration cursor
  * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
  * @cgrp: the target cgroup to iterate css's of
  *
  * Should be called under cgroup_mutex.
  */
 #define for_each_css(css, ssid, cgrp)                                   \
         for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)        \
                 if (!((css) = rcu_dereference_check(                    \
                                 (cgrp)->subsys[(ssid)],                 \
                                 lockdep_is_held(&cgroup_mutex)))) { }   \
                 else
 
 /**
  * do_each_subsys_mask - filter for_each_subsys with a bitmask
  * @ss: the iteration cursor
  * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
  * @ss_mask: the bitmask
  *
  * The block will only run for cases where the ssid-th bit (1 << ssid) of
  * @ss_mask is set.
  */
 #define do_each_subsys_mask(ss, ssid, ss_mask) do {                     \
         unsigned long __ss_mask = (ss_mask);                            \
         if (!CGROUP_HAS_SUBSYS_CONFIG) {                                \
                 (ssid) = 0;                                             \
                 break;                                                  \
         }                                                               \
         for_each_set_bit(ssid, &__ss_mask, CGROUP_SUBSYS_COUNT) {       \
                 (ss) = cgroup_subsys[ssid];                             \
                 {
 
 #define while_each_subsys_mask()                                        \
                 }                                                       \
         }                                                               \
 } while (false)
 
 /* iterate over child cgrps, lock should be held throughout iteration */
 #define cgroup_for_each_live_child(child, cgrp)                         \
         list_for_each_entry((child), &(cgrp)->self.children, self.sibling) \
                 if (({ lockdep_assert_held(&cgroup_mutex);              \
                        cgroup_is_dead(child); }))                       \
                         ;                                               \
                 else
 
 /* walk live descendants in pre order */
 #define cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)          \
         css_for_each_descendant_pre((d_css), cgroup_css((cgrp), NULL))  \
                 if (({ lockdep_assert_held(&cgroup_mutex);              \
                        (dsct) = (d_css)->cgroup;                        \
                        cgroup_is_dead(dsct); }))                        \
                         ;                                               \
                 else
 
 /* walk live descendants in postorder */
 #define cgroup_for_each_live_descendant_post(dsct, d_css, cgrp)         \
         css_for_each_descendant_post((d_css), cgroup_css((cgrp), NULL)) \
                 if (({ lockdep_assert_held(&cgroup_mutex);              \
                        (dsct) = (d_css)->cgroup;                        \
                        cgroup_is_dead(dsct); }))                        \
                         ;                                               \
                 else
 
 /*
  * The default css_set - used by init and its children prior to any
  * hierarchies being mounted. It contains a pointer to the root state
  * for each subsystem. Also used to anchor the list of css_sets. Not
  * reference-counted, to improve performance when child cgroups
  * haven't been created.
  */
 struct css_set init_css_set = {
         .refcount               = REFCOUNT_INIT(1),
         .dom_cset               = &init_css_set,
         .tasks                  = LIST_HEAD_INIT(init_css_set.tasks),
         .mg_tasks               = LIST_HEAD_INIT(init_css_set.mg_tasks),
         .dying_tasks            = LIST_HEAD_INIT(init_css_set.dying_tasks),
         .task_iters             = LIST_HEAD_INIT(init_css_set.task_iters),
         .threaded_csets         = LIST_HEAD_INIT(init_css_set.threaded_csets),
         .cgrp_links             = LIST_HEAD_INIT(init_css_set.cgrp_links),
         .mg_src_preload_node    = LIST_HEAD_INIT(init_css_set.mg_src_preload_node),
         .mg_dst_preload_node    = LIST_HEAD_INIT(init_css_set.mg_dst_preload_node),
         .mg_node                = LIST_HEAD_INIT(init_css_set.mg_node),
 
         /*
          * The following field is re-initialized when this cset gets linked
          * in cgroup_init().  However, let's initialize the field
          * statically too so that the default cgroup can be accessed safely
          * early during boot.
          */
         .dfl_cgrp               = &cgrp_dfl_root.cgrp,
 };
 
 static int css_set_count        = 1;    /* 1 for init_css_set */
 
 static bool css_set_threaded(struct css_set *cset)
 {
         return cset->dom_cset != cset;
 }
 
 /**
  * css_set_populated - does a css_set contain any tasks?
  * @cset: target css_set
  *
  * css_set_populated() should be the same as !!cset->nr_tasks at steady
  * state. However, css_set_populated() can be called while a task is being
  * added to or removed from the linked list before the nr_tasks is
  * properly updated. Hence, we can't just look at ->nr_tasks here.
  */
 static bool css_set_populated(struct css_set *cset)
 {
         lockdep_assert_held(&css_set_lock);
 
         return !list_empty(&cset->tasks) || !list_empty(&cset->mg_tasks);
 }
 
 /**
  * cgroup_update_populated - update the populated count of a cgroup
  * @cgrp: the target cgroup
  * @populated: inc or dec populated count
  *
  * One of the css_sets associated with @cgrp is either getting its first
  * task or losing the last.  Update @cgrp->nr_populated_* accordingly.  The
  * count is propagated towards root so that a given cgroup's
  * nr_populated_children is zero iff none of its descendants contain any
  * tasks.
  *
  * @cgrp's interface file "cgroup.populated" is zero if both
  * @cgrp->nr_populated_csets and @cgrp->nr_populated_children are zero and
  * 1 otherwise.  When the sum changes from or to zero, userland is notified
  * that the content of the interface file has changed.  This can be used to
  * detect when @cgrp and its descendants become populated or empty.
  */
 static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 {
         struct cgroup *child = NULL;
         int adj = populated ? 1 : -1;
 
         lockdep_assert_held(&css_set_lock);
 
         do {
                 bool was_populated = cgroup_is_populated(cgrp);
 
                 if (!child) {
                         cgrp->nr_populated_csets += adj;
                 } else {
                         if (cgroup_is_threaded(child))
                                 cgrp->nr_populated_threaded_children += adj;
                         else
                                 cgrp->nr_populated_domain_children += adj;
                 }
 
                 if (was_populated == cgroup_is_populated(cgrp))
                         break;
 
                 cgroup1_check_for_release(cgrp);
                 TRACE_CGROUP_PATH(notify_populated, cgrp,
                                   cgroup_is_populated(cgrp));
                 cgroup_file_notify(&cgrp->events_file);
 
                 child = cgrp;
                 cgrp = cgroup_parent(cgrp);
         } while (cgrp);
 }
 
 /**
  * css_set_update_populated - update populated state of a css_set
  * @cset: target css_set
  * @populated: whether @cset is populated or depopulated
  *
  * @cset is either getting the first task or losing the last.  Update the
  * populated counters of all associated cgroups accordingly.
  */
 static void css_set_update_populated(struct css_set *cset, bool populated)
 {
         struct cgrp_cset_link *link;
 
         lockdep_assert_held(&css_set_lock);
 
         list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
                 cgroup_update_populated(link->cgrp, populated);
 }
 
 /*
  * @task is leaving, advance task iterators which are pointing to it so
  * that they can resume at the next position.  Advancing an iterator might
  * remove it from the list, use safe walk.  See css_task_iter_skip() for
  * details.
  */
 static void css_set_skip_task_iters(struct css_set *cset,
                                     struct task_struct *task)
 {
         struct css_task_iter *it, *pos;
 
         list_for_each_entry_safe(it, pos, &cset->task_iters, iters_node)
                 css_task_iter_skip(it, task);
 }
 
 /**
  * css_set_move_task - move a task from one css_set to another
  * @task: task being moved
  * @from_cset: css_set @task currently belongs to (may be NULL)
  * @to_cset: new css_set @task is being moved to (may be NULL)
  * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
  *
  * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
  * css_set, @from_cset can be NULL.  If @task is being disassociated
  * instead of moved, @to_cset can be NULL.
  *
  * This function automatically handles populated counter updates and
  * css_task_iter adjustments but the caller is responsible for managing
  * @from_cset and @to_cset's reference counts.
  */
 static void css_set_move_task(struct task_struct *task,
                               struct css_set *from_cset, struct css_set *to_cset,
                               bool use_mg_tasks)
 {
         lockdep_assert_held(&css_set_lock);
 
         if (to_cset && !css_set_populated(to_cset))
                 css_set_update_populated(to_cset, true);
 
         if (from_cset) {
                 WARN_ON_ONCE(list_empty(&task->cg_list));
 
                 css_set_skip_task_iters(from_cset, task);
                 list_del_init(&task->cg_list);
                 if (!css_set_populated(from_cset))
                         css_set_update_populated(from_cset, false);
         } else {
                 WARN_ON_ONCE(!list_empty(&task->cg_list));
         }
 
         if (to_cset) {
                 /*
                  * We are synchronized through cgroup_threadgroup_rwsem
                  * against PF_EXITING setting such that we can't race
                  * against cgroup_exit()/cgroup_free() dropping the css_set.
                  */
                 WARN_ON_ONCE(task->flags & PF_EXITING);
 
                 cgroup_move_task(task, to_cset);
                 list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
                                                              &to_cset->tasks);
         }
 }
 
 /*
  * hash table for cgroup groups. This improves the performance to find
  * an existing css_set. This hash doesn't (currently) take into
  * account cgroups in empty hierarchies.
  */
 #define CSS_SET_HASH_BITS       7
 static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);
 
 static unsigned long css_set_hash(struct cgroup_subsys_state **css)
 {
         unsigned long key = 0UL;
         struct cgroup_subsys *ss;
         int i;
 
         for_each_subsys(ss, i)
                 key += (unsigned long)css[i];
         key = (key >> 16) ^ key;
 
         return key;
 }
 
 void put_css_set_locked(struct css_set *cset)
 {
         struct cgrp_cset_link *link, *tmp_link;
         struct cgroup_subsys *ss;
         int ssid;
 
         lockdep_assert_held(&css_set_lock);
 
         if (!refcount_dec_and_test(&cset->refcount))
                 return;
 
         WARN_ON_ONCE(!list_empty(&cset->threaded_csets));
 
         /* This css_set is dead. Unlink it and release cgroup and css refs */
         for_each_subsys(ss, ssid) {
                 list_del(&cset->e_cset_node[ssid]);
                 css_put(cset->subsys[ssid]);
         }
         hash_del(&cset->hlist);
         css_set_count--;
 
         list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
                 list_del(&link->cset_link);
                 list_del(&link->cgrp_link);
                 if (cgroup_parent(link->cgrp))
                         cgroup_put(link->cgrp);
                 kfree(link);
         }
 
         if (css_set_threaded(cset)) {
                 list_del(&cset->threaded_csets_node);
                 put_css_set_locked(cset->dom_cset);
         }
 
         kfree_rcu(cset, rcu_head);
 }
 
 /**
  * compare_css_sets - helper function for find_existing_css_set().
  * @cset: candidate css_set being tested
  * @old_cset: existing css_set for a task
  * @new_cgrp: cgroup that's being entered by the task
  * @template: desired set of css pointers in css_set (pre-calculated)
  *
  * Returns true if "cset" matches "old_cset" except for the hierarchy
  * which "new_cgrp" belongs to, for which it should match "new_cgrp".
  */
 static bool compare_css_sets(struct css_set *cset,
                              struct css_set *old_cset,
                              struct cgroup *new_cgrp,
                              struct cgroup_subsys_state *template[])
 {
         struct cgroup *new_dfl_cgrp;
         struct list_head *l1, *l2;
 
         /*
          * On the default hierarchy, there can be csets which are
          * associated with the same set of cgroups but different csses.
          * Let's first ensure that csses match.
          */
         if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
                 return false;
 
 
         /* @cset's domain should match the default cgroup's */
         if (cgroup_on_dfl(new_cgrp))
                 new_dfl_cgrp = new_cgrp;
         else
                 new_dfl_cgrp = old_cset->dfl_cgrp;
 
         if (new_dfl_cgrp->dom_cgrp != cset->dom_cset->dfl_cgrp)
                 return false;
 
         /*
          * Compare cgroup pointers in order to distinguish between
          * different cgroups in hierarchies.  As different cgroups may
          * share the same effective css, this comparison is always
          * necessary.
          */
         l1 = &cset->cgrp_links;
         l2 = &old_cset->cgrp_links;
         while (1) {
                 struct cgrp_cset_link *link1, *link2;
                 struct cgroup *cgrp1, *cgrp2;
 
                 l1 = l1->next;
                 l2 = l2->next;
                 /* See if we reached the end - both lists are equal length. */
                 if (l1 == &cset->cgrp_links) {
                         BUG_ON(l2 != &old_cset->cgrp_links);
                         break;
                 } else {
                         BUG_ON(l2 == &old_cset->cgrp_links);
                 }
                 /* Locate the cgroups associated with these links. */
                 link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
                 link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
                 cgrp1 = link1->cgrp;
                 cgrp2 = link2->cgrp;
                 /* Hierarchies should be linked in the same order. */
                 BUG_ON(cgrp1->root != cgrp2->root);
 
                 /*
                  * If this hierarchy is the hierarchy of the cgroup
                  * that's changing, then we need to check that this
                  * css_set points to the new cgroup; if it's any other
                  * hierarchy, then this css_set should point to the
                  * same cgroup as the old css_set.
                  */
                 if (cgrp1->root == new_cgrp->root) {
                         if (cgrp1 != new_cgrp)
                                 return false;
                 } else {
                         if (cgrp1 != cgrp2)
                                 return false;
                 }
         }
         return true;
 }
 
 /**
  * find_existing_css_set - init css array and find the matching css_set
  * @old_cset: the css_set that we're using before the cgroup transition
  * @cgrp: the cgroup that we're moving into
  * @template: out param for the new set of csses, should be clear on entry
  */
 static struct css_set *find_existing_css_set(struct css_set *old_cset,
                                         struct cgroup *cgrp,
                                         struct cgroup_subsys_state **template)
 {
         struct cgroup_root *root = cgrp->root;
         struct cgroup_subsys *ss;
         struct css_set *cset;
         unsigned long key;
         int i;
 
         /*
          * Build the set of subsystem state objects that we want to see in the
          * new css_set. While subsystems can change globally, the entries here
          * won't change, so no need for locking.
          */
         for_each_subsys(ss, i) {
                 if (root->subsys_mask & (1UL << i)) {
                         /*
                          * @ss is in this hierarchy, so we want the
                          * effective css from @cgrp.
                          */
                         template[i] = cgroup_e_css_by_mask(cgrp, ss);
                 } else {
                         /*
                          * @ss is not in this hierarchy, so we don't want
                          * to change the css.
                          */
                         template[i] = old_cset->subsys[i];
                 }
         }
 
         key = css_set_hash(template);
         hash_for_each_possible(css_set_table, cset, hlist, key) {
                 if (!compare_css_sets(cset, old_cset, cgrp, template))
                         continue;
 
                 /* This css_set matches what we need */
                 return cset;
         }
 
         /* No existing cgroup group matched */
         return NULL;
 }
 
 static void free_cgrp_cset_links(struct list_head *links_to_free)
 {
         struct cgrp_cset_link *link, *tmp_link;
 
         list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
                 list_del(&link->cset_link);
                 kfree(link);
         }
 }
 
 /**
  * allocate_cgrp_cset_links - allocate cgrp_cset_links
  * @count: the number of links to allocate
  * @tmp_links: list_head the allocated links are put on
  *
  * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
  * through ->cset_link.  Returns 0 on success or -errno.
  */
 static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
 {
         struct cgrp_cset_link *link;
         int i;
 
         INIT_LIST_HEAD(tmp_links);
 
         for (i = 0; i < count; i++) {
                 link = kzalloc(sizeof(*link), GFP_KERNEL);
                 if (!link) {
                         free_cgrp_cset_links(tmp_links);
                         return -ENOMEM;
                 }
                 list_add(&link->cset_link, tmp_links);
         }
         return 0;
 }
 
 /**
  * link_css_set - a helper function to link a css_set to a cgroup
  * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
  * @cset: the css_set to be linked
  * @cgrp: the destination cgroup
  */
 static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
                          struct cgroup *cgrp)
 {
         struct cgrp_cset_link *link;
 
         BUG_ON(list_empty(tmp_links));
 
         if (cgroup_on_dfl(cgrp))
                 cset->dfl_cgrp = cgrp;
 
         link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
         link->cset = cset;
         link->cgrp = cgrp;
 
         /*
          * Always add links to the tail of the lists so that the lists are
          * in chronological order.
          */
         list_move_tail(&link->cset_link, &cgrp->cset_links);
         list_add_tail(&link->cgrp_link, &cset->cgrp_links);
 
         if (cgroup_parent(cgrp))
                 cgroup_get_live(cgrp);
 }
 
 /**
  * find_css_set - return a new css_set with one cgroup updated
  * @old_cset: the baseline css_set
  * @cgrp: the cgroup to be updated
  *
  * Return a new css_set that's equivalent to @old_cset, but with @cgrp
  * substituted into the appropriate hierarchy.
  */
 static struct css_set *find_css_set(struct css_set *old_cset,
                                     struct cgroup *cgrp)
 {
         struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
         struct css_set *cset;
         struct list_head tmp_links;
         struct cgrp_cset_link *link;
         struct cgroup_subsys *ss;
         unsigned long key;
         int ssid;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* First see if we already have a cgroup group that matches
          * the desired set */
         spin_lock_irq(&css_set_lock);
         cset = find_existing_css_set(old_cset, cgrp, template);
         if (cset)
                 get_css_set(cset);
         spin_unlock_irq(&css_set_lock);
 
         if (cset)
                 return cset;
 
         cset = kzalloc(sizeof(*cset), GFP_KERNEL);
         if (!cset)
                 return NULL;
 
         /* Allocate all the cgrp_cset_link objects that we'll need */
         if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
                 kfree(cset);
                 return NULL;
         }
 
         refcount_set(&cset->refcount, 1);
         cset->dom_cset = cset;
         INIT_LIST_HEAD(&cset->tasks);
         INIT_LIST_HEAD(&cset->mg_tasks);
         INIT_LIST_HEAD(&cset->dying_tasks);
         INIT_LIST_HEAD(&cset->task_iters);
         INIT_LIST_HEAD(&cset->threaded_csets);
         INIT_HLIST_NODE(&cset->hlist);
         INIT_LIST_HEAD(&cset->cgrp_links);
         INIT_LIST_HEAD(&cset->mg_src_preload_node);
         INIT_LIST_HEAD(&cset->mg_dst_preload_node);
         INIT_LIST_HEAD(&cset->mg_node);
 
         /* Copy the set of subsystem state objects generated in
          * find_existing_css_set() */
         memcpy(cset->subsys, template, sizeof(cset->subsys));
 
         spin_lock_irq(&css_set_lock);
         /* Add reference counts and links from the new css_set. */
         list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
                 struct cgroup *c = link->cgrp;
 
                 if (c->root == cgrp->root)
                         c = cgrp;
                 link_css_set(&tmp_links, cset, c);
         }
 
         BUG_ON(!list_empty(&tmp_links));
 
         css_set_count++;
 
         /* Add @cset to the hash table */
         key = css_set_hash(cset->subsys);
         hash_add(css_set_table, &cset->hlist, key);
 
         for_each_subsys(ss, ssid) {
                 struct cgroup_subsys_state *css = cset->subsys[ssid];
 
                 list_add_tail(&cset->e_cset_node[ssid],
                               &css->cgroup->e_csets[ssid]);
                 css_get(css);
         }
 
         spin_unlock_irq(&css_set_lock);
 
         /*
          * If @cset should be threaded, look up the matching dom_cset and
          * link them up.  We first fully initialize @cset then look for the
          * dom_cset.  It's simpler this way and safe as @cset is guaranteed
          * to stay empty until we return.
          */
         if (cgroup_is_threaded(cset->dfl_cgrp)) {
                 struct css_set *dcset;
 
                 dcset = find_css_set(cset, cset->dfl_cgrp->dom_cgrp);
                 if (!dcset) {
                         put_css_set(cset);
                         return NULL;
                 }
 
                 spin_lock_irq(&css_set_lock);
                 cset->dom_cset = dcset;
                 list_add_tail(&cset->threaded_csets_node,
                               &dcset->threaded_csets);
                 spin_unlock_irq(&css_set_lock);
         }
 
         return cset;
 }
 
 struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
 {
         struct cgroup *root_cgrp = kernfs_root_to_node(kf_root)->priv;
 
         return root_cgrp->root;
 }
 
 void cgroup_favor_dynmods(struct cgroup_root *root, bool favor)
 {
         bool favoring = root->flags & CGRP_ROOT_FAVOR_DYNMODS;
 
         /* see the comment above CGRP_ROOT_FAVOR_DYNMODS definition */
         if (favor && !favoring) {
                 rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
                 root->flags |= CGRP_ROOT_FAVOR_DYNMODS;
         } else if (!favor && favoring) {
                 rcu_sync_exit(&cgroup_threadgroup_rwsem.rss);
                 root->flags &= ~CGRP_ROOT_FAVOR_DYNMODS;
         }
 }
 
 static int cgroup_init_root_id(struct cgroup_root *root)
 {
         int id;
 
         lockdep_assert_held(&cgroup_mutex);
 
         id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
         if (id < 0)
                 return id;
 
         root->hierarchy_id = id;
         return 0;
 }
 
 static void cgroup_exit_root_id(struct cgroup_root *root)
 {
         lockdep_assert_held(&cgroup_mutex);
 
         idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
 }
 
 void cgroup_free_root(struct cgroup_root *root)
 {
         kfree_rcu(root, rcu);
 }
 
 static void cgroup_destroy_root(struct cgroup_root *root)
 {
         struct cgroup *cgrp = &root->cgrp;
         struct cgrp_cset_link *link, *tmp_link;
 
         trace_cgroup_destroy_root(root);
 
         cgroup_lock_and_drain_offline(&cgrp_dfl_root.cgrp);
 
         BUG_ON(atomic_read(&root->nr_cgrps));
         BUG_ON(!list_empty(&cgrp->self.children));
 
         /* Rebind all subsystems back to the default hierarchy */
         WARN_ON(rebind_subsystems(&cgrp_dfl_root, root->subsys_mask));
 
         /*
          * Release all the links from cset_links to this hierarchy's
          * root cgroup
          */
         spin_lock_irq(&css_set_lock);
 
         list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
                 list_del(&link->cset_link);
                 list_del(&link->cgrp_link);
                 kfree(link);
         }
 
         spin_unlock_irq(&css_set_lock);
 
         WARN_ON_ONCE(list_empty(&root->root_list));
         list_del_rcu(&root->root_list);
         cgroup_root_count--;
 
         if (!have_favordynmods)
                 cgroup_favor_dynmods(root, false);
 
         cgroup_exit_root_id(root);
 
         cgroup_unlock();
 
         cgroup_rstat_exit(cgrp);
         kernfs_destroy_root(root->kf_root);
         cgroup_free_root(root);
 }
 
 /*
  * Returned cgroup is without refcount but it's valid as long as cset pins it.
  */
 static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset,
                                             struct cgroup_root *root)
 {
         struct cgroup *res_cgroup = NULL;
 
         if (cset == &init_css_set) {
                 res_cgroup = &root->cgrp;
         } else if (root == &cgrp_dfl_root) {
                 res_cgroup = cset->dfl_cgrp;
         } else {
                 struct cgrp_cset_link *link;
                 lockdep_assert_held(&css_set_lock);
 
                 list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                         struct cgroup *c = link->cgrp;
 
                         if (c->root == root) {
                                 res_cgroup = c;
                                 break;
                         }
                 }
         }
 
         /*
          * If cgroup_mutex is not held, the cgrp_cset_link will be freed
          * before we remove the cgroup root from the root_list. Consequently,
          * when accessing a cgroup root, the cset_link may have already been
          * freed, resulting in a NULL res_cgroup. However, by holding the
          * cgroup_mutex, we ensure that res_cgroup can't be NULL.
          * If we don't hold cgroup_mutex in the caller, we must do the NULL
          * check.
          */
         return res_cgroup;
 }
 
 /*
  * look up cgroup associated with current task's cgroup namespace on the
  * specified hierarchy
  */
 static struct cgroup *
 current_cgns_cgroup_from_root(struct cgroup_root *root)
 {
         struct cgroup *res = NULL;
         struct css_set *cset;
 
         lockdep_assert_held(&css_set_lock);
 
         rcu_read_lock();
 
         cset = current->nsproxy->cgroup_ns->root_cset;
         res = __cset_cgroup_from_root(cset, root);
 
         rcu_read_unlock();
 
         /*
          * The namespace_sem is held by current, so the root cgroup can't
          * be umounted. Therefore, we can ensure that the res is non-NULL.
          */
         WARN_ON_ONCE(!res);
         return res;
 }
 
 /*
  * Look up cgroup associated with current task's cgroup namespace on the default
  * hierarchy.
  *
  * Unlike current_cgns_cgroup_from_root(), this doesn't need locks:
  * - Internal rcu_read_lock is unnecessary because we don't dereference any rcu
  *   pointers.
  * - css_set_lock is not needed because we just read cset->dfl_cgrp.
  * - As a bonus returned cgrp is pinned with the current because it cannot
  *   switch cgroup_ns asynchronously.
  */
 static struct cgroup *current_cgns_cgroup_dfl(void)
 {
         struct css_set *cset;
 
         if (current->nsproxy) {
                 cset = current->nsproxy->cgroup_ns->root_cset;
                 return __cset_cgroup_from_root(cset, &cgrp_dfl_root);
         } else {
                 /*
                  * NOTE: This function may be called from bpf_cgroup_from_id()
                  * on a task which has already passed exit_task_namespaces() and
                  * nsproxy == NULL. Fall back to cgrp_dfl_root which will make all
                  * cgroups visible for lookups.
                  */
                 return &cgrp_dfl_root.cgrp;
         }
 }
 
 /* look up cgroup associated with given css_set on the specified hierarchy */
 static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
                                             struct cgroup_root *root)
 {
         lockdep_assert_held(&css_set_lock);
 
         return __cset_cgroup_from_root(cset, root);
 }
 
 /*
  * Return the cgroup for "task" from the given hierarchy. Must be
  * called with css_set_lock held to prevent task's groups from being modified.
  * Must be called with either cgroup_mutex or rcu read lock to prevent the
  * cgroup root from being destroyed.
  */
 struct cgroup *task_cgroup_from_root(struct task_struct *task,
                                      struct cgroup_root *root)
 {
         /*
          * No need to lock the task - since we hold css_set_lock the
          * task can't change groups.
          */
         return cset_cgroup_from_root(task_css_set(task), root);
 }
 
 /*
  * A task must hold cgroup_mutex to modify cgroups.
  *
  * Any task can increment and decrement the count field without lock.
  * So in general, code holding cgroup_mutex can't rely on the count
  * field not changing.  However, if the count goes to zero, then only
  * cgroup_attach_task() can increment it again.  Because a count of zero
  * means that no tasks are currently attached, therefore there is no
  * way a task attached to that cgroup can fork (the other way to
  * increment the count).  So code holding cgroup_mutex can safely
  * assume that if the count is zero, it will stay zero. Similarly, if
  * a task holds cgroup_mutex on a cgroup with zero count, it
  * knows that the cgroup won't be removed, as cgroup_rmdir()
  * needs that mutex.
  *
  * A cgroup can only be deleted if both its 'count' of using tasks
  * is zero, and its list of 'children' cgroups is empty.  Since all
  * tasks in the system use _some_ cgroup, and since there is always at
  * least one task in the system (init, pid == 1), therefore, root cgroup
  * always has either children cgroups and/or using tasks.  So we don't
  * need a special hack to ensure that root cgroup cannot be deleted.
  *
  * P.S.  One more locking exception.  RCU is used to guard the
  * update of a tasks cgroup pointer by cgroup_attach_task()
  */
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
 
 static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
                               char *buf)
 {
         struct cgroup_subsys *ss = cft->ss;
 
         if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
             !(cgrp->root->flags & CGRP_ROOT_NOPREFIX)) {
                 const char *dbg = (cft->flags & CFTYPE_DEBUG) ? ".__DEBUG__." : "";
 
                 snprintf(buf, CGROUP_FILE_NAME_MAX, "%s%s.%s",
                          dbg, cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
                          cft->name);
         } else {
                 strscpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
         }
         return buf;
 }
 
 /**
  * cgroup_file_mode - deduce file mode of a control file
  * @cft: the control file in question
  *
  * S_IRUGO for read, S_IWUSR for write.
  */
 static umode_t cgroup_file_mode(const struct cftype *cft)
 {
         umode_t mode = 0;
 
         if (cft->read_u64 || cft->read_s64 || cft->seq_show)
                 mode |= S_IRUGO;
 
         if (cft->write_u64 || cft->write_s64 || cft->write) {
                 if (cft->flags & CFTYPE_WORLD_WRITABLE)
                         mode |= S_IWUGO;
                 else
                         mode |= S_IWUSR;
         }
 
         return mode;
 }
 
 /**
  * cgroup_calc_subtree_ss_mask - calculate subtree_ss_mask
  * @subtree_control: the new subtree_control mask to consider
  * @this_ss_mask: available subsystems
  *
  * On the default hierarchy, a subsystem may request other subsystems to be
  * enabled together through its ->depends_on mask.  In such cases, more
  * subsystems than specified in "cgroup.subtree_control" may be enabled.
  *
  * This function calculates which subsystems need to be enabled if
  * @subtree_control is to be applied while restricted to @this_ss_mask.
  */
 static u16 cgroup_calc_subtree_ss_mask(u16 subtree_control, u16 this_ss_mask)
 {
         u16 cur_ss_mask = subtree_control;
         struct cgroup_subsys *ss;
         int ssid;
 
         lockdep_assert_held(&cgroup_mutex);
 
         cur_ss_mask |= cgrp_dfl_implicit_ss_mask;
 
         while (true) {
                 u16 new_ss_mask = cur_ss_mask;
 
                 do_each_subsys_mask(ss, ssid, cur_ss_mask) {
                         new_ss_mask |= ss->depends_on;
                 } while_each_subsys_mask();
 
                 /*
                  * Mask out subsystems which aren't available.  This can
                  * happen only if some depended-upon subsystems were bound
                  * to non-default hierarchies.
                  */
                 new_ss_mask &= this_ss_mask;
 
                 if (new_ss_mask == cur_ss_mask)
                         break;
                 cur_ss_mask = new_ss_mask;
         }
 
         return cur_ss_mask;
 }
 
 /**
  * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
  * @kn: the kernfs_node being serviced
  *
  * This helper undoes cgroup_kn_lock_live() and should be invoked before
  * the method finishes if locking succeeded.  Note that once this function
  * returns the cgroup returned by cgroup_kn_lock_live() may become
  * inaccessible any time.  If the caller intends to continue to access the
  * cgroup, it should pin it before invoking this function.
  */
 void cgroup_kn_unlock(struct kernfs_node *kn)
 {
         struct cgroup *cgrp;
 
         if (kernfs_type(kn) == KERNFS_DIR)
                 cgrp = kn->priv;
         else
                 cgrp = kn->parent->priv;
 
         cgroup_unlock();
 
         kernfs_unbreak_active_protection(kn);
         cgroup_put(cgrp);
 }
 
 /**
  * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
  * @kn: the kernfs_node being serviced
  * @drain_offline: perform offline draining on the cgroup
  *
  * This helper is to be used by a cgroup kernfs method currently servicing
  * @kn.  It breaks the active protection, performs cgroup locking and
  * verifies that the associated cgroup is alive.  Returns the cgroup if
  * alive; otherwise, %NULL.  A successful return should be undone by a
  * matching cgroup_kn_unlock() invocation.  If @drain_offline is %true, the
  * cgroup is drained of offlining csses before return.
  *
  * Any cgroup kernfs method implementation which requires locking the
  * associated cgroup should use this helper.  It avoids nesting cgroup
  * locking under kernfs active protection and allows all kernfs operations
  * including self-removal.
  */
 struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn, bool drain_offline)
 {
         struct cgroup *cgrp;
 
         if (kernfs_type(kn) == KERNFS_DIR)
                 cgrp = kn->priv;
         else
                 cgrp = kn->parent->priv;
 
         /*
          * We're gonna grab cgroup_mutex which nests outside kernfs
          * active_ref.  cgroup liveliness check alone provides enough
          * protection against removal.  Ensure @cgrp stays accessible and
          * break the active_ref protection.
          */
         if (!cgroup_tryget(cgrp))
                 return NULL;
         kernfs_break_active_protection(kn);
 
         if (drain_offline)
                 cgroup_lock_and_drain_offline(cgrp);
         else
                 cgroup_lock();
 
         if (!cgroup_is_dead(cgrp))
                 return cgrp;
 
         cgroup_kn_unlock(kn);
         return NULL;
 }
 
 static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 {
         char name[CGROUP_FILE_NAME_MAX];
 
         lockdep_assert_held(&cgroup_mutex);
 
         if (cft->file_offset) {
                 struct cgroup_subsys_state *css = cgroup_css(cgrp, cft->ss);
                 struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
                 spin_lock_irq(&cgroup_file_kn_lock);
                 cfile->kn = NULL;
                 spin_unlock_irq(&cgroup_file_kn_lock);
 
                 del_timer_sync(&cfile->notify_timer);
         }
 
         kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
 }
 
 /**
  * css_clear_dir - remove subsys files in a cgroup directory
  * @css: target css
  */
 static void css_clear_dir(struct cgroup_subsys_state *css)
 {
         struct cgroup *cgrp = css->cgroup;
         struct cftype *cfts;
 
         if (!(css->flags & CSS_VISIBLE))
                 return;
 
         css->flags &= ~CSS_VISIBLE;
 
         if (!css->ss) {
                 if (cgroup_on_dfl(cgrp)) {
                         cgroup_addrm_files(css, cgrp,
                                            cgroup_base_files, false);
                         if (cgroup_psi_enabled())
                                 cgroup_addrm_files(css, cgrp,
                                                    cgroup_psi_files, false);
                 } else {
                         cgroup_addrm_files(css, cgrp,
                                            cgroup1_base_files, false);
                 }
         } else {
                 list_for_each_entry(cfts, &css->ss->cfts, node)
                         cgroup_addrm_files(css, cgrp, cfts, false);
         }
 }
 
 /**
  * css_populate_dir - create subsys files in a cgroup directory
  * @css: target css
  *
  * On failure, no file is added.
  */
 static int css_populate_dir(struct cgroup_subsys_state *css)
 {
         struct cgroup *cgrp = css->cgroup;
         struct cftype *cfts, *failed_cfts;
         int ret;
 
         if (css->flags & CSS_VISIBLE)
                 return 0;
 
         if (!css->ss) {
                 if (cgroup_on_dfl(cgrp)) {
                         ret = cgroup_addrm_files(css, cgrp,
                                                  cgroup_base_files, true);
                         if (ret < 0)
                                 return ret;
 
                         if (cgroup_psi_enabled()) {
                                 ret = cgroup_addrm_files(css, cgrp,
                                                          cgroup_psi_files, true);
                                 if (ret < 0) {
                                         cgroup_addrm_files(css, cgrp,
                                                            cgroup_base_files, false);
                                         return ret;
                                 }
                         }
                 } else {
                         ret = cgroup_addrm_files(css, cgrp,
                                                  cgroup1_base_files, true);
                         if (ret < 0)
                                 return ret;
                 }
         } else {
                 list_for_each_entry(cfts, &css->ss->cfts, node) {
                         ret = cgroup_addrm_files(css, cgrp, cfts, true);
                         if (ret < 0) {
                                 failed_cfts = cfts;
                                 goto err;
                         }
                 }
         }
 
         css->flags |= CSS_VISIBLE;
 
         return 0;
 err:
         list_for_each_entry(cfts, &css->ss->cfts, node) {
                 if (cfts == failed_cfts)
                         break;
                 cgroup_addrm_files(css, cgrp, cfts, false);
         }
         return ret;
 }
 
 int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 {
         struct cgroup *dcgrp = &dst_root->cgrp;
         struct cgroup_subsys *ss;
         int ssid, ret;
         u16 dfl_disable_ss_mask = 0;
 
         lockdep_assert_held(&cgroup_mutex);
 
         do_each_subsys_mask(ss, ssid, ss_mask) {
                 /*
                  * If @ss has non-root csses attached to it, can't move.
                  * If @ss is an implicit controller, it is exempt from this
                  * rule and can be stolen.
                  */
                 if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)) &&
                     !ss->implicit_on_dfl)
                         return -EBUSY;
 
                 /* can't move between two non-dummy roots either */
                 if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
                         return -EBUSY;
 
                 /*
                  * Collect ssid's that need to be disabled from default
                  * hierarchy.
                  */
                 if (ss->root == &cgrp_dfl_root)
                         dfl_disable_ss_mask |= 1 << ssid;
 
         } while_each_subsys_mask();
 
         if (dfl_disable_ss_mask) {
                 struct cgroup *scgrp = &cgrp_dfl_root.cgrp;
 
                 /*
                  * Controllers from default hierarchy that need to be rebound
                  * are all disabled together in one go.
                  */
                 cgrp_dfl_root.subsys_mask &= ~dfl_disable_ss_mask;
                 WARN_ON(cgroup_apply_control(scgrp));
                 cgroup_finalize_control(scgrp, 0);
         }
 
         do_each_subsys_mask(ss, ssid, ss_mask) {
                 struct cgroup_root *src_root = ss->root;
                 struct cgroup *scgrp = &src_root->cgrp;
                 struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
                 struct css_set *cset, *cset_pos;
                 struct css_task_iter *it;
 
                 WARN_ON(!css || cgroup_css(dcgrp, ss));
 
                 if (src_root != &cgrp_dfl_root) {
                         /* disable from the source */
                         src_root->subsys_mask &= ~(1 << ssid);
                         WARN_ON(cgroup_apply_control(scgrp));
                         cgroup_finalize_control(scgrp, 0);
                 }
 
                 /* rebind */
                 RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
                 rcu_assign_pointer(dcgrp->subsys[ssid], css);
                 ss->root = dst_root;
 
                 spin_lock_irq(&css_set_lock);
                 css->cgroup = dcgrp;
                 WARN_ON(!list_empty(&dcgrp->e_csets[ss->id]));
                 list_for_each_entry_safe(cset, cset_pos, &scgrp->e_csets[ss->id],
                                          e_cset_node[ss->id]) {
                         list_move_tail(&cset->e_cset_node[ss->id],
                                        &dcgrp->e_csets[ss->id]);
                         /*
                          * all css_sets of scgrp together in same order to dcgrp,
                          * patch in-flight iterators to preserve correct iteration.
                          * since the iterator is always advanced right away and
                          * finished when it->cset_pos meets it->cset_head, so only
                          * update it->cset_head is enough here.
                          */
                         list_for_each_entry(it, &cset->task_iters, iters_node)
                                 if (it->cset_head == &scgrp->e_csets[ss->id])
                                         it->cset_head = &dcgrp->e_csets[ss->id];
                 }
                 spin_unlock_irq(&css_set_lock);
 
                 if (ss->css_rstat_flush) {
                         list_del_rcu(&css->rstat_css_node);
                         synchronize_rcu();
                         list_add_rcu(&css->rstat_css_node,
                                      &dcgrp->rstat_css_list);
                 }
 
                 /* default hierarchy doesn't enable controllers by default */
                 dst_root->subsys_mask |= 1 << ssid;
                 if (dst_root == &cgrp_dfl_root) {
                         static_branch_enable(cgroup_subsys_on_dfl_key[ssid]);
                 } else {
                         dcgrp->subtree_control |= 1 << ssid;
                         static_branch_disable(cgroup_subsys_on_dfl_key[ssid]);
                 }
 
                 ret = cgroup_apply_control(dcgrp);
                 if (ret)
                         pr_warn("partial failure to rebind %s controller (err=%d)\n",
                                 ss->name, ret);
 
                 if (ss->bind)
                         ss->bind(css);
         } while_each_subsys_mask();
 
         kernfs_activate(dcgrp->kn);
         return 0;
 }
 
 int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
                      struct kernfs_root *kf_root)
 {
         int len = 0;
         char *buf = NULL;
         struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
         struct cgroup *ns_cgroup;
 
         buf = kmalloc(PATH_MAX, GFP_KERNEL);
         if (!buf)
                 return -ENOMEM;
 
         spin_lock_irq(&css_set_lock);
         ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
         len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
         spin_unlock_irq(&css_set_lock);
 
         if (len == -E2BIG)
                 len = -ERANGE;
         else if (len > 0) {
                 seq_escape(sf, buf, " \t\n\\");
                 len = 0;
         }
         kfree(buf);
         return len;
 }
 
 enum cgroup2_param {
         Opt_nsdelegate,
         Opt_favordynmods,
         Opt_memory_localevents,
         Opt_memory_recursiveprot,
         Opt_memory_hugetlb_accounting,
         Opt_pids_localevents,
         nr__cgroup2_params
 };
 
 static const struct fs_parameter_spec cgroup2_fs_parameters[] = {
         fsparam_flag("nsdelegate",              Opt_nsdelegate),
         fsparam_flag("favordynmods",            Opt_favordynmods),
         fsparam_flag("memory_localevents",      Opt_memory_localevents),
         fsparam_flag("memory_recursiveprot",    Opt_memory_recursiveprot),
         fsparam_flag("memory_hugetlb_accounting", Opt_memory_hugetlb_accounting),
         fsparam_flag("pids_localevents",        Opt_pids_localevents),
         {}
 };
 
 static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
         struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
         struct fs_parse_result result;
         int opt;
 
         opt = fs_parse(fc, cgroup2_fs_parameters, param, &result);
         if (opt < 0)
                 return opt;
 
         switch (opt) {
         case Opt_nsdelegate:
                 ctx->flags |= CGRP_ROOT_NS_DELEGATE;
                 return 0;
         case Opt_favordynmods:
                 ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
                 return 0;
         case Opt_memory_localevents:
                 ctx->flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
                 return 0;
         case Opt_memory_recursiveprot:
                 ctx->flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
                 return 0;
         case Opt_memory_hugetlb_accounting:
                 ctx->flags |= CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
                 return 0;
         case Opt_pids_localevents:
                 ctx->flags |= CGRP_ROOT_PIDS_LOCAL_EVENTS;
                 return 0;
         }
         return -EINVAL;
 }
 
 static void apply_cgroup_root_flags(unsigned int root_flags)
 {
         if (current->nsproxy->cgroup_ns == &init_cgroup_ns) {
                 if (root_flags & CGRP_ROOT_NS_DELEGATE)
                         cgrp_dfl_root.flags |= CGRP_ROOT_NS_DELEGATE;
                 else
                         cgrp_dfl_root.flags &= ~CGRP_ROOT_NS_DELEGATE;
 
                 cgroup_favor_dynmods(&cgrp_dfl_root,
                                      root_flags & CGRP_ROOT_FAVOR_DYNMODS);
 
                 if (root_flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
                         cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
                 else
                         cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 
                 if (root_flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
                         cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
                 else
                         cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_RECURSIVE_PROT;
 
                 if (root_flags & CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING)
                         cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
                 else
                         cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING;
 
                 if (root_flags & CGRP_ROOT_PIDS_LOCAL_EVENTS)
                         cgrp_dfl_root.flags |= CGRP_ROOT_PIDS_LOCAL_EVENTS;
                 else
                         cgrp_dfl_root.flags &= ~CGRP_ROOT_PIDS_LOCAL_EVENTS;
         }
 }
 
 static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root)
 {
         if (cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE)
                 seq_puts(seq, ",nsdelegate");
         if (cgrp_dfl_root.flags & CGRP_ROOT_FAVOR_DYNMODS)
                 seq_puts(seq, ",favordynmods");
         if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
                 seq_puts(seq, ",memory_localevents");
         if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
                 seq_puts(seq, ",memory_recursiveprot");
         if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_HUGETLB_ACCOUNTING)
                 seq_puts(seq, ",memory_hugetlb_accounting");
         if (cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS)
                 seq_puts(seq, ",pids_localevents");
         return 0;
 }
 
 static int cgroup_reconfigure(struct fs_context *fc)
 {
         struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
 
         apply_cgroup_root_flags(ctx->flags);
         return 0;
 }
 
 static void init_cgroup_housekeeping(struct cgroup *cgrp)
 {
         struct cgroup_subsys *ss;
         int ssid;
 
         INIT_LIST_HEAD(&cgrp->self.sibling);
         INIT_LIST_HEAD(&cgrp->self.children);
         INIT_LIST_HEAD(&cgrp->cset_links);
         INIT_LIST_HEAD(&cgrp->pidlists);
         mutex_init(&cgrp->pidlist_mutex);
         cgrp->self.cgroup = cgrp;
         cgrp->self.flags |= CSS_ONLINE;
         cgrp->dom_cgrp = cgrp;
         cgrp->max_descendants = INT_MAX;
         cgrp->max_depth = INT_MAX;
         INIT_LIST_HEAD(&cgrp->rstat_css_list);
         prev_cputime_init(&cgrp->prev_cputime);
 
         for_each_subsys(ss, ssid)
                 INIT_LIST_HEAD(&cgrp->e_csets[ssid]);
 
         init_waitqueue_head(&cgrp->offline_waitq);
         INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
 }
 
 void init_cgroup_root(struct cgroup_fs_context *ctx)
 {
         struct cgroup_root *root = ctx->root;
         struct cgroup *cgrp = &root->cgrp;
 
         INIT_LIST_HEAD_RCU(&root->root_list);
         atomic_set(&root->nr_cgrps, 1);
         cgrp->root = root;
         init_cgroup_housekeeping(cgrp);
 
         /* DYNMODS must be modified through cgroup_favor_dynmods() */
         root->flags = ctx->flags & ~CGRP_ROOT_FAVOR_DYNMODS;
         if (ctx->release_agent)
                 strscpy(root->release_agent_path, ctx->release_agent, PATH_MAX);
         if (ctx->name)
                 strscpy(root->name, ctx->name, MAX_CGROUP_ROOT_NAMELEN);
         if (ctx->cpuset_clone_children)
                 set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
 }
 
 int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
 {
         LIST_HEAD(tmp_links);
         struct cgroup *root_cgrp = &root->cgrp;
         struct kernfs_syscall_ops *kf_sops;
         struct css_set *cset;
         int i, ret;
 
         lockdep_assert_held(&cgroup_mutex);
 
         ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release,
                               0, GFP_KERNEL);
         if (ret)
                 goto out;
 
         /*
          * We're accessing css_set_count without locking css_set_lock here,
          * but that's OK - it can only be increased by someone holding
          * cgroup_lock, and that's us.  Later rebinding may disable
          * controllers on the default hierarchy and thus create new csets,
          * which can't be more than the existing ones.  Allocate 2x.
          */
         ret = allocate_cgrp_cset_links(2 * css_set_count, &tmp_links);
         if (ret)
                 goto cancel_ref;
 
         ret = cgroup_init_root_id(root);
         if (ret)
                 goto cancel_ref;
 
         kf_sops = root == &cgrp_dfl_root ?
                 &cgroup_kf_syscall_ops : &cgroup1_kf_syscall_ops;
 
         root->kf_root = kernfs_create_root(kf_sops,
                                            KERNFS_ROOT_CREATE_DEACTIVATED |
                                            KERNFS_ROOT_SUPPORT_EXPORTOP |
                                            KERNFS_ROOT_SUPPORT_USER_XATTR,
                                            root_cgrp);
         if (IS_ERR(root->kf_root)) {
                 ret = PTR_ERR(root->kf_root);
                 goto exit_root_id;
         }
         root_cgrp->kn = kernfs_root_to_node(root->kf_root);
         WARN_ON_ONCE(cgroup_ino(root_cgrp) != 1);
         root_cgrp->ancestors[0] = root_cgrp;
 
         ret = css_populate_dir(&root_cgrp->self);
         if (ret)
                 goto destroy_root;
 
         ret = cgroup_rstat_init(root_cgrp);
         if (ret)
                 goto destroy_root;
 
         ret = rebind_subsystems(root, ss_mask);
         if (ret)
                 goto exit_stats;
 
         ret = cgroup_bpf_inherit(root_cgrp);
         WARN_ON_ONCE(ret);
 
         trace_cgroup_setup_root(root);
 
         /*
          * There must be no failure case after here, since rebinding takes
          * care of subsystems' refcounts, which are explicitly dropped in
          * the failure exit path.
          */
         list_add_rcu(&root->root_list, &cgroup_roots);
         cgroup_root_count++;
 
         /*
          * Link the root cgroup in this hierarchy into all the css_set
          * objects.
          */
         spin_lock_irq(&css_set_lock);
         hash_for_each(css_set_table, i, cset, hlist) {
                 link_css_set(&tmp_links, cset, root_cgrp);
                 if (css_set_populated(cset))
                         cgroup_update_populated(root_cgrp, true);
         }
         spin_unlock_irq(&css_set_lock);
 
         BUG_ON(!list_empty(&root_cgrp->self.children));
         BUG_ON(atomic_read(&root->nr_cgrps) != 1);
 
         ret = 0;
         goto out;
 
 exit_stats:
         cgroup_rstat_exit(root_cgrp);
 destroy_root:
         kernfs_destroy_root(root->kf_root);
         root->kf_root = NULL;
 exit_root_id:
         cgroup_exit_root_id(root);
 cancel_ref:
         percpu_ref_exit(&root_cgrp->self.refcnt);
 out:
         free_cgrp_cset_links(&tmp_links);
         return ret;
 }
 
 int cgroup_do_get_tree(struct fs_context *fc)
 {
         struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
         int ret;
 
         ctx->kfc.root = ctx->root->kf_root;
         if (fc->fs_type == &cgroup2_fs_type)
                 ctx->kfc.magic = CGROUP2_SUPER_MAGIC;
         else
                 ctx->kfc.magic = CGROUP_SUPER_MAGIC;
         ret = kernfs_get_tree(fc);
 
         /*
          * In non-init cgroup namespace, instead of root cgroup's dentry,
          * we return the dentry corresponding to the cgroupns->root_cgrp.
          */
         if (!ret && ctx->ns != &init_cgroup_ns) {
                 struct dentry *nsdentry;
                 struct super_block *sb = fc->root->d_sb;
                 struct cgroup *cgrp;
 
                 cgroup_lock();
                 spin_lock_irq(&css_set_lock);
 
                 cgrp = cset_cgroup_from_root(ctx->ns->root_cset, ctx->root);
 
                 spin_unlock_irq(&css_set_lock);
                 cgroup_unlock();
 
                 nsdentry = kernfs_node_dentry(cgrp->kn, sb);
                 dput(fc->root);
                 if (IS_ERR(nsdentry)) {
                         deactivate_locked_super(sb);
                         ret = PTR_ERR(nsdentry);
                         nsdentry = NULL;
                 }
                 fc->root = nsdentry;
         }
 
         if (!ctx->kfc.new_sb_created)
                 cgroup_put(&ctx->root->cgrp);
 
         return ret;
 }
 
 /*
  * Destroy a cgroup filesystem context.
  */
 static void cgroup_fs_context_free(struct fs_context *fc)
 {
         struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
 
         kfree(ctx->name);
         kfree(ctx->release_agent);
         put_cgroup_ns(ctx->ns);
         kernfs_free_fs_context(fc);
         kfree(ctx);
 }
 
 static int cgroup_get_tree(struct fs_context *fc)
 {
         struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
         int ret;
 
         WRITE_ONCE(cgrp_dfl_visible, true);
         cgroup_get_live(&cgrp_dfl_root.cgrp);
         ctx->root = &cgrp_dfl_root;
 
         ret = cgroup_do_get_tree(fc);
         if (!ret)
                 apply_cgroup_root_flags(ctx->flags);
         return ret;
 }
 
 static const struct fs_context_operations cgroup_fs_context_ops = {
         .free           = cgroup_fs_context_free,
         .parse_param    = cgroup2_parse_param,
         .get_tree       = cgroup_get_tree,
         .reconfigure    = cgroup_reconfigure,
 };
 
 static const struct fs_context_operations cgroup1_fs_context_ops = {
         .free           = cgroup_fs_context_free,
         .parse_param    = cgroup1_parse_param,
         .get_tree       = cgroup1_get_tree,
         .reconfigure    = cgroup1_reconfigure,
 };
 
 /*
  * Initialise the cgroup filesystem creation/reconfiguration context.  Notably,
  * we select the namespace we're going to use.
  */
 static int cgroup_init_fs_context(struct fs_context *fc)
 {
         struct cgroup_fs_context *ctx;
 
         ctx = kzalloc(sizeof(struct cgroup_fs_context), GFP_KERNEL);
         if (!ctx)
                 return -ENOMEM;
 
         ctx->ns = current->nsproxy->cgroup_ns;
         get_cgroup_ns(ctx->ns);
         fc->fs_private = &ctx->kfc;
         if (fc->fs_type == &cgroup2_fs_type)
                 fc->ops = &cgroup_fs_context_ops;
         else
                 fc->ops = &cgroup1_fs_context_ops;
         put_user_ns(fc->user_ns);
         fc->user_ns = get_user_ns(ctx->ns->user_ns);
         fc->global = true;
 
         if (have_favordynmods)
                 ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
 
         return 0;
 }
 
 static void cgroup_kill_sb(struct super_block *sb)
 {
         struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
         struct cgroup_root *root = cgroup_root_from_kf(kf_root);
 
         /*
          * If @root doesn't have any children, start killing it.
          * This prevents new mounts by disabling percpu_ref_tryget_live().
          *
          * And don't kill the default root.
          */
         if (list_empty(&root->cgrp.self.children) && root != &cgrp_dfl_root &&
             !percpu_ref_is_dying(&root->cgrp.self.refcnt)) {
                 cgroup_bpf_offline(&root->cgrp);
                 percpu_ref_kill(&root->cgrp.self.refcnt);
         }
         cgroup_put(&root->cgrp);
         kernfs_kill_sb(sb);
 }
 
 struct file_system_type cgroup_fs_type = {
         .name                   = "cgroup",
         .init_fs_context        = cgroup_init_fs_context,
         .parameters             = cgroup1_fs_parameters,
         .kill_sb                = cgroup_kill_sb,
         .fs_flags               = FS_USERNS_MOUNT,
 };
 
 static struct file_system_type cgroup2_fs_type = {
         .name                   = "cgroup2",
         .init_fs_context        = cgroup_init_fs_context,
         .parameters             = cgroup2_fs_parameters,
         .kill_sb                = cgroup_kill_sb,
         .fs_flags               = FS_USERNS_MOUNT,
 };
 
 #ifdef CONFIG_CPUSETS
 static const struct fs_context_operations cpuset_fs_context_ops = {
         .get_tree       = cgroup1_get_tree,
         .free           = cgroup_fs_context_free,
 };
 
 /*
  * This is ugly, but preserves the userspace API for existing cpuset
  * users. If someone tries to mount the "cpuset" filesystem, we
  * silently switch it to mount "cgroup" instead
  */
 static int cpuset_init_fs_context(struct fs_context *fc)
 {
         char *agent = kstrdup("/sbin/cpuset_release_agent", GFP_USER);
         struct cgroup_fs_context *ctx;
         int err;
 
         err = cgroup_init_fs_context(fc);
         if (err) {
                 kfree(agent);
                 return err;
         }
 
         fc->ops = &cpuset_fs_context_ops;
 
         ctx = cgroup_fc2context(fc);
         ctx->subsys_mask = 1 << cpuset_cgrp_id;
         ctx->flags |= CGRP_ROOT_NOPREFIX;
         ctx->release_agent = agent;
 
         get_filesystem(&cgroup_fs_type);
         put_filesystem(fc->fs_type);
         fc->fs_type = &cgroup_fs_type;
 
         return 0;
 }
 
 static struct file_system_type cpuset_fs_type = {
         .name                   = "cpuset",
         .init_fs_context        = cpuset_init_fs_context,
         .fs_flags               = FS_USERNS_MOUNT,
 };
 #endif
 
 int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,
                           struct cgroup_namespace *ns)
 {
         struct cgroup *root = cset_cgroup_from_root(ns->root_cset, cgrp->root);
 
         return kernfs_path_from_node(cgrp->kn, root->kn, buf, buflen);
 }
 
 int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,
                    struct cgroup_namespace *ns)
 {
         int ret;
 
         cgroup_lock();
         spin_lock_irq(&css_set_lock);
 
         ret = cgroup_path_ns_locked(cgrp, buf, buflen, ns);
 
         spin_unlock_irq(&css_set_lock);
         cgroup_unlock();
 
         return ret;
 }
 EXPORT_SYMBOL_GPL(cgroup_path_ns);
 
 /**
  * cgroup_attach_lock - Lock for ->attach()
  * @lock_threadgroup: whether to down_write cgroup_threadgroup_rwsem
  *
  * cgroup migration sometimes needs to stabilize threadgroups against forks and
  * exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
  * implementations (e.g. cpuset), also need to disable CPU hotplug.
  * Unfortunately, letting ->attach() operations acquire cpus_read_lock() can
  * lead to deadlocks.
  *
  * Bringing up a CPU may involve creating and destroying tasks which requires
  * read-locking threadgroup_rwsem, so threadgroup_rwsem nests inside
  * cpus_read_lock(). If we call an ->attach() which acquires the cpus lock while
  * write-locking threadgroup_rwsem, the locking order is reversed and we end up
  * waiting for an on-going CPU hotplug operation which in turn is waiting for
  * the threadgroup_rwsem to be released to create new tasks. For more details:
  *
  *   http://lkml.kernel.org/r/20220711174629.uehfmqegcwn2lqzu@wubuntu
  *
  * Resolve the situation by always acquiring cpus_read_lock() before optionally
  * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
  * CPU hotplug is disabled on entry.
  */
 void cgroup_attach_lock(bool lock_threadgroup)
 {
         cpus_read_lock();
         if (lock_threadgroup)
                 percpu_down_write(&cgroup_threadgroup_rwsem);
 }
 
 /**
  * cgroup_attach_unlock - Undo cgroup_attach_lock()
  * @lock_threadgroup: whether to up_write cgroup_threadgroup_rwsem
  */
 void cgroup_attach_unlock(bool lock_threadgroup)
 {
         if (lock_threadgroup)
                 percpu_up_write(&cgroup_threadgroup_rwsem);
         cpus_read_unlock();
 }
 
 /**
  * cgroup_migrate_add_task - add a migration target task to a migration context
  * @task: target task
  * @mgctx: target migration context
  *
  * Add @task, which is a migration target, to @mgctx->tset.  This function
  * becomes noop if @task doesn't need to be migrated.  @task's css_set
  * should have been added as a migration source and @task->cg_list will be
  * moved from the css_set's tasks list to mg_tasks one.
  */
 static void cgroup_migrate_add_task(struct task_struct *task,
                                     struct cgroup_mgctx *mgctx)
 {
         struct css_set *cset;
 
         lockdep_assert_held(&css_set_lock);
 
         /* @task either already exited or can't exit until the end */
         if (task->flags & PF_EXITING)
                 return;
 
         /* cgroup_threadgroup_rwsem protects racing against forks */
         WARN_ON_ONCE(list_empty(&task->cg_list));
 
         cset = task_css_set(task);
         if (!cset->mg_src_cgrp)
                 return;
 
         mgctx->tset.nr_tasks++;
 
         list_move_tail(&task->cg_list, &cset->mg_tasks);
         if (list_empty(&cset->mg_node))
                 list_add_tail(&cset->mg_node,
                               &mgctx->tset.src_csets);
         if (list_empty(&cset->mg_dst_cset->mg_node))
                 list_add_tail(&cset->mg_dst_cset->mg_node,
                               &mgctx->tset.dst_csets);
 }
 
 /**
  * cgroup_taskset_first - reset taskset and return the first task
  * @tset: taskset of interest
  * @dst_cssp: output variable for the destination css
  *
  * @tset iteration is initialized and the first task is returned.
  */
 struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset,
                                          struct cgroup_subsys_state **dst_cssp)
 {
         tset->cur_cset = list_first_entry(tset->csets, struct css_set, mg_node);
         tset->cur_task = NULL;
 
         return cgroup_taskset_next(tset, dst_cssp);
 }
 
 /**
  * cgroup_taskset_next - iterate to the next task in taskset
  * @tset: taskset of interest
  * @dst_cssp: output variable for the destination css
  *
  * Return the next task in @tset.  Iteration must have been initialized
  * with cgroup_taskset_first().
  */
 struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
                                         struct cgroup_subsys_state **dst_cssp)
 {
         struct css_set *cset = tset->cur_cset;
         struct task_struct *task = tset->cur_task;
 
         while (CGROUP_HAS_SUBSYS_CONFIG && &cset->mg_node != tset->csets) {
                 if (!task)
                         task = list_first_entry(&cset->mg_tasks,
                                                 struct task_struct, cg_list);
                 else
                         task = list_next_entry(task, cg_list);
 
                 if (&task->cg_list != &cset->mg_tasks) {
                         tset->cur_cset = cset;
                         tset->cur_task = task;
 
                         /*
                          * This function may be called both before and
                          * after cgroup_migrate_execute().  The two cases
                          * can be distinguished by looking at whether @cset
                          * has its ->mg_dst_cset set.
                          */
                         if (cset->mg_dst_cset)
                                 *dst_cssp = cset->mg_dst_cset->subsys[tset->ssid];
                         else
                                 *dst_cssp = cset->subsys[tset->ssid];
 
                         return task;
                 }
 
                 cset = list_next_entry(cset, mg_node);
                 task = NULL;
         }
 
         return NULL;
 }
 
 /**
  * cgroup_migrate_execute - migrate a taskset
  * @mgctx: migration context
  *
  * Migrate tasks in @mgctx as setup by migration preparation functions.
  * This function fails iff one of the ->can_attach callbacks fails and
  * guarantees that either all or none of the tasks in @mgctx are migrated.
  * @mgctx is consumed regardless of success.
  */
 static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
 {
         struct cgroup_taskset *tset = &mgctx->tset;
         struct cgroup_subsys *ss;
         struct task_struct *task, *tmp_task;
         struct css_set *cset, *tmp_cset;
         int ssid, failed_ssid, ret;
 
         /* check that we can legitimately attach to the cgroup */
         if (tset->nr_tasks) {
                 do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
                         if (ss->can_attach) {
                                 tset->ssid = ssid;
                                 ret = ss->can_attach(tset);
                                 if (ret) {
                                         failed_ssid = ssid;
                                         goto out_cancel_attach;
                                 }
                         }
                 } while_each_subsys_mask();
         }
 
         /*
          * Now that we're guaranteed success, proceed to move all tasks to
          * the new cgroup.  There are no failure cases after here, so this
          * is the commit point.
          */
         spin_lock_irq(&css_set_lock);
         list_for_each_entry(cset, &tset->src_csets, mg_node) {
                 list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
                         struct css_set *from_cset = task_css_set(task);
                         struct css_set *to_cset = cset->mg_dst_cset;
 
                         get_css_set(to_cset);
                         to_cset->nr_tasks++;
                         css_set_move_task(task, from_cset, to_cset, true);
                         from_cset->nr_tasks--;
                         /*
                          * If the source or destination cgroup is frozen,
                          * the task might require to change its state.
                          */
                         cgroup_freezer_migrate_task(task, from_cset->dfl_cgrp,
                                                     to_cset->dfl_cgrp);
                         put_css_set_locked(from_cset);
 
                 }
         }
         spin_unlock_irq(&css_set_lock);
 
         /*
          * Migration is committed, all target tasks are now on dst_csets.
          * Nothing is sensitive to fork() after this point.  Notify
          * controllers that migration is complete.
          */
         tset->csets = &tset->dst_csets;
 
         if (tset->nr_tasks) {
                 do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
                         if (ss->attach) {
                                 tset->ssid = ssid;
                                 ss->attach(tset);
                         }
                 } while_each_subsys_mask();
         }
 
         ret = 0;
         goto out_release_tset;
 
 out_cancel_attach:
         if (tset->nr_tasks) {
                 do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
                         if (ssid == failed_ssid)
                                 break;
                         if (ss->cancel_attach) {
                                 tset->ssid = ssid;
                                 ss->cancel_attach(tset);
                         }
                 } while_each_subsys_mask();
         }
 out_release_tset:
         spin_lock_irq(&css_set_lock);
         list_splice_init(&tset->dst_csets, &tset->src_csets);
         list_for_each_entry_safe(cset, tmp_cset, &tset->src_csets, mg_node) {
                 list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
                 list_del_init(&cset->mg_node);
         }
         spin_unlock_irq(&css_set_lock);
 
         /*
          * Re-initialize the cgroup_taskset structure in case it is reused
          * again in another cgroup_migrate_add_task()/cgroup_migrate_execute()
          * iteration.
          */
         tset->nr_tasks = 0;
         tset->csets    = &tset->src_csets;
         return ret;
 }
 
 /**
  * cgroup_migrate_vet_dst - verify whether a cgroup can be migration destination
  * @dst_cgrp: destination cgroup to test
  *
  * On the default hierarchy, except for the mixable, (possible) thread root
  * and threaded cgroups, subtree_control must be zero for migration
  * destination cgroups with tasks so that child cgroups don't compete
  * against tasks.
  */
 int cgroup_migrate_vet_dst(struct cgroup *dst_cgrp)
 {
         /* v1 doesn't have any restriction */
         if (!cgroup_on_dfl(dst_cgrp))
                 return 0;
 
         /* verify @dst_cgrp can host resources */
         if (!cgroup_is_valid_domain(dst_cgrp->dom_cgrp))
                 return -EOPNOTSUPP;
 
         /*
          * If @dst_cgrp is already or can become a thread root or is
          * threaded, it doesn't matter.
          */
         if (cgroup_can_be_thread_root(dst_cgrp) || cgroup_is_threaded(dst_cgrp))
                 return 0;
 
         /* apply no-internal-process constraint */
         if (dst_cgrp->subtree_control)
                 return -EBUSY;
 
         return 0;
 }
 
 /**
  * cgroup_migrate_finish - cleanup after attach
  * @mgctx: migration context
  *
  * Undo cgroup_migrate_add_src() and cgroup_migrate_prepare_dst().  See
  * those functions for details.
  */
 void cgroup_migrate_finish(struct cgroup_mgctx *mgctx)
 {
         struct css_set *cset, *tmp_cset;
 
         lockdep_assert_held(&cgroup_mutex);
 
         spin_lock_irq(&css_set_lock);
 
         list_for_each_entry_safe(cset, tmp_cset, &mgctx->preloaded_src_csets,
                                  mg_src_preload_node) {
                 cset->mg_src_cgrp = NULL;
                 cset->mg_dst_cgrp = NULL;
                 cset->mg_dst_cset = NULL;
                 list_del_init(&cset->mg_src_preload_node);
                 put_css_set_locked(cset);
         }
 
         list_for_each_entry_safe(cset, tmp_cset, &mgctx->preloaded_dst_csets,
                                  mg_dst_preload_node) {
                 cset->mg_src_cgrp = NULL;
                 cset->mg_dst_cgrp = NULL;
                 cset->mg_dst_cset = NULL;
                 list_del_init(&cset->mg_dst_preload_node);
                 put_css_set_locked(cset);
         }
 
         spin_unlock_irq(&css_set_lock);
 }
 
 /**
  * cgroup_migrate_add_src - add a migration source css_set
  * @src_cset: the source css_set to add
  * @dst_cgrp: the destination cgroup
  * @mgctx: migration context
  *
  * Tasks belonging to @src_cset are about to be migrated to @dst_cgrp.  Pin
  * @src_cset and add it to @mgctx->src_csets, which should later be cleaned
  * up by cgroup_migrate_finish().
  *
  * This function may be called without holding cgroup_threadgroup_rwsem
  * even if the target is a process.  Threads may be created and destroyed
  * but as long as cgroup_mutex is not dropped, no new css_set can be put
  * into play and the preloaded css_sets are guaranteed to cover all
  * migrations.
  */
 void cgroup_migrate_add_src(struct css_set *src_cset,
                             struct cgroup *dst_cgrp,
                             struct cgroup_mgctx *mgctx)
 {
         struct cgroup *src_cgrp;
 
         lockdep_assert_held(&cgroup_mutex);
         lockdep_assert_held(&css_set_lock);
 
         /*
          * If ->dead, @src_set is associated with one or more dead cgroups
          * and doesn't contain any migratable tasks.  Ignore it early so
          * that the rest of migration path doesn't get confused by it.
          */
         if (src_cset->dead)
                 return;
 
         if (!list_empty(&src_cset->mg_src_preload_node))
                 return;
 
         src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
         WARN_ON(src_cset->mg_src_cgrp);
         WARN_ON(src_cset->mg_dst_cgrp);
         WARN_ON(!list_empty(&src_cset->mg_tasks));
         WARN_ON(!list_empty(&src_cset->mg_node));
 
         src_cset->mg_src_cgrp = src_cgrp;
         src_cset->mg_dst_cgrp = dst_cgrp;
         get_css_set(src_cset);
         list_add_tail(&src_cset->mg_src_preload_node, &mgctx->preloaded_src_csets);
 }
 
 /**
  * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
  * @mgctx: migration context
  *
  * Tasks are about to be moved and all the source css_sets have been
  * preloaded to @mgctx->preloaded_src_csets.  This function looks up and
  * pins all destination css_sets, links each to its source, and append them
  * to @mgctx->preloaded_dst_csets.
  *
  * This function must be called after cgroup_migrate_add_src() has been
  * called on each migration source css_set.  After migration is performed
  * using cgroup_migrate(), cgroup_migrate_finish() must be called on
  * @mgctx.
  */
 int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx)
 {
         struct css_set *src_cset, *tmp_cset;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* look up the dst cset for each src cset and link it to src */
         list_for_each_entry_safe(src_cset, tmp_cset, &mgctx->preloaded_src_csets,
                                  mg_src_preload_node) {
                 struct css_set *dst_cset;
                 struct cgroup_subsys *ss;
                 int ssid;
 
                 dst_cset = find_css_set(src_cset, src_cset->mg_dst_cgrp);
                 if (!dst_cset)
                         return -ENOMEM;
 
                 WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
 
                 /*
                  * If src cset equals dst, it's noop.  Drop the src.
                  * cgroup_migrate() will skip the cset too.  Note that we
                  * can't handle src == dst as some nodes are used by both.
                  */
                 if (src_cset == dst_cset) {
                         src_cset->mg_src_cgrp = NULL;
                         src_cset->mg_dst_cgrp = NULL;
                         list_del_init(&src_cset->mg_src_preload_node);
                         put_css_set(src_cset);
                         put_css_set(dst_cset);
                         continue;
                 }
 
                 src_cset->mg_dst_cset = dst_cset;
 
                 if (list_empty(&dst_cset->mg_dst_preload_node))
                         list_add_tail(&dst_cset->mg_dst_preload_node,
                                       &mgctx->preloaded_dst_csets);
                 else
                         put_css_set(dst_cset);
 
                 for_each_subsys(ss, ssid)
                         if (src_cset->subsys[ssid] != dst_cset->subsys[ssid])
                                 mgctx->ss_mask |= 1 << ssid;
         }
 
         return 0;
 }
 
 /**
  * cgroup_migrate - migrate a process or task to a cgroup
  * @leader: the leader of the process or the task to migrate
  * @threadgroup: whether @leader points to the whole process or a single task
  * @mgctx: migration context
  *
  * Migrate a process or task denoted by @leader.  If migrating a process,
  * the caller must be holding cgroup_threadgroup_rwsem.  The caller is also
  * responsible for invoking cgroup_migrate_add_src() and
  * cgroup_migrate_prepare_dst() on the targets before invoking this
  * function and following up with cgroup_migrate_finish().
  *
  * As long as a controller's ->can_attach() doesn't fail, this function is
  * guaranteed to succeed.  This means that, excluding ->can_attach()
  * failure, when migrating multiple targets, the success or failure can be
  * decided for all targets by invoking group_migrate_prepare_dst() before
  * actually starting migrating.
  */
 int cgroup_migrate(struct task_struct *leader, bool threadgroup,
                    struct cgroup_mgctx *mgctx)
 {
         struct task_struct *task;
 
         /*
          * The following thread iteration should be inside an RCU critical
          * section to prevent tasks from being freed while taking the snapshot.
          * spin_lock_irq() implies RCU critical section here.
          */
         spin_lock_irq(&css_set_lock);
         task = leader;
         do {
                 cgroup_migrate_add_task(task, mgctx);
                 if (!threadgroup)
                         break;
         } while_each_thread(leader, task);
         spin_unlock_irq(&css_set_lock);
 
         return cgroup_migrate_execute(mgctx);
 }
 
 /**
  * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
  * @dst_cgrp: the cgroup to attach to
  * @leader: the task or the leader of the threadgroup to be attached
  * @threadgroup: attach the whole threadgroup?
  *
  * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
  */
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
                        bool threadgroup)
 {
         DEFINE_CGROUP_MGCTX(mgctx);
         struct task_struct *task;
         int ret = 0;
 
         /* look up all src csets */
         spin_lock_irq(&css_set_lock);
         rcu_read_lock();
         task = leader;
         do {
                 cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
                 if (!threadgroup)
                         break;
         } while_each_thread(leader, task);
         rcu_read_unlock();
         spin_unlock_irq(&css_set_lock);
 
         /* prepare dst csets and commit */
         ret = cgroup_migrate_prepare_dst(&mgctx);
         if (!ret)
                 ret = cgroup_migrate(leader, threadgroup, &mgctx);
 
         cgroup_migrate_finish(&mgctx);
 
         if (!ret)
                 TRACE_CGROUP_PATH(attach_task, dst_cgrp, leader, threadgroup);
 
         return ret;
 }
 
 struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
                                              bool *threadgroup_locked)
 {
         struct task_struct *tsk;
         pid_t pid;
 
         if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
                 return ERR_PTR(-EINVAL);
 
         /*
          * If we migrate a single thread, we don't care about threadgroup
          * stability. If the thread is `current`, it won't exit(2) under our
          * hands or change PID through exec(2). We exclude
          * cgroup_update_dfl_csses and other cgroup_{proc,thread}s_write
          * callers by cgroup_mutex.
          * Therefore, we can skip the global lock.
          */
         lockdep_assert_held(&cgroup_mutex);
         *threadgroup_locked = pid || threadgroup;
         cgroup_attach_lock(*threadgroup_locked);
 
         rcu_read_lock();
         if (pid) {
                 tsk = find_task_by_vpid(pid);
                 if (!tsk) {
                         tsk = ERR_PTR(-ESRCH);
                         goto out_unlock_threadgroup;
                 }
         } else {
                 tsk = current;
         }
 
         if (threadgroup)
                 tsk = tsk->group_leader;
 
         /*
          * kthreads may acquire PF_NO_SETAFFINITY during initialization.
          * If userland migrates such a kthread to a non-root cgroup, it can
          * become trapped in a cpuset, or RT kthread may be born in a
          * cgroup with no rt_runtime allocated.  Just say no.
          */
         if (tsk->no_cgroup_migration || (tsk->flags & PF_NO_SETAFFINITY)) {
                 tsk = ERR_PTR(-EINVAL);
                 goto out_unlock_threadgroup;
         }
 
         get_task_struct(tsk);
         goto out_unlock_rcu;
 
 out_unlock_threadgroup:
         cgroup_attach_unlock(*threadgroup_locked);
         *threadgroup_locked = false;
 out_unlock_rcu:
         rcu_read_unlock();
         return tsk;
 }
 
 void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked)
 {
         struct cgroup_subsys *ss;
         int ssid;
 
         /* release reference from cgroup_procs_write_start() */
         put_task_struct(task);
 
         cgroup_attach_unlock(threadgroup_locked);
 
         for_each_subsys(ss, ssid)
                 if (ss->post_attach)
                         ss->post_attach();
 }
 
 static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
 {
         struct cgroup_subsys *ss;
         bool printed = false;
         int ssid;
 
         do_each_subsys_mask(ss, ssid, ss_mask) {
                 if (printed)
                         seq_putc(seq, ' ');
                 seq_puts(seq, ss->name);
                 printed = true;
         } while_each_subsys_mask();
         if (printed)
                 seq_putc(seq, '\n');
 }
 
 /* show controllers which are enabled from the parent */
 static int cgroup_controllers_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
         cgroup_print_ss_mask(seq, cgroup_control(cgrp));
         return 0;
 }
 
 /* show controllers which are enabled for a given cgroup's children */
 static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
         cgroup_print_ss_mask(seq, cgrp->subtree_control);
         return 0;
 }
 
 /**
  * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
  * @cgrp: root of the subtree to update csses for
  *
  * @cgrp's control masks have changed and its subtree's css associations
  * need to be updated accordingly.  This function looks up all css_sets
  * which are attached to the subtree, creates the matching updated css_sets
  * and migrates the tasks to the new ones.
  */
 static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 {
         DEFINE_CGROUP_MGCTX(mgctx);
         struct cgroup_subsys_state *d_css;
         struct cgroup *dsct;
         struct css_set *src_cset;
         bool has_tasks;
         int ret;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* look up all csses currently attached to @cgrp's subtree */
         spin_lock_irq(&css_set_lock);
         cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
                 struct cgrp_cset_link *link;
 
                 /*
                  * As cgroup_update_dfl_csses() is only called by
                  * cgroup_apply_control(). The csses associated with the
                  * given cgrp will not be affected by changes made to
                  * its subtree_control file. We can skip them.
                  */
                 if (dsct == cgrp)
                         continue;
 
                 list_for_each_entry(link, &dsct->cset_links, cset_link)
                         cgroup_migrate_add_src(link->cset, dsct, &mgctx);
         }
         spin_unlock_irq(&css_set_lock);
 
         /*
          * We need to write-lock threadgroup_rwsem while migrating tasks.
          * However, if there are no source csets for @cgrp, changing its
          * controllers isn't gonna produce any task migrations and the
          * write-locking can be skipped safely.
          */
         has_tasks = !list_empty(&mgctx.preloaded_src_csets);
         cgroup_attach_lock(has_tasks);
 
         /* NULL dst indicates self on default hierarchy */
         ret = cgroup_migrate_prepare_dst(&mgctx);
         if (ret)
                 goto out_finish;
 
         spin_lock_irq(&css_set_lock);
         list_for_each_entry(src_cset, &mgctx.preloaded_src_csets,
                             mg_src_preload_node) {
                 struct task_struct *task, *ntask;
 
                 /* all tasks in src_csets need to be migrated */
                 list_for_each_entry_safe(task, ntask, &src_cset->tasks, cg_list)
                         cgroup_migrate_add_task(task, &mgctx);
         }
         spin_unlock_irq(&css_set_lock);
 
         ret = cgroup_migrate_execute(&mgctx);
 out_finish:
         cgroup_migrate_finish(&mgctx);
         cgroup_attach_unlock(has_tasks);
         return ret;
 }
 
 /**
  * cgroup_lock_and_drain_offline - lock cgroup_mutex and drain offlined csses
  * @cgrp: root of the target subtree
  *
  * Because css offlining is asynchronous, userland may try to re-enable a
  * controller while the previous css is still around.  This function grabs
  * cgroup_mutex and drains the previous css instances of @cgrp's subtree.
  */
 void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
         __acquires(&cgroup_mutex)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
         struct cgroup_subsys *ss;
         int ssid;
 
 restart:
         cgroup_lock();
 
         cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
                 for_each_subsys(ss, ssid) {
                         struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
                         DEFINE_WAIT(wait);
 
                         if (!css || !percpu_ref_is_dying(&css->refcnt))
                                 continue;
 
                         cgroup_get_live(dsct);
                         prepare_to_wait(&dsct->offline_waitq, &wait,
                                         TASK_UNINTERRUPTIBLE);
 
                         cgroup_unlock();
                         schedule();
                         finish_wait(&dsct->offline_waitq, &wait);
 
                         cgroup_put(dsct);
                         goto restart;
                 }
         }
 }
 
 /**
  * cgroup_save_control - save control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
  * Save ->subtree_control, ->subtree_ss_mask and ->dom_cgrp to the
  * respective old_ prefixed fields for @cgrp's subtree including @cgrp
  * itself.
  */
 static void cgroup_save_control(struct cgroup *cgrp)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
 
         cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
                 dsct->old_subtree_control = dsct->subtree_control;
                 dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
                 dsct->old_dom_cgrp = dsct->dom_cgrp;
         }
 }
 
 /**
  * cgroup_propagate_control - refresh control masks of a subtree
  * @cgrp: root of the target subtree
  *
  * For @cgrp and its subtree, ensure ->subtree_ss_mask matches
  * ->subtree_control and propagate controller availability through the
  * subtree so that descendants don't have unavailable controllers enabled.
  */
 static void cgroup_propagate_control(struct cgroup *cgrp)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
 
         cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
                 dsct->subtree_control &= cgroup_control(dsct);
                 dsct->subtree_ss_mask =
                         cgroup_calc_subtree_ss_mask(dsct->subtree_control,
                                                     cgroup_ss_mask(dsct));
         }
 }
 
 /**
  * cgroup_restore_control - restore control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
  * Restore ->subtree_control, ->subtree_ss_mask and ->dom_cgrp from the
  * respective old_ prefixed fields for @cgrp's subtree including @cgrp
  * itself.
  */
 static void cgroup_restore_control(struct cgroup *cgrp)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
 
         cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
                 dsct->subtree_control = dsct->old_subtree_control;
                 dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
                 dsct->dom_cgrp = dsct->old_dom_cgrp;
         }
 }
 
 static bool css_visible(struct cgroup_subsys_state *css)
 {
         struct cgroup_subsys *ss = css->ss;
         struct cgroup *cgrp = css->cgroup;
 
         if (cgroup_control(cgrp) & (1 << ss->id))
                 return true;
         if (!(cgroup_ss_mask(cgrp) & (1 << ss->id)))
                 return false;
         return cgroup_on_dfl(cgrp) && ss->implicit_on_dfl;
 }
 
 /**
  * cgroup_apply_control_enable - enable or show csses according to control
  * @cgrp: root of the target subtree
  *
  * Walk @cgrp's subtree and create new csses or make the existing ones
  * visible.  A css is created invisible if it's being implicitly enabled
  * through dependency.  An invisible css is made visible when the userland
  * explicitly enables it.
  *
  * Returns 0 on success, -errno on failure.  On failure, csses which have
  * been processed already aren't cleaned up.  The caller is responsible for
  * cleaning up with cgroup_apply_control_disable().
  */
 static int cgroup_apply_control_enable(struct cgroup *cgrp)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
         struct cgroup_subsys *ss;
         int ssid, ret;
 
         cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
                 for_each_subsys(ss, ssid) {
                         struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
 
                         if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
                                 continue;
 
                         if (!css) {
                                 css = css_create(dsct, ss);
                                 if (IS_ERR(css))
                                         return PTR_ERR(css);
                         }
 
                         WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
 
                         if (css_visible(css)) {
                                 ret = css_populate_dir(css);
                                 if (ret)
                                         return ret;
                         }
                 }
         }
 
         return 0;
 }
 
 /**
  * cgroup_apply_control_disable - kill or hide csses according to control
  * @cgrp: root of the target subtree
  *
  * Walk @cgrp's subtree and kill and hide csses so that they match
  * cgroup_ss_mask() and cgroup_visible_mask().
  *
  * A css is hidden when the userland requests it to be disabled while other
  * subsystems are still depending on it.  The css must not actively control
  * resources and be in the vanilla state if it's made visible again later.
  * Controllers which may be depended upon should provide ->css_reset() for
  * this purpose.
  */
 static void cgroup_apply_control_disable(struct cgroup *cgrp)
 {
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
         struct cgroup_subsys *ss;
         int ssid;
 
         cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
                 for_each_subsys(ss, ssid) {
                         struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
 
                         if (!css)
                                 continue;
 
                         WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
 
                         if (css->parent &&
                             !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
                                 kill_css(css);
                         } else if (!css_visible(css)) {
                                 css_clear_dir(css);
                                 if (ss->css_reset)
                                         ss->css_reset(css);
                         }
                 }
         }
 }
 
 /**
  * cgroup_apply_control - apply control mask updates to the subtree
  * @cgrp: root of the target subtree
  *
  * subsystems can be enabled and disabled in a subtree using the following
  * steps.
  *
  * 1. Call cgroup_save_control() to stash the current state.
  * 2. Update ->subtree_control masks in the subtree as desired.
  * 3. Call cgroup_apply_control() to apply the changes.
  * 4. Optionally perform other related operations.
  * 5. Call cgroup_finalize_control() to finish up.
  *
  * This function implements step 3 and propagates the mask changes
  * throughout @cgrp's subtree, updates csses accordingly and perform
  * process migrations.
  */
 static int cgroup_apply_control(struct cgroup *cgrp)
 {
         int ret;
 
         cgroup_propagate_control(cgrp);
 
         ret = cgroup_apply_control_enable(cgrp);
         if (ret)
                 return ret;
 
         /*
          * At this point, cgroup_e_css_by_mask() results reflect the new csses
          * making the following cgroup_update_dfl_csses() properly update
          * css associations of all tasks in the subtree.
          */
         return cgroup_update_dfl_csses(cgrp);
 }
 
 /**
  * cgroup_finalize_control - finalize control mask update
  * @cgrp: root of the target subtree
  * @ret: the result of the update
  *
  * Finalize control mask update.  See cgroup_apply_control() for more info.
  */
 static void cgroup_finalize_control(struct cgroup *cgrp, int ret)
 {
         if (ret) {
                 cgroup_restore_control(cgrp);
                 cgroup_propagate_control(cgrp);
         }
 
         cgroup_apply_control_disable(cgrp);
 }
 
 static int cgroup_vet_subtree_control_enable(struct cgroup *cgrp, u16 enable)
 {
         u16 domain_enable = enable & ~cgrp_dfl_threaded_ss_mask;
 
         /* if nothing is getting enabled, nothing to worry about */
         if (!enable)
                 return 0;
 
         /* can @cgrp host any resources? */
         if (!cgroup_is_valid_domain(cgrp->dom_cgrp))
                 return -EOPNOTSUPP;
 
         /* mixables don't care */
         if (cgroup_is_mixable(cgrp))
                 return 0;
 
         if (domain_enable) {
                 /* can't enable domain controllers inside a thread subtree */
                 if (cgroup_is_thread_root(cgrp) || cgroup_is_threaded(cgrp))
                         return -EOPNOTSUPP;
         } else {
                 /*
                  * Threaded controllers can handle internal competitions
                  * and are always allowed inside a (prospective) thread
                  * subtree.
                  */
                 if (cgroup_can_be_thread_root(cgrp) || cgroup_is_threaded(cgrp))
                         return 0;
         }
 
         /*
          * Controllers can't be enabled for a cgroup with tasks to avoid
          * child cgroups competing against tasks.
          */
         if (cgroup_has_tasks(cgrp))
                 return -EBUSY;
 
         return 0;
 }
 
 /* change the enabled child controllers for a cgroup in the default hierarchy */
 static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                                             char *buf, size_t nbytes,
                                             loff_t off)
 {
         u16 enable = 0, disable = 0;
         struct cgroup *cgrp, *child;
         struct cgroup_subsys *ss;
         char *tok;
         int ssid, ret;
 
         /*
          * Parse input - space separated list of subsystem names prefixed
          * with either + or -.
          */
         buf = strstrip(buf);
         while ((tok = strsep(&buf, " "))) {
                 if (tok[0] == '\0')
                         continue;
                 do_each_subsys_mask(ss, ssid, ~cgrp_dfl_inhibit_ss_mask) {
                         if (!cgroup_ssid_enabled(ssid) ||
                             strcmp(tok + 1, ss->name))
                                 continue;
 
                         if (*tok == '+') {
                                 enable |= 1 << ssid;
                                 disable &= ~(1 << ssid);
                         } else if (*tok == '-') {
                                 disable |= 1 << ssid;
                                 enable &= ~(1 << ssid);
                         } else {
                                 return -EINVAL;
                         }
                         break;
                 } while_each_subsys_mask();
                 if (ssid == CGROUP_SUBSYS_COUNT)
                         return -EINVAL;
         }
 
         cgrp = cgroup_kn_lock_live(of->kn, true);
         if (!cgrp)
                 return -ENODEV;
 
         for_each_subsys(ss, ssid) {
                 if (enable & (1 << ssid)) {
                         if (cgrp->subtree_control & (1 << ssid)) {
                                 enable &= ~(1 << ssid);
                                 continue;
                         }
 
                         if (!(cgroup_control(cgrp) & (1 << ssid))) {
                                 ret = -ENOENT;
                                 goto out_unlock;
                         }
                 } else if (disable & (1 << ssid)) {
                         if (!(cgrp->subtree_control & (1 << ssid))) {
                                 disable &= ~(1 << ssid);
                                 continue;
                         }
 
                         /* a child has it enabled? */
                         cgroup_for_each_live_child(child, cgrp) {
                                 if (child->subtree_control & (1 << ssid)) {
                                         ret = -EBUSY;
                                         goto out_unlock;
                                 }
                         }
                 }
         }
 
         if (!enable && !disable) {
                 ret = 0;
                 goto out_unlock;
         }
 
         ret = cgroup_vet_subtree_control_enable(cgrp, enable);
         if (ret)
                 goto out_unlock;
 
         /* save and update control masks and prepare csses */
         cgroup_save_control(cgrp);
 
         cgrp->subtree_control |= enable;
         cgrp->subtree_control &= ~disable;
 
         ret = cgroup_apply_control(cgrp);
         cgroup_finalize_control(cgrp, ret);
         if (ret)
                 goto out_unlock;
 
         kernfs_activate(cgrp->kn);
 out_unlock:
         cgroup_kn_unlock(of->kn);
         return ret ?: nbytes;
 }
 
 /**
  * cgroup_enable_threaded - make @cgrp threaded
  * @cgrp: the target cgroup
  *
  * Called when "threaded" is written to the cgroup.type interface file and
  * tries to make @cgrp threaded and join the parent's resource domain.
  * This function is never called on the root cgroup as cgroup.type doesn't
  * exist on it.
  */
 static int cgroup_enable_threaded(struct cgroup *cgrp)
 {
         struct cgroup *parent = cgroup_parent(cgrp);
         struct cgroup *dom_cgrp = parent->dom_cgrp;
         struct cgroup *dsct;
         struct cgroup_subsys_state *d_css;
         int ret;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* noop if already threaded */
         if (cgroup_is_threaded(cgrp))
                 return 0;
 
         /*
          * If @cgroup is populated or has domain controllers enabled, it
          * can't be switched.  While the below cgroup_can_be_thread_root()
          * test can catch the same conditions, that's only when @parent is
          * not mixable, so let's check it explicitly.
          */
         if (cgroup_is_populated(cgrp) ||
             cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
                 return -EOPNOTSUPP;
 
         /* we're joining the parent's domain, ensure its validity */
         if (!cgroup_is_valid_domain(dom_cgrp) ||
             !cgroup_can_be_thread_root(dom_cgrp))
                 return -EOPNOTSUPP;
 
         /*
          * The following shouldn't cause actual migrations and should
          * always succeed.
          */
         cgroup_save_control(cgrp);
 
         cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)
                 if (dsct == cgrp || cgroup_is_threaded(dsct))
                         dsct->dom_cgrp = dom_cgrp;
 
         ret = cgroup_apply_control(cgrp);
         if (!ret)
                 parent->nr_threaded_children++;
 
         cgroup_finalize_control(cgrp, ret);
         return ret;
 }
 
 static int cgroup_type_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
         if (cgroup_is_threaded(cgrp))
                 seq_puts(seq, "threaded\n");
         else if (!cgroup_is_valid_domain(cgrp))
                 seq_puts(seq, "domain invalid\n");
         else if (cgroup_is_thread_root(cgrp))
                 seq_puts(seq, "domain threaded\n");
         else
                 seq_puts(seq, "domain\n");
 
         return 0;
 }
 
 static ssize_t cgroup_type_write(struct kernfs_open_file *of, char *buf,
                                  size_t nbytes, loff_t off)
 {
         struct cgroup *cgrp;
         int ret;
 
         /* only switching to threaded mode is supported */
         if (strcmp(strstrip(buf), "threaded"))
                 return -EINVAL;
 
         /* drain dying csses before we re-apply (threaded) subtree control */
         cgrp = cgroup_kn_lock_live(of->kn, true);
         if (!cgrp)
                 return -ENOENT;
 
         /* threaded can only be enabled */
         ret = cgroup_enable_threaded(cgrp);
 
         cgroup_kn_unlock(of->kn);
         return ret ?: nbytes;
 }
 
 static int cgroup_max_descendants_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         int descendants = READ_ONCE(cgrp->max_descendants);
 
         if (descendants == INT_MAX)
                 seq_puts(seq, "max\n");
         else
                 seq_printf(seq, "%d\n", descendants);
 
         return 0;
 }
 
 static ssize_t cgroup_max_descendants_write(struct kernfs_open_file *of,
                                            char *buf, size_t nbytes, loff_t off)
 {
         struct cgroup *cgrp;
         int descendants;
         ssize_t ret;
 
         buf = strstrip(buf);
         if (!strcmp(buf, "max")) {
                 descendants = INT_MAX;
         } else {
                 ret = kstrtoint(buf, 0, &descendants);
                 if (ret)
                         return ret;
         }
 
         if (descendants < 0)
                 return -ERANGE;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENOENT;
 
         cgrp->max_descendants = descendants;
 
         cgroup_kn_unlock(of->kn);
 
         return nbytes;
 }
 
 static int cgroup_max_depth_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         int depth = READ_ONCE(cgrp->max_depth);
 
         if (depth == INT_MAX)
                 seq_puts(seq, "max\n");
         else
                 seq_printf(seq, "%d\n", depth);
 
         return 0;
 }
 
 static ssize_t cgroup_max_depth_write(struct kernfs_open_file *of,
                                       char *buf, size_t nbytes, loff_t off)
 {
         struct cgroup *cgrp;
         ssize_t ret;
         int depth;
 
         buf = strstrip(buf);
         if (!strcmp(buf, "max")) {
                 depth = INT_MAX;
         } else {
                 ret = kstrtoint(buf, 0, &depth);
                 if (ret)
                         return ret;
         }
 
         if (depth < 0)
                 return -ERANGE;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENOENT;
 
         cgrp->max_depth = depth;
 
         cgroup_kn_unlock(of->kn);
 
         return nbytes;
 }
 
 static int cgroup_events_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
         seq_printf(seq, "populated %d\n", cgroup_is_populated(cgrp));
         seq_printf(seq, "frozen %d\n", test_bit(CGRP_FROZEN, &cgrp->flags));
 
         return 0;
 }
 
 static int cgroup_stat_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgroup = seq_css(seq)->cgroup;
 
         seq_printf(seq, "nr_descendants %d\n",
                    cgroup->nr_descendants);
         seq_printf(seq, "nr_dying_descendants %d\n",
                    cgroup->nr_dying_descendants);
 
         return 0;
 }
 
 #ifdef CONFIG_CGROUP_SCHED
 /**
  * cgroup_tryget_css - try to get a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get @cgrp's css associated with @ss.  If the css doesn't exist
  * or is offline, %NULL is returned.
  */
 static struct cgroup_subsys_state *cgroup_tryget_css(struct cgroup *cgrp,
                                                      struct cgroup_subsys *ss)
 {
         struct cgroup_subsys_state *css;
 
         rcu_read_lock();
         css = cgroup_css(cgrp, ss);
         if (css && !css_tryget_online(css))
                 css = NULL;
         rcu_read_unlock();
 
         return css;
 }
 
 static int cgroup_extra_stat_show(struct seq_file *seq, int ssid)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct cgroup_subsys *ss = cgroup_subsys[ssid];
         struct cgroup_subsys_state *css;
         int ret;
 
         if (!ss->css_extra_stat_show)
                 return 0;
 
         css = cgroup_tryget_css(cgrp, ss);
         if (!css)
                 return 0;
 
         ret = ss->css_extra_stat_show(seq, css);
         css_put(css);
         return ret;
 }
 
 static int cgroup_local_stat_show(struct seq_file *seq,
                                   struct cgroup *cgrp, int ssid)
 {
         struct cgroup_subsys *ss = cgroup_subsys[ssid];
         struct cgroup_subsys_state *css;
         int ret;
 
         if (!ss->css_local_stat_show)
                 return 0;
 
         css = cgroup_tryget_css(cgrp, ss);
         if (!css)
                 return 0;
 
         ret = ss->css_local_stat_show(seq, css);
         css_put(css);
         return ret;
 }
 #endif
 
 static int cpu_stat_show(struct seq_file *seq, void *v)
 {
         int ret = 0;
 
         cgroup_base_stat_cputime_show(seq);
 #ifdef CONFIG_CGROUP_SCHED
         ret = cgroup_extra_stat_show(seq, cpu_cgrp_id);
 #endif
         return ret;
 }
 
 static int cpu_local_stat_show(struct seq_file *seq, void *v)
 {
         struct cgroup __maybe_unused *cgrp = seq_css(seq)->cgroup;
         int ret = 0;
 
 #ifdef CONFIG_CGROUP_SCHED
         ret = cgroup_local_stat_show(seq, cgrp, cpu_cgrp_id);
 #endif
         return ret;
 }
 
 #ifdef CONFIG_PSI
 static int cgroup_io_pressure_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct psi_group *psi = cgroup_psi(cgrp);
 
         return psi_show(seq, psi, PSI_IO);
 }
 static int cgroup_memory_pressure_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct psi_group *psi = cgroup_psi(cgrp);
 
         return psi_show(seq, psi, PSI_MEM);
 }
 static int cgroup_cpu_pressure_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct psi_group *psi = cgroup_psi(cgrp);
 
         return psi_show(seq, psi, PSI_CPU);
 }
 
 static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
                               size_t nbytes, enum psi_res res)
 {
         struct cgroup_file_ctx *ctx = of->priv;
         struct psi_trigger *new;
         struct cgroup *cgrp;
         struct psi_group *psi;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENODEV;
 
         cgroup_get(cgrp);
         cgroup_kn_unlock(of->kn);
 
         /* Allow only one trigger per file descriptor */
         if (ctx->psi.trigger) {
                 cgroup_put(cgrp);
                 return -EBUSY;
         }
 
         psi = cgroup_psi(cgrp);
         new = psi_trigger_create(psi, buf, res, of->file, of);
         if (IS_ERR(new)) {
                 cgroup_put(cgrp);
                 return PTR_ERR(new);
         }
 
         smp_store_release(&ctx->psi.trigger, new);
         cgroup_put(cgrp);
 
         return nbytes;
 }
 
 static ssize_t cgroup_io_pressure_write(struct kernfs_open_file *of,
                                           char *buf, size_t nbytes,
                                           loff_t off)
 {
         return pressure_write(of, buf, nbytes, PSI_IO);
 }
 
 static ssize_t cgroup_memory_pressure_write(struct kernfs_open_file *of,
                                           char *buf, size_t nbytes,
                                           loff_t off)
 {
         return pressure_write(of, buf, nbytes, PSI_MEM);
 }
 
 static ssize_t cgroup_cpu_pressure_write(struct kernfs_open_file *of,
                                           char *buf, size_t nbytes,
                                           loff_t off)
 {
         return pressure_write(of, buf, nbytes, PSI_CPU);
 }
 
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 static int cgroup_irq_pressure_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct psi_group *psi = cgroup_psi(cgrp);
 
         return psi_show(seq, psi, PSI_IRQ);
 }
 
 static ssize_t cgroup_irq_pressure_write(struct kernfs_open_file *of,
                                          char *buf, size_t nbytes,
                                          loff_t off)
 {
         return pressure_write(of, buf, nbytes, PSI_IRQ);
 }
 #endif
 
 static int cgroup_pressure_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
         struct psi_group *psi = cgroup_psi(cgrp);
 
         seq_printf(seq, "%d\n", psi->enabled);
 
         return 0;
 }
 
 static ssize_t cgroup_pressure_write(struct kernfs_open_file *of,
                                      char *buf, size_t nbytes,
                                      loff_t off)
 {
         ssize_t ret;
         int enable;
         struct cgroup *cgrp;
         struct psi_group *psi;
 
         ret = kstrtoint(strstrip(buf), 0, &enable);
         if (ret)
                 return ret;
 
         if (enable < 0 || enable > 1)
                 return -ERANGE;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENOENT;
 
         psi = cgroup_psi(cgrp);
         if (psi->enabled != enable) {
                 int i;
 
                 /* show or hide {cpu,memory,io,irq}.pressure files */
                 for (i = 0; i < NR_PSI_RESOURCES; i++)
                         cgroup_file_show(&cgrp->psi_files[i], enable);
 
                 psi->enabled = enable;
                 if (enable)
                         psi_cgroup_restart(psi);
         }
 
         cgroup_kn_unlock(of->kn);
 
         return nbytes;
 }
 
 static __poll_t cgroup_pressure_poll(struct kernfs_open_file *of,
                                           poll_table *pt)
 {
         struct cgroup_file_ctx *ctx = of->priv;
 
         return psi_trigger_poll(&ctx->psi.trigger, of->file, pt);
 }
 
 static void cgroup_pressure_release(struct kernfs_open_file *of)
 {
         struct cgroup_file_ctx *ctx = of->priv;
 
         psi_trigger_destroy(ctx->psi.trigger);
 }
 
 bool cgroup_psi_enabled(void)
 {
         if (static_branch_likely(&psi_disabled))
                 return false;
 
         return (cgroup_feature_disable_mask & (1 << OPT_FEATURE_PRESSURE)) == 0;
 }
 
 #else /* CONFIG_PSI */
 bool cgroup_psi_enabled(void)
 {
         return false;
 }
 
 #endif /* CONFIG_PSI */
 
 static int cgroup_freeze_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
         seq_printf(seq, "%d\n", cgrp->freezer.freeze);
 
         return 0;
 }
 
 static ssize_t cgroup_freeze_write(struct kernfs_open_file *of,
                                    char *buf, size_t nbytes, loff_t off)
 {
         struct cgroup *cgrp;
         ssize_t ret;
         int freeze;
 
         ret = kstrtoint(strstrip(buf), 0, &freeze);
         if (ret)
                 return ret;
 
         if (freeze < 0 || freeze > 1)
                 return -ERANGE;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENOENT;
 
         cgroup_freeze(cgrp, freeze);
 
         cgroup_kn_unlock(of->kn);
 
         return nbytes;
 }
 
 static void __cgroup_kill(struct cgroup *cgrp)
 {
         struct css_task_iter it;
         struct task_struct *task;
 
         lockdep_assert_held(&cgroup_mutex);
 
         spin_lock_irq(&css_set_lock);
         set_bit(CGRP_KILL, &cgrp->flags);
         spin_unlock_irq(&css_set_lock);
 
         css_task_iter_start(&cgrp->self, CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED, &it);
         while ((task = css_task_iter_next(&it))) {
                 /* Ignore kernel threads here. */
                 if (task->flags & PF_KTHREAD)
                         continue;
 
                 /* Skip tasks that are already dying. */
                 if (__fatal_signal_pending(task))
                         continue;
 
                 send_sig(SIGKILL, task, 0);
         }
         css_task_iter_end(&it);
 
         spin_lock_irq(&css_set_lock);
         clear_bit(CGRP_KILL, &cgrp->flags);
         spin_unlock_irq(&css_set_lock);
 }
 
 static void cgroup_kill(struct cgroup *cgrp)
 {
         struct cgroup_subsys_state *css;
         struct cgroup *dsct;
 
         lockdep_assert_held(&cgroup_mutex);
 
         cgroup_for_each_live_descendant_pre(dsct, css, cgrp)
                 __cgroup_kill(dsct);
 }
 
 static ssize_t cgroup_kill_write(struct kernfs_open_file *of, char *buf,
                                  size_t nbytes, loff_t off)
 {
         ssize_t ret = 0;
         int kill;
         struct cgroup *cgrp;
 
         ret = kstrtoint(strstrip(buf), 0, &kill);
         if (ret)
                 return ret;
 
         if (kill != 1)
                 return -ERANGE;
 
         cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!cgrp)
                 return -ENOENT;
 
         /*
          * Killing is a process directed operation, i.e. the whole thread-group
          * is taken down so act like we do for cgroup.procs and only make this
          * writable in non-threaded cgroups.
          */
         if (cgroup_is_threaded(cgrp))
                 ret = -EOPNOTSUPP;
         else
                 cgroup_kill(cgrp);
 
         cgroup_kn_unlock(of->kn);
 
         return ret ?: nbytes;
 }
 
 static int cgroup_file_open(struct kernfs_open_file *of)
 {
         struct cftype *cft = of_cft(of);
         struct cgroup_file_ctx *ctx;
         int ret;
 
         ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
         if (!ctx)
                 return -ENOMEM;
 
         ctx->ns = current->nsproxy->cgroup_ns;
         get_cgroup_ns(ctx->ns);
         of->priv = ctx;
 
         if (!cft->open)
                 return 0;
 
         ret = cft->open(of);
         if (ret) {
                 put_cgroup_ns(ctx->ns);
                 kfree(ctx);
         }
         return ret;
 }
 
 static void cgroup_file_release(struct kernfs_open_file *of)
 {
         struct cftype *cft = of_cft(of);
         struct cgroup_file_ctx *ctx = of->priv;
 
         if (cft->release)
                 cft->release(of);
         put_cgroup_ns(ctx->ns);
         kfree(ctx);
 }
 
 static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
                                  size_t nbytes, loff_t off)
 {
         struct cgroup_file_ctx *ctx = of->priv;
         struct cgroup *cgrp = of->kn->parent->priv;
         struct cftype *cft = of_cft(of);
         struct cgroup_subsys_state *css;
         int ret;
 
         if (!nbytes)
                 return 0;
 
         /*
          * If namespaces are delegation boundaries, disallow writes to
          * files in an non-init namespace root from inside the namespace
          * except for the files explicitly marked delegatable -
          * cgroup.procs and cgroup.subtree_control.
          */
         if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
             !(cft->flags & CFTYPE_NS_DELEGATABLE) &&
             ctx->ns != &init_cgroup_ns && ctx->ns->root_cset->dfl_cgrp == cgrp)
                 return -EPERM;
 
         if (cft->write)
                 return cft->write(of, buf, nbytes, off);
 
         /*
          * kernfs guarantees that a file isn't deleted with operations in
          * flight, which means that the matching css is and stays alive and
          * doesn't need to be pinned.  The RCU locking is not necessary
          * either.  It's just for the convenience of using cgroup_css().
          */
         rcu_read_lock();
         css = cgroup_css(cgrp, cft->ss);
         rcu_read_unlock();
 
         if (cft->write_u64) {
                 unsigned long long v;
                 ret = kstrtoull(buf, 0, &v);
                 if (!ret)
                         ret = cft->write_u64(css, cft, v);
         } else if (cft->write_s64) {
                 long long v;
                 ret = kstrtoll(buf, 0, &v);
                 if (!ret)
                         ret = cft->write_s64(css, cft, v);
         } else {
                 ret = -EINVAL;
         }
 
         return ret ?: nbytes;
 }
 
 static __poll_t cgroup_file_poll(struct kernfs_open_file *of, poll_table *pt)
 {
         struct cftype *cft = of_cft(of);
 
         if (cft->poll)
                 return cft->poll(of, pt);
 
         return kernfs_generic_poll(of, pt);
 }
 
 static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
 {
         return seq_cft(seq)->seq_start(seq, ppos);
 }
 
 static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
 {
         return seq_cft(seq)->seq_next(seq, v, ppos);
 }
 
 static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
 {
         if (seq_cft(seq)->seq_stop)
                 seq_cft(seq)->seq_stop(seq, v);
 }
 
 static int cgroup_seqfile_show(struct seq_file *m, void *arg)
 {
         struct cftype *cft = seq_cft(m);
         struct cgroup_subsys_state *css = seq_css(m);
 
         if (cft->seq_show)
                 return cft->seq_show(m, arg);
 
         if (cft->read_u64)
                 seq_printf(m, "%llu\n", cft->read_u64(css, cft));
         else if (cft->read_s64)
                 seq_printf(m, "%lld\n", cft->read_s64(css, cft));
         else
                 return -EINVAL;
         return 0;
 }
 
 static struct kernfs_ops cgroup_kf_single_ops = {
         .atomic_write_len       = PAGE_SIZE,
         .open                   = cgroup_file_open,
         .release                = cgroup_file_release,
         .write                  = cgroup_file_write,
         .poll                   = cgroup_file_poll,
         .seq_show               = cgroup_seqfile_show,
 };
 
 static struct kernfs_ops cgroup_kf_ops = {
         .atomic_write_len       = PAGE_SIZE,
         .open                   = cgroup_file_open,
         .release                = cgroup_file_release,
         .write                  = cgroup_file_write,
         .poll                   = cgroup_file_poll,
         .seq_start              = cgroup_seqfile_start,
         .seq_next               = cgroup_seqfile_next,
         .seq_stop               = cgroup_seqfile_stop,
         .seq_show               = cgroup_seqfile_show,
 };
 
 static void cgroup_file_notify_timer(struct timer_list *timer)
 {
         cgroup_file_notify(container_of(timer, struct cgroup_file,
                                         notify_timer));
 }
 
 static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
                            struct cftype *cft)
 {
         char name[CGROUP_FILE_NAME_MAX];
         struct kernfs_node *kn;
         struct lock_class_key *key = NULL;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         key = &cft->lockdep_key;
 #endif
         kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
                                   cgroup_file_mode(cft),
                                   current_fsuid(), current_fsgid(),
                                   0, cft->kf_ops, cft,
                                   NULL, key);
         if (IS_ERR(kn))
                 return PTR_ERR(kn);
 
         if (cft->file_offset) {
                 struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
                 timer_setup(&cfile->notify_timer, cgroup_file_notify_timer, 0);
 
                 spin_lock_irq(&cgroup_file_kn_lock);
                 cfile->kn = kn;
                 spin_unlock_irq(&cgroup_file_kn_lock);
         }
 
         return 0;
 }
 
 /**
  * cgroup_addrm_files - add or remove files to a cgroup directory
  * @css: the target css
  * @cgrp: the target cgroup (usually css->cgroup)
  * @cfts: array of cftypes to be added
  * @is_add: whether to add or remove
  *
  * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
  * For removals, this function never fails.
  */
 static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                               struct cgroup *cgrp, struct cftype cfts[],
                               bool is_add)
 {
         struct cftype *cft, *cft_end = NULL;
         int ret = 0;
 
         lockdep_assert_held(&cgroup_mutex);
 
 restart:
         for (cft = cfts; cft != cft_end && cft->name[0] != '\0'; cft++) {
                 /* does cft->flags tell us to skip this file on @cgrp? */
                 if ((cft->flags & __CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
                         continue;
                 if ((cft->flags & __CFTYPE_NOT_ON_DFL) && cgroup_on_dfl(cgrp))
                         continue;
                 if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
                         continue;
                 if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
                         continue;
                 if ((cft->flags & CFTYPE_DEBUG) && !cgroup_debug)
                         continue;
                 if (is_add) {
                         ret = cgroup_add_file(css, cgrp, cft);
                         if (ret) {
                                 pr_warn("%s: failed to add %s, err=%d\n",
                                         __func__, cft->name, ret);
                                 cft_end = cft;
                                 is_add = false;
                                 goto restart;
                         }
                 } else {
                         cgroup_rm_file(cgrp, cft);
                 }
         }
         return ret;
 }
 
 static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
 {
         struct cgroup_subsys *ss = cfts[0].ss;
         struct cgroup *root = &ss->root->cgrp;
         struct cgroup_subsys_state *css;
         int ret = 0;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* add/rm files for all cgroups created before */
         css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
                 struct cgroup *cgrp = css->cgroup;
 
                 if (!(css->flags & CSS_VISIBLE))
                         continue;
 
                 ret = cgroup_addrm_files(css, cgrp, cfts, is_add);
                 if (ret)
                         break;
         }
 
         if (is_add && !ret)
                 kernfs_activate(root->kn);
         return ret;
 }
 
 static void cgroup_exit_cftypes(struct cftype *cfts)
 {
         struct cftype *cft;
 
         for (cft = cfts; cft->name[0] != '\0'; cft++) {
                 /* free copy for custom atomic_write_len, see init_cftypes() */
                 if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
                         kfree(cft->kf_ops);
                 cft->kf_ops = NULL;
                 cft->ss = NULL;
 
                 /* revert flags set by cgroup core while adding @cfts */
                 cft->flags &= ~(__CFTYPE_ONLY_ON_DFL | __CFTYPE_NOT_ON_DFL |
                                 __CFTYPE_ADDED);
         }
 }
 
 static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
         struct cftype *cft;
         int ret = 0;
 
         for (cft = cfts; cft->name[0] != '\0'; cft++) {
                 struct kernfs_ops *kf_ops;
 
                 WARN_ON(cft->ss || cft->kf_ops);
 
                 if (cft->flags & __CFTYPE_ADDED) {
                         ret = -EBUSY;
                         break;
                 }
 
                 if (cft->seq_start)
                         kf_ops = &cgroup_kf_ops;
                 else
                         kf_ops = &cgroup_kf_single_ops;
 
                 /*
                  * Ugh... if @cft wants a custom max_write_len, we need to
                  * make a copy of kf_ops to set its atomic_write_len.
                  */
                 if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
                         kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
                         if (!kf_ops) {
                                 ret = -ENOMEM;
                                 break;
                         }
                         kf_ops->atomic_write_len = cft->max_write_len;
                 }
 
                 cft->kf_ops = kf_ops;
                 cft->ss = ss;
                 cft->flags |= __CFTYPE_ADDED;
         }
 
         if (ret)
                 cgroup_exit_cftypes(cfts);
         return ret;
 }
 
 static void cgroup_rm_cftypes_locked(struct cftype *cfts)
 {
         lockdep_assert_held(&cgroup_mutex);
 
         list_del(&cfts->node);
         cgroup_apply_cftypes(cfts, false);
         cgroup_exit_cftypes(cfts);
 }
 
 /**
  * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Unregister @cfts.  Files described by @cfts are removed from all
  * existing cgroups and all future cgroups won't have them either.  This
  * function can be called anytime whether @cfts' subsys is attached or not.
  *
  * Returns 0 on successful unregistration, -ENOENT if @cfts is not
  * registered.
  */
 int cgroup_rm_cftypes(struct cftype *cfts)
 {
         if (!cfts || cfts[0].name[0] == '\0')
                 return 0;
 
         if (!(cfts[0].flags & __CFTYPE_ADDED))
                 return -ENOENT;
 
         cgroup_lock();
         cgroup_rm_cftypes_locked(cfts);
         cgroup_unlock();
         return 0;
 }
 
 /**
  * cgroup_add_cftypes - add an array of cftypes to a subsystem
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Register @cfts to @ss.  Files described by @cfts are created for all
  * existing cgroups to which @ss is attached and all future cgroups will
  * have them too.  This function can be called anytime whether @ss is
  * attached or not.
  *
  * Returns 0 on successful registration, -errno on failure.  Note that this
  * function currently returns 0 as long as @cfts registration is successful
  * even if some file creation attempts on existing cgroups fail.
  */
 static int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
         int ret;
 
         if (!cgroup_ssid_enabled(ss->id))
                 return 0;
 
         if (!cfts || cfts[0].name[0] == '\0')
                 return 0;
 
         ret = cgroup_init_cftypes(ss, cfts);
         if (ret)
                 return ret;
 
         cgroup_lock();
 
         list_add_tail(&cfts->node, &ss->cfts);
         ret = cgroup_apply_cftypes(cfts, true);
         if (ret)
                 cgroup_rm_cftypes_locked(cfts);
 
         cgroup_unlock();
         return ret;
 }
 
 /**
  * cgroup_add_dfl_cftypes - add an array of cftypes for default hierarchy
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Similar to cgroup_add_cftypes() but the added files are only used for
  * the default hierarchy.
  */
 int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
         struct cftype *cft;
 
         for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
                 cft->flags |= __CFTYPE_ONLY_ON_DFL;
         return cgroup_add_cftypes(ss, cfts);
 }
 
 /**
  * cgroup_add_legacy_cftypes - add an array of cftypes for legacy hierarchies
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Similar to cgroup_add_cftypes() but the added files are only used for
  * the legacy hierarchies.
  */
 int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
         struct cftype *cft;
 
         for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
                 cft->flags |= __CFTYPE_NOT_ON_DFL;
         return cgroup_add_cftypes(ss, cfts);
 }
 
 /**
  * cgroup_file_notify - generate a file modified event for a cgroup_file
  * @cfile: target cgroup_file
  *
  * @cfile must have been obtained by setting cftype->file_offset.
  */
 void cgroup_file_notify(struct cgroup_file *cfile)
 {
         unsigned long flags;
 
         spin_lock_irqsave(&cgroup_file_kn_lock, flags);
         if (cfile->kn) {
                 unsigned long last = cfile->notified_at;
                 unsigned long next = last + CGROUP_FILE_NOTIFY_MIN_INTV;
 
                 if (time_in_range(jiffies, last, next)) {
                         timer_reduce(&cfile->notify_timer, next);
                 } else {
                         kernfs_notify(cfile->kn);
                         cfile->notified_at = jiffies;
                 }
         }
         spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);
 }
 
 /**
  * cgroup_file_show - show or hide a hidden cgroup file
  * @cfile: target cgroup_file obtained by setting cftype->file_offset
  * @show: whether to show or hide
  */
 void cgroup_file_show(struct cgroup_file *cfile, bool show)
 {
         struct kernfs_node *kn;
 
         spin_lock_irq(&cgroup_file_kn_lock);
         kn = cfile->kn;
         kernfs_get(kn);
         spin_unlock_irq(&cgroup_file_kn_lock);
 
         if (kn)
                 kernfs_show(kn, show);
 
         kernfs_put(kn);
 }
 
 /**
  * css_next_child - find the next child of a given css
  * @pos: the current position (%NULL to initiate traversal)
  * @parent: css whose children to walk
  *
  * This function returns the next child of @parent and should be called
  * under either cgroup_mutex or RCU read lock.  The only requirement is
  * that @parent and @pos are accessible.  The next sibling is guaranteed to
  * be returned regardless of their states.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
                                            struct cgroup_subsys_state *parent)
 {
         struct cgroup_subsys_state *next;
 
         cgroup_assert_mutex_or_rcu_locked();
 
         /*
          * @pos could already have been unlinked from the sibling list.
          * Once a cgroup is removed, its ->sibling.next is no longer
          * updated when its next sibling changes.  CSS_RELEASED is set when
          * @pos is taken off list, at which time its next pointer is valid,
          * and, as releases are serialized, the one pointed to by the next
          * pointer is guaranteed to not have started release yet.  This
          * implies that if we observe !CSS_RELEASED on @pos in this RCU
          * critical section, the one pointed to by its next pointer is
          * guaranteed to not have finished its RCU grace period even if we
          * have dropped rcu_read_lock() in-between iterations.
          *
          * If @pos has CSS_RELEASED set, its next pointer can't be
          * dereferenced; however, as each css is given a monotonically
          * increasing unique serial number and always appended to the
          * sibling list, the next one can be found by walking the parent's
          * children until the first css with higher serial number than
          * @pos's.  While this path can be slower, it happens iff iteration
          * races against release and the race window is very small.
          */
         if (!pos) {
                 next = list_entry_rcu(parent->children.next, struct cgroup_subsys_state, sibling);
         } else if (likely(!(pos->flags & CSS_RELEASED))) {
                 next = list_entry_rcu(pos->sibling.next, struct cgroup_subsys_state, sibling);
         } else {
                 list_for_each_entry_rcu(next, &parent->children, sibling,
                                         lockdep_is_held(&cgroup_mutex))
                         if (next->serial_nr > pos->serial_nr)
                                 break;
         }
 
         /*
          * @next, if not pointing to the head, can be dereferenced and is
          * the next sibling.
          */
         if (&next->sibling != &parent->children)
                 return next;
         return NULL;
 }
 
 /**
  * css_next_descendant_pre - find the next descendant for pre-order walk
  * @pos: the current position (%NULL to initiate traversal)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_pre().  Find the next descendant
  * to visit for pre-order traversal of @root's descendants.  @root is
  * included in the iteration and the first node to be visited.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct next descendant as long
  * as both @pos and @root are accessible and @pos is a descendant of @root.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *
 css_next_descendant_pre(struct cgroup_subsys_state *pos,
                         struct cgroup_subsys_state *root)
 {
         struct cgroup_subsys_state *next;
 
         cgroup_assert_mutex_or_rcu_locked();
 
         /* if first iteration, visit @root */
         if (!pos)
                 return root;
 
         /* visit the first child if exists */
         next = css_next_child(NULL, pos);
         if (next)
                 return next;
 
         /* no child, visit my or the closest ancestor's next sibling */
         while (pos != root) {
                 next = css_next_child(pos, pos->parent);
                 if (next)
                         return next;
                 pos = pos->parent;
         }
 
         return NULL;
 }
 EXPORT_SYMBOL_GPL(css_next_descendant_pre);
 
 /**
  * css_rightmost_descendant - return the rightmost descendant of a css
  * @pos: css of interest
  *
  * Return the rightmost descendant of @pos.  If there's no descendant, @pos
  * is returned.  This can be used during pre-order traversal to skip
  * subtree of @pos.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct rightmost descendant as
  * long as @pos is accessible.
  */
 struct cgroup_subsys_state *
 css_rightmost_descendant(struct cgroup_subsys_state *pos)
 {
         struct cgroup_subsys_state *last, *tmp;
 
         cgroup_assert_mutex_or_rcu_locked();
 
         do {
                 last = pos;
                 /* ->prev isn't RCU safe, walk ->next till the end */
                 pos = NULL;
                 css_for_each_child(tmp, last)
                         pos = tmp;
         } while (pos);
 
         return last;
 }
 
 static struct cgroup_subsys_state *
 css_leftmost_descendant(struct cgroup_subsys_state *pos)
 {
         struct cgroup_subsys_state *last;
 
         do {
                 last = pos;
                 pos = css_next_child(NULL, pos);
         } while (pos);
 
         return last;
 }
 
 /**
  * css_next_descendant_post - find the next descendant for post-order walk
  * @pos: the current position (%NULL to initiate traversal)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_post().  Find the next descendant
  * to visit for post-order traversal of @root's descendants.  @root is
  * included in the iteration and the last node to be visited.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct next descendant as long
  * as both @pos and @cgroup are accessible and @pos is a descendant of
  * @cgroup.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *
 css_next_descendant_post(struct cgroup_subsys_state *pos,
                          struct cgroup_subsys_state *root)
 {
         struct cgroup_subsys_state *next;
 
         cgroup_assert_mutex_or_rcu_locked();
 
         /* if first iteration, visit leftmost descendant which may be @root */
         if (!pos)
                 return css_leftmost_descendant(root);
 
         /* if we visited @root, we're done */
         if (pos == root)
                 return NULL;
 
         /* if there's an unvisited sibling, visit its leftmost descendant */
         next = css_next_child(pos, pos->parent);
         if (next)
                 return css_leftmost_descendant(next);
 
         /* no sibling left, visit parent */
         return pos->parent;
 }
 
 /**
  * css_has_online_children - does a css have online children
  * @css: the target css
  *
  * Returns %true if @css has any online children; otherwise, %false.  This
  * function can be called from any context but the caller is responsible
  * for synchronizing against on/offlining as necessary.
  */
 bool css_has_online_children(struct cgroup_subsys_state *css)
 {
         struct cgroup_subsys_state *child;
         bool ret = false;
 
         rcu_read_lock();
         css_for_each_child(child, css) {
                 if (child->flags & CSS_ONLINE) {
                         ret = true;
                         break;
                 }
         }
         rcu_read_unlock();
         return ret;
 }
 
 static struct css_set *css_task_iter_next_css_set(struct css_task_iter *it)
 {
         struct list_head *l;
         struct cgrp_cset_link *link;
         struct css_set *cset;
 
         lockdep_assert_held(&css_set_lock);
 
         /* find the next threaded cset */
         if (it->tcset_pos) {
                 l = it->tcset_pos->next;
 
                 if (l != it->tcset_head) {
                         it->tcset_pos = l;
                         return container_of(l, struct css_set,
                                             threaded_csets_node);
                 }
 
                 it->tcset_pos = NULL;
         }
 
         /* find the next cset */
         l = it->cset_pos;
         l = l->next;
         if (l == it->cset_head) {
                 it->cset_pos = NULL;
                 return NULL;
         }
 
         if (it->ss) {
                 cset = container_of(l, struct css_set, e_cset_node[it->ss->id]);
         } else {
                 link = list_entry(l, struct cgrp_cset_link, cset_link);
                 cset = link->cset;
         }
 
         it->cset_pos = l;
 
         /* initialize threaded css_set walking */
         if (it->flags & CSS_TASK_ITER_THREADED) {
                 if (it->cur_dcset)
                         put_css_set_locked(it->cur_dcset);
                 it->cur_dcset = cset;
                 get_css_set(cset);
 
                 it->tcset_head = &cset->threaded_csets;
                 it->tcset_pos = &cset->threaded_csets;
         }
 
         return cset;
 }
 
 /**
  * css_task_iter_advance_css_set - advance a task iterator to the next css_set
  * @it: the iterator to advance
  *
  * Advance @it to the next css_set to walk.
  */
 static void css_task_iter_advance_css_set(struct css_task_iter *it)
 {
         struct css_set *cset;
 
         lockdep_assert_held(&css_set_lock);
 
         /* Advance to the next non-empty css_set and find first non-empty tasks list*/
         while ((cset = css_task_iter_next_css_set(it))) {
                 if (!list_empty(&cset->tasks)) {
                         it->cur_tasks_head = &cset->tasks;
                         break;
                 } else if (!list_empty(&cset->mg_tasks)) {
                         it->cur_tasks_head = &cset->mg_tasks;
                         break;
                 } else if (!list_empty(&cset->dying_tasks)) {
                         it->cur_tasks_head = &cset->dying_tasks;
                         break;
                 }
         }
         if (!cset) {
                 it->task_pos = NULL;
                 return;
         }
         it->task_pos = it->cur_tasks_head->next;
 
         /*
          * We don't keep css_sets locked across iteration steps and thus
          * need to take steps to ensure that iteration can be resumed after
          * the lock is re-acquired.  Iteration is performed at two levels -
          * css_sets and tasks in them.
          *
          * Once created, a css_set never leaves its cgroup lists, so a
          * pinned css_set is guaranteed to stay put and we can resume
          * iteration afterwards.
          *
          * Tasks may leave @cset across iteration steps.  This is resolved
          * by registering each iterator with the css_set currently being
          * walked and making css_set_move_task() advance iterators whose
          * next task is leaving.
          */
         if (it->cur_cset) {
                 list_del(&it->iters_node);
                 put_css_set_locked(it->cur_cset);
         }
         get_css_set(cset);
         it->cur_cset = cset;
         list_add(&it->iters_node, &cset->task_iters);
 }
 
 static void css_task_iter_skip(struct css_task_iter *it,
                                struct task_struct *task)
 {
         lockdep_assert_held(&css_set_lock);
 
         if (it->task_pos == &task->cg_list) {
                 it->task_pos = it->task_pos->next;
                 it->flags |= CSS_TASK_ITER_SKIPPED;
         }
 }
 
 static void css_task_iter_advance(struct css_task_iter *it)
 {
         struct task_struct *task;
 
         lockdep_assert_held(&css_set_lock);
 repeat:
         if (it->task_pos) {
                 /*
                  * Advance iterator to find next entry. We go through cset
                  * tasks, mg_tasks and dying_tasks, when consumed we move onto
                  * the next cset.
                  */
                 if (it->flags & CSS_TASK_ITER_SKIPPED)
                         it->flags &= ~CSS_TASK_ITER_SKIPPED;
                 else
                         it->task_pos = it->task_pos->next;
 
                 if (it->task_pos == &it->cur_cset->tasks) {
                         it->cur_tasks_head = &it->cur_cset->mg_tasks;
                         it->task_pos = it->cur_tasks_head->next;
                 }
                 if (it->task_pos == &it->cur_cset->mg_tasks) {
                         it->cur_tasks_head = &it->cur_cset->dying_tasks;
                         it->task_pos = it->cur_tasks_head->next;
                 }
                 if (it->task_pos == &it->cur_cset->dying_tasks)
                         css_task_iter_advance_css_set(it);
         } else {
                 /* called from start, proceed to the first cset */
                 css_task_iter_advance_css_set(it);
         }
 
         if (!it->task_pos)
                 return;
 
         task = list_entry(it->task_pos, struct task_struct, cg_list);
 
         if (it->flags & CSS_TASK_ITER_PROCS) {
                 /* if PROCS, skip over tasks which aren't group leaders */
                 if (!thread_group_leader(task))
                         goto repeat;
 
                 /* and dying leaders w/o live member threads */
                 if (it->cur_tasks_head == &it->cur_cset->dying_tasks &&
                     !atomic_read(&task->signal->live))
                         goto repeat;
         } else {
                 /* skip all dying ones */
                 if (it->cur_tasks_head == &it->cur_cset->dying_tasks)
                         goto repeat;
         }
 }
 
 /**
  * css_task_iter_start - initiate task iteration
  * @css: the css to walk tasks of
  * @flags: CSS_TASK_ITER_* flags
  * @it: the task iterator to use
  *
  * Initiate iteration through the tasks of @css.  The caller can call
  * css_task_iter_next() to walk through the tasks until the function
  * returns NULL.  On completion of iteration, css_task_iter_end() must be
  * called.
  */
 void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags,
                          struct css_task_iter *it)
 {
         unsigned long irqflags;
 
         memset(it, 0, sizeof(*it));
 
         spin_lock_irqsave(&css_set_lock, irqflags);
 
         it->ss = css->ss;
         it->flags = flags;
 
         if (CGROUP_HAS_SUBSYS_CONFIG && it->ss)
                 it->cset_pos = &css->cgroup->e_csets[css->ss->id];
         else
                 it->cset_pos = &css->cgroup->cset_links;
 
         it->cset_head = it->cset_pos;
 
         css_task_iter_advance(it);
 
         spin_unlock_irqrestore(&css_set_lock, irqflags);
 }
 
 /**
  * css_task_iter_next - return the next task for the iterator
  * @it: the task iterator being iterated
  *
  * The "next" function for task iteration.  @it should have been
  * initialized via css_task_iter_start().  Returns NULL when the iteration
  * reaches the end.
  */
 struct task_struct *css_task_iter_next(struct css_task_iter *it)
 {
         unsigned long irqflags;
 
         if (it->cur_task) {
                 put_task_struct(it->cur_task);
                 it->cur_task = NULL;
         }
 
         spin_lock_irqsave(&css_set_lock, irqflags);
 
         /* @it may be half-advanced by skips, finish advancing */
         if (it->flags & CSS_TASK_ITER_SKIPPED)
                 css_task_iter_advance(it);
 
         if (it->task_pos) {
                 it->cur_task = list_entry(it->task_pos, struct task_struct,
                                           cg_list);
                 get_task_struct(it->cur_task);
                 css_task_iter_advance(it);
         }
 
         spin_unlock_irqrestore(&css_set_lock, irqflags);
 
         return it->cur_task;
 }
 
 /**
  * css_task_iter_end - finish task iteration
  * @it: the task iterator to finish
  *
  * Finish task iteration started by css_task_iter_start().
  */
 void css_task_iter_end(struct css_task_iter *it)
 {
         unsigned long irqflags;
 
         if (it->cur_cset) {
                 spin_lock_irqsave(&css_set_lock, irqflags);
                 list_del(&it->iters_node);
                 put_css_set_locked(it->cur_cset);
                 spin_unlock_irqrestore(&css_set_lock, irqflags);
         }
 
         if (it->cur_dcset)
                 put_css_set(it->cur_dcset);
 
         if (it->cur_task)
                 put_task_struct(it->cur_task);
 }
 
 static void cgroup_procs_release(struct kernfs_open_file *of)
 {
         struct cgroup_file_ctx *ctx = of->priv;
 
         if (ctx->procs.started)
                 css_task_iter_end(&ctx->procs.iter);
 }
 
 static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
 {
         struct kernfs_open_file *of = s->private;
         struct cgroup_file_ctx *ctx = of->priv;
 
         if (pos)
                 (*pos)++;
 
         return css_task_iter_next(&ctx->procs.iter);
 }
 
 static void *__cgroup_procs_start(struct seq_file *s, loff_t *pos,
                                   unsigned int iter_flags)
 {
         struct kernfs_open_file *of = s->private;
         struct cgroup *cgrp = seq_css(s)->cgroup;
         struct cgroup_file_ctx *ctx = of->priv;
         struct css_task_iter *it = &ctx->procs.iter;
 
         /*
          * When a seq_file is seeked, it's always traversed sequentially
          * from position 0, so we can simply keep iterating on !0 *pos.
          */
         if (!ctx->procs.started) {
                 if (WARN_ON_ONCE((*pos)))
                         return ERR_PTR(-EINVAL);
                 css_task_iter_start(&cgrp->self, iter_flags, it);
                 ctx->procs.started = true;
         } else if (!(*pos)) {
                 css_task_iter_end(it);
                 css_task_iter_start(&cgrp->self, iter_flags, it);
         } else
                 return it->cur_task;
 
         return cgroup_procs_next(s, NULL, NULL);
 }
 
 static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
 {
         struct cgroup *cgrp = seq_css(s)->cgroup;
 
         /*
          * All processes of a threaded subtree belong to the domain cgroup
          * of the subtree.  Only threads can be distributed across the
          * subtree.  Reject reads on cgroup.procs in the subtree proper.
          * They're always empty anyway.
          */
         if (cgroup_is_threaded(cgrp))
                 return ERR_PTR(-EOPNOTSUPP);
 
         return __cgroup_procs_start(s, pos, CSS_TASK_ITER_PROCS |
                                             CSS_TASK_ITER_THREADED);
 }
 
 static int cgroup_procs_show(struct seq_file *s, void *v)
 {
         seq_printf(s, "%d\n", task_pid_vnr(v));
         return 0;
 }
 
 static int cgroup_may_write(const struct cgroup *cgrp, struct super_block *sb)
 {
         int ret;
         struct inode *inode;
 
         lockdep_assert_held(&cgroup_mutex);
 
         inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
         if (!inode)
                 return -ENOMEM;
 
         ret = inode_permission(&nop_mnt_idmap, inode, MAY_WRITE);
         iput(inode);
         return ret;
 }
 
 static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
                                          struct cgroup *dst_cgrp,
                                          struct super_block *sb,
                                          struct cgroup_namespace *ns)
 {
         struct cgroup *com_cgrp = src_cgrp;
         int ret;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* find the common ancestor */
         while (!cgroup_is_descendant(dst_cgrp, com_cgrp))
                 com_cgrp = cgroup_parent(com_cgrp);
 
         /* %current should be authorized to migrate to the common ancestor */
         ret = cgroup_may_write(com_cgrp, sb);
         if (ret)
                 return ret;
 
         /*
          * If namespaces are delegation boundaries, %current must be able
          * to see both source and destination cgroups from its namespace.
          */
         if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
             (!cgroup_is_descendant(src_cgrp, ns->root_cset->dfl_cgrp) ||
              !cgroup_is_descendant(dst_cgrp, ns->root_cset->dfl_cgrp)))
                 return -ENOENT;
 
         return 0;
 }
 
 static int cgroup_attach_permissions(struct cgroup *src_cgrp,
                                      struct cgroup *dst_cgrp,
                                      struct super_block *sb, bool threadgroup,
                                      struct cgroup_namespace *ns)
 {
         int ret = 0;
 
         ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb, ns);
         if (ret)
                 return ret;
 
         ret = cgroup_migrate_vet_dst(dst_cgrp);
         if (ret)
                 return ret;
 
         if (!threadgroup && (src_cgrp->dom_cgrp != dst_cgrp->dom_cgrp))
                 ret = -EOPNOTSUPP;
 
         return ret;
 }
 
 static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
                                     bool threadgroup)
 {
         struct cgroup_file_ctx *ctx = of->priv;
         struct cgroup *src_cgrp, *dst_cgrp;
         struct task_struct *task;
         const struct cred *saved_cred;
         ssize_t ret;
         bool threadgroup_locked;
 
         dst_cgrp = cgroup_kn_lock_live(of->kn, false);
         if (!dst_cgrp)
                 return -ENODEV;
 
         task = cgroup_procs_write_start(buf, threadgroup, &threadgroup_locked);
         ret = PTR_ERR_OR_ZERO(task);
         if (ret)
                 goto out_unlock;
 
         /* find the source cgroup */
         spin_lock_irq(&css_set_lock);
         src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
         spin_unlock_irq(&css_set_lock);
 
         /*
          * Process and thread migrations follow same delegation rule. Check
          * permissions using the credentials from file open to protect against
          * inherited fd attacks.
          */
         saved_cred = override_creds(of->file->f_cred);
         ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
                                         of->file->f_path.dentry->d_sb,
                                         threadgroup, ctx->ns);
         revert_creds(saved_cred);
         if (ret)
                 goto out_finish;
 
         ret = cgroup_attach_task(dst_cgrp, task, threadgroup);
 
 out_finish:
         cgroup_procs_write_finish(task, threadgroup_locked);
 out_unlock:
         cgroup_kn_unlock(of->kn);
 
         return ret;
 }
 
 static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
                                   char *buf, size_t nbytes, loff_t off)
 {
         return __cgroup_procs_write(of, buf, true) ?: nbytes;
 }
 
 static void *cgroup_threads_start(struct seq_file *s, loff_t *pos)
 {
         return __cgroup_procs_start(s, pos, 0);
 }
 
 static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
                                     char *buf, size_t nbytes, loff_t off)
 {
         return __cgroup_procs_write(of, buf, false) ?: nbytes;
 }
 
 /* cgroup core interface files for the default hierarchy */
 static struct cftype cgroup_base_files[] = {
         {
                 .name = "cgroup.type",
                 .flags = CFTYPE_NOT_ON_ROOT,
                 .seq_show = cgroup_type_show,
                 .write = cgroup_type_write,
         },
         {
                 .name = "cgroup.procs",
                 .flags = CFTYPE_NS_DELEGATABLE,
                 .file_offset = offsetof(struct cgroup, procs_file),
                 .release = cgroup_procs_release,
                 .seq_start = cgroup_procs_start,
                 .seq_next = cgroup_procs_next,
                 .seq_show = cgroup_procs_show,
                 .write = cgroup_procs_write,
         },
         {
                 .name = "cgroup.threads",
                 .flags = CFTYPE_NS_DELEGATABLE,
                 .release = cgroup_procs_release,
                 .seq_start = cgroup_threads_start,
                 .seq_next = cgroup_procs_next,
                 .seq_show = cgroup_procs_show,
                 .write = cgroup_threads_write,
         },
         {
                 .name = "cgroup.controllers",
                 .seq_show = cgroup_controllers_show,
         },
         {
                 .name = "cgroup.subtree_control",
                 .flags = CFTYPE_NS_DELEGATABLE,
                 .seq_show = cgroup_subtree_control_show,
                 .write = cgroup_subtree_control_write,
         },
         {
                 .name = "cgroup.events",
                 .flags = CFTYPE_NOT_ON_ROOT,
                 .file_offset = offsetof(struct cgroup, events_file),
                 .seq_show = cgroup_events_show,
         },
         {
                 .name = "cgroup.max.descendants",
                 .seq_show = cgroup_max_descendants_show,
                 .write = cgroup_max_descendants_write,
         },
         {
                 .name = "cgroup.max.depth",
                 .seq_show = cgroup_max_depth_show,
                 .write = cgroup_max_depth_write,
         },
         {
                 .name = "cgroup.stat",
                 .seq_show = cgroup_stat_show,
         },
         {
                 .name = "cgroup.freeze",
                 .flags = CFTYPE_NOT_ON_ROOT,
                 .seq_show = cgroup_freeze_show,
                 .write = cgroup_freeze_write,
         },
         {
                 .name = "cgroup.kill",
                 .flags = CFTYPE_NOT_ON_ROOT,
                 .write = cgroup_kill_write,
         },
         {
                 .name = "cpu.stat",
                 .seq_show = cpu_stat_show,
         },
         {
                 .name = "cpu.stat.local",
                 .seq_show = cpu_local_stat_show,
         },
         { }     /* terminate */
 };
 
 static struct cftype cgroup_psi_files[] = {
 #ifdef CONFIG_PSI
         {
                 .name = "io.pressure",
                 .file_offset = offsetof(struct cgroup, psi_files[PSI_IO]),
                 .seq_show = cgroup_io_pressure_show,
                 .write = cgroup_io_pressure_write,
                 .poll = cgroup_pressure_poll,
                 .release = cgroup_pressure_release,
         },
         {
                 .name = "memory.pressure",
                 .file_offset = offsetof(struct cgroup, psi_files[PSI_MEM]),
                 .seq_show = cgroup_memory_pressure_show,
                 .write = cgroup_memory_pressure_write,
                 .poll = cgroup_pressure_poll,
                 .release = cgroup_pressure_release,
         },
         {
                 .name = "cpu.pressure",
                 .file_offset = offsetof(struct cgroup, psi_files[PSI_CPU]),
                 .seq_show = cgroup_cpu_pressure_show,
                 .write = cgroup_cpu_pressure_write,
                 .poll = cgroup_pressure_poll,
                 .release = cgroup_pressure_release,
         },
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
         {
                 .name = "irq.pressure",
                 .file_offset = offsetof(struct cgroup, psi_files[PSI_IRQ]),
                 .seq_show = cgroup_irq_pressure_show,
                 .write = cgroup_irq_pressure_write,
                 .poll = cgroup_pressure_poll,
                 .release = cgroup_pressure_release,
         },
 #endif
         {
                 .name = "cgroup.pressure",
                 .seq_show = cgroup_pressure_show,
                 .write = cgroup_pressure_write,
         },
 #endif /* CONFIG_PSI */
         { }     /* terminate */
 };
 
 /*
  * css destruction is four-stage process.
  *
  * 1. Destruction starts.  Killing of the percpu_ref is initiated.
  *    Implemented in kill_css().
  *
  * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
  *    and thus css_tryget_online() is guaranteed to fail, the css can be
  *    offlined by invoking offline_css().  After offlining, the base ref is
  *    put.  Implemented in css_killed_work_fn().
  *
  * 3. When the percpu_ref reaches zero, the only possible remaining
  *    accessors are inside RCU read sections.  css_release() schedules the
  *    RCU callback.
  *
  * 4. After the grace period, the css can be freed.  Implemented in
  *    css_free_rwork_fn().
  *
  * It is actually hairier because both step 2 and 4 require process context
  * and thus involve punting to css->destroy_work adding two additional
  * steps to the already complex sequence.
  */
 static void css_free_rwork_fn(struct work_struct *work)
 {
         struct cgroup_subsys_state *css = container_of(to_rcu_work(work),
                                 struct cgroup_subsys_state, destroy_rwork);
         struct cgroup_subsys *ss = css->ss;
         struct cgroup *cgrp = css->cgroup;
 
         percpu_ref_exit(&css->refcnt);
 
         if (ss) {
                 /* css free path */
                 struct cgroup_subsys_state *parent = css->parent;
                 int id = css->id;
 
                 ss->css_free(css);
                 cgroup_idr_remove(&ss->css_idr, id);
                 cgroup_put(cgrp);
 
                 if (parent)
                         css_put(parent);
         } else {
                 /* cgroup free path */
                 atomic_dec(&cgrp->root->nr_cgrps);
                 if (!cgroup_on_dfl(cgrp))
                         cgroup1_pidlist_destroy_all(cgrp);
                 cancel_work_sync(&cgrp->release_agent_work);
                 bpf_cgrp_storage_free(cgrp);
 
                 if (cgroup_parent(cgrp)) {
                         /*
                          * We get a ref to the parent, and put the ref when
                          * this cgroup is being freed, so it's guaranteed
                          * that the parent won't be destroyed before its
                          * children.
                          */
                         cgroup_put(cgroup_parent(cgrp));
                         kernfs_put(cgrp->kn);
                         psi_cgroup_free(cgrp);
                         cgroup_rstat_exit(cgrp);
                         kfree(cgrp);
                 } else {
                         /*
                          * This is root cgroup's refcnt reaching zero,
                          * which indicates that the root should be
                          * released.
                          */
                         cgroup_destroy_root(cgrp->root);
                 }
         }
 }
 
 static void css_release_work_fn(struct work_struct *work)
 {
         struct cgroup_subsys_state *css =
                 container_of(work, struct cgroup_subsys_state, destroy_work);
         struct cgroup_subsys *ss = css->ss;
         struct cgroup *cgrp = css->cgroup;
 
         cgroup_lock();
 
         css->flags |= CSS_RELEASED;
         list_del_rcu(&css->sibling);
 
         if (ss) {
                 /* css release path */
                 if (!list_empty(&css->rstat_css_node)) {
                         cgroup_rstat_flush(cgrp);
                         list_del_rcu(&css->rstat_css_node);
                 }
 
                 cgroup_idr_replace(&ss->css_idr, NULL, css->id);
                 if (ss->css_released)
                         ss->css_released(css);
         } else {
                 struct cgroup *tcgrp;
 
                 /* cgroup release path */
                 TRACE_CGROUP_PATH(release, cgrp);
 
                 cgroup_rstat_flush(cgrp);
 
                 spin_lock_irq(&css_set_lock);
                 for (tcgrp = cgroup_parent(cgrp); tcgrp;
                      tcgrp = cgroup_parent(tcgrp))
                         tcgrp->nr_dying_descendants--;
                 spin_unlock_irq(&css_set_lock);
 
                 /*
                  * There are two control paths which try to determine
                  * cgroup from dentry without going through kernfs -
                  * cgroupstats_build() and css_tryget_online_from_dir().
                  * Those are supported by RCU protecting clearing of
                  * cgrp->kn->priv backpointer.
                  */
                 if (cgrp->kn)
                         RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv,
                                          NULL);
         }
 
         cgroup_unlock();
 
         INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
         queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 }
 
 static void css_release(struct percpu_ref *ref)
 {
         struct cgroup_subsys_state *css =
                 container_of(ref, struct cgroup_subsys_state, refcnt);
 
         INIT_WORK(&css->destroy_work, css_release_work_fn);
         queue_work(cgroup_destroy_wq, &css->destroy_work);
 }
 
 static void init_and_link_css(struct cgroup_subsys_state *css,
                               struct cgroup_subsys *ss, struct cgroup *cgrp)
 {
         lockdep_assert_held(&cgroup_mutex);
 
         cgroup_get_live(cgrp);
 
         memset(css, 0, sizeof(*css));
         css->cgroup = cgrp;
         css->ss = ss;
         css->id = -1;
         INIT_LIST_HEAD(&css->sibling);
         INIT_LIST_HEAD(&css->children);
         INIT_LIST_HEAD(&css->rstat_css_node);
         css->serial_nr = css_serial_nr_next++;
         atomic_set(&css->online_cnt, 0);
 
         if (cgroup_parent(cgrp)) {
                 css->parent = cgroup_css(cgroup_parent(cgrp), ss);
                 css_get(css->parent);
         }
 
         if (ss->css_rstat_flush)
                 list_add_rcu(&css->rstat_css_node, &cgrp->rstat_css_list);
 
         BUG_ON(cgroup_css(cgrp, ss));
 }
 
 /* invoke ->css_online() on a new CSS and mark it online if successful */
 static int online_css(struct cgroup_subsys_state *css)
 {
         struct cgroup_subsys *ss = css->ss;
         int ret = 0;
 
         lockdep_assert_held(&cgroup_mutex);
 
         if (ss->css_online)
                 ret = ss->css_online(css);
         if (!ret) {
                 css->flags |= CSS_ONLINE;
                 rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
 
                 atomic_inc(&css->online_cnt);
                 if (css->parent)
                         atomic_inc(&css->parent->online_cnt);
         }
         return ret;
 }
 
 /* if the CSS is online, invoke ->css_offline() on it and mark it offline */
 static void offline_css(struct cgroup_subsys_state *css)
 {
         struct cgroup_subsys *ss = css->ss;
 
         lockdep_assert_held(&cgroup_mutex);
 
         if (!(css->flags & CSS_ONLINE))
                 return;
 
         if (ss->css_offline)
                 ss->css_offline(css);
 
         css->flags &= ~CSS_ONLINE;
         RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
 
         wake_up_all(&css->cgroup->offline_waitq);
 }
 
 /**
  * css_create - create a cgroup_subsys_state
  * @cgrp: the cgroup new css will be associated with
  * @ss: the subsys of new css
  *
  * Create a new css associated with @cgrp - @ss pair.  On success, the new
  * css is online and installed in @cgrp.  This function doesn't create the
  * interface files.  Returns 0 on success, -errno on failure.
  */
 static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
                                               struct cgroup_subsys *ss)
 {
         struct cgroup *parent = cgroup_parent(cgrp);
         struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
         struct cgroup_subsys_state *css;
         int err;
 
         lockdep_assert_held(&cgroup_mutex);
 
         css = ss->css_alloc(parent_css);
         if (!css)
                 css = ERR_PTR(-ENOMEM);
         if (IS_ERR(css))
                 return css;
 
         init_and_link_css(css, ss, cgrp);
 
         err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
         if (err)
                 goto err_free_css;
 
         err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
         if (err < 0)
                 goto err_free_css;
         css->id = err;
 
         /* @css is ready to be brought online now, make it visible */
         list_add_tail_rcu(&css->sibling, &parent_css->children);
         cgroup_idr_replace(&ss->css_idr, css, css->id);
 
         err = online_css(css);
         if (err)
                 goto err_list_del;
 
         return css;
 
 err_list_del:
         list_del_rcu(&css->sibling);
 err_free_css:
         list_del_rcu(&css->rstat_css_node);
         INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
         queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
         return ERR_PTR(err);
 }
 
 /*
  * The returned cgroup is fully initialized including its control mask, but
  * it doesn't have the control mask applied.
  */
 static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
                                     umode_t mode)
 {
         struct cgroup_root *root = parent->root;
         struct cgroup *cgrp, *tcgrp;
         struct kernfs_node *kn;
         int level = parent->level + 1;
         int ret;
 
         /* allocate the cgroup and its ID, 0 is reserved for the root */
         cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);
         if (!cgrp)
                 return ERR_PTR(-ENOMEM);
 
         ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
         if (ret)
                 goto out_free_cgrp;
 
         ret = cgroup_rstat_init(cgrp);
         if (ret)
                 goto out_cancel_ref;
 
         /* create the directory */
         kn = kernfs_create_dir_ns(parent->kn, name, mode,
                                   current_fsuid(), current_fsgid(),
                                   cgrp, NULL);
         if (IS_ERR(kn)) {
                 ret = PTR_ERR(kn);
                 goto out_stat_exit;
         }
         cgrp->kn = kn;
 
         init_cgroup_housekeeping(cgrp);
 
         cgrp->self.parent = &parent->self;
         cgrp->root = root;
         cgrp->level = level;
 
         ret = psi_cgroup_alloc(cgrp);
         if (ret)
                 goto out_kernfs_remove;
 
         ret = cgroup_bpf_inherit(cgrp);
         if (ret)
                 goto out_psi_free;
 
         /*
          * New cgroup inherits effective freeze counter, and
          * if the parent has to be frozen, the child has too.
          */
         cgrp->freezer.e_freeze = parent->freezer.e_freeze;
         if (cgrp->freezer.e_freeze) {
                 /*
                  * Set the CGRP_FREEZE flag, so when a process will be
                  * attached to the child cgroup, it will become frozen.
                  * At this point the new cgroup is unpopulated, so we can
                  * consider it frozen immediately.
                  */
                 set_bit(CGRP_FREEZE, &cgrp->flags);
                 set_bit(CGRP_FROZEN, &cgrp->flags);
         }
 
         spin_lock_irq(&css_set_lock);
         for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
                 cgrp->ancestors[tcgrp->level] = tcgrp;
 
                 if (tcgrp != cgrp) {
                         tcgrp->nr_descendants++;
 
                         /*
                          * If the new cgroup is frozen, all ancestor cgroups
                          * get a new frozen descendant, but their state can't
                          * change because of this.
                          */
                         if (cgrp->freezer.e_freeze)
                                 tcgrp->freezer.nr_frozen_descendants++;
                 }
         }
         spin_unlock_irq(&css_set_lock);
 
         if (notify_on_release(parent))
                 set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
 
         if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
                 set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
 
         cgrp->self.serial_nr = css_serial_nr_next++;
 
         /* allocation complete, commit to creation */
         list_add_tail_rcu(&cgrp->self.sibling, &cgroup_parent(cgrp)->self.children);
         atomic_inc(&root->nr_cgrps);
         cgroup_get_live(parent);
 
         /*
          * On the default hierarchy, a child doesn't automatically inherit
          * subtree_control from the parent.  Each is configured manually.
          */
         if (!cgroup_on_dfl(cgrp))
                 cgrp->subtree_control = cgroup_control(cgrp);
 
         cgroup_propagate_control(cgrp);
 
         return cgrp;
 
 out_psi_free:
         psi_cgroup_free(cgrp);
 out_kernfs_remove:
         kernfs_remove(cgrp->kn);
 out_stat_exit:
         cgroup_rstat_exit(cgrp);
 out_cancel_ref:
         percpu_ref_exit(&cgrp->self.refcnt);
 out_free_cgrp:
         kfree(cgrp);
         return ERR_PTR(ret);
 }
 
 static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 {
         struct cgroup *cgroup;
         int ret = false;
         int level = 1;
 
         lockdep_assert_held(&cgroup_mutex);
 
         for (cgroup = parent; cgroup; cgroup = cgroup_parent(cgroup)) {
                 if (cgroup->nr_descendants >= cgroup->max_descendants)
                         goto fail;
 
                 if (level > cgroup->max_depth)
                         goto fail;
 
                 level++;
         }
 
         ret = true;
 fail:
         return ret;
 }
 
 int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 {
         struct cgroup *parent, *cgrp;
         int ret;
 
         /* do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable */
         if (strchr(name, '\n'))
                 return -EINVAL;
 
         parent = cgroup_kn_lock_live(parent_kn, false);
         if (!parent)
                 return -ENODEV;
 
         if (!cgroup_check_hierarchy_limits(parent)) {
                 ret = -EAGAIN;
                 goto out_unlock;
         }
 
         cgrp = cgroup_create(parent, name, mode);
         if (IS_ERR(cgrp)) {
                 ret = PTR_ERR(cgrp);
                 goto out_unlock;
         }
 
         /*
          * This extra ref will be put in cgroup_free_fn() and guarantees
          * that @cgrp->kn is always accessible.
          */
         kernfs_get(cgrp->kn);
 
         ret = css_populate_dir(&cgrp->self);
         if (ret)
                 goto out_destroy;
 
         ret = cgroup_apply_control_enable(cgrp);
         if (ret)
                 goto out_destroy;
 
         TRACE_CGROUP_PATH(mkdir, cgrp);
 
         /* let's create and online css's */
         kernfs_activate(cgrp->kn);
 
         ret = 0;
         goto out_unlock;
 
 out_destroy:
         cgroup_destroy_locked(cgrp);
 out_unlock:
         cgroup_kn_unlock(parent_kn);
         return ret;
 }
 
 /*
  * This is called when the refcnt of a css is confirmed to be killed.
  * css_tryget_online() is now guaranteed to fail.  Tell the subsystem to
  * initiate destruction and put the css ref from kill_css().
  */
 static void css_killed_work_fn(struct work_struct *work)
 {
         struct cgroup_subsys_state *css =
                 container_of(work, struct cgroup_subsys_state, destroy_work);
 
         cgroup_lock();
 
         do {
                 offline_css(css);
                 css_put(css);
                 /* @css can't go away while we're holding cgroup_mutex */
                 css = css->parent;
         } while (css && atomic_dec_and_test(&css->online_cnt));
 
         cgroup_unlock();
 }
 
 /* css kill confirmation processing requires process context, bounce */
 static void css_killed_ref_fn(struct percpu_ref *ref)
 {
         struct cgroup_subsys_state *css =
                 container_of(ref, struct cgroup_subsys_state, refcnt);
 
         if (atomic_dec_and_test(&css->online_cnt)) {
                 INIT_WORK(&css->destroy_work, css_killed_work_fn);
                 queue_work(cgroup_destroy_wq, &css->destroy_work);
         }
 }
 
 /**
  * kill_css - destroy a css
  * @css: css to destroy
  *
  * This function initiates destruction of @css by removing cgroup interface
  * files and putting its base reference.  ->css_offline() will be invoked
  * asynchronously once css_tryget_online() is guaranteed to fail and when
  * the reference count reaches zero, @css will be released.
  */
 static void kill_css(struct cgroup_subsys_state *css)
 {
         lockdep_assert_held(&cgroup_mutex);
 
         if (css->flags & CSS_DYING)
                 return;
 
         css->flags |= CSS_DYING;
 
         /*
          * This must happen before css is disassociated with its cgroup.
          * See seq_css() for details.
          */
         css_clear_dir(css);
 
         /*
          * Killing would put the base ref, but we need to keep it alive
          * until after ->css_offline().
          */
         css_get(css);
 
         /*
          * cgroup core guarantees that, by the time ->css_offline() is
          * invoked, no new css reference will be given out via
          * css_tryget_online().  We can't simply call percpu_ref_kill() and
          * proceed to offlining css's because percpu_ref_kill() doesn't
          * guarantee that the ref is seen as killed on all CPUs on return.
          *
          * Use percpu_ref_kill_and_confirm() to get notifications as each
          * css is confirmed to be seen as killed on all CPUs.
          */
         percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
 }
 
 /**
  * cgroup_destroy_locked - the first stage of cgroup destruction
  * @cgrp: cgroup to be destroyed
  *
  * css's make use of percpu refcnts whose killing latency shouldn't be
  * exposed to userland and are RCU protected.  Also, cgroup core needs to
  * guarantee that css_tryget_online() won't succeed by the time
  * ->css_offline() is invoked.  To satisfy all the requirements,
  * destruction is implemented in the following two steps.
  *
  * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
  *     userland visible parts and start killing the percpu refcnts of
  *     css's.  Set up so that the next stage will be kicked off once all
  *     the percpu refcnts are confirmed to be killed.
  *
  * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
  *     rest of destruction.  Once all cgroup references are gone, the
  *     cgroup is RCU-freed.
  *
  * This function implements s1.  After this step, @cgrp is gone as far as
  * the userland is concerned and a new cgroup with the same name may be
  * created.  As cgroup doesn't care about the names internally, this
  * doesn't cause any problem.
  */
 static int cgroup_destroy_locked(struct cgroup *cgrp)
         __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
         struct cgroup *tcgrp, *parent = cgroup_parent(cgrp);
         struct cgroup_subsys_state *css;
         struct cgrp_cset_link *link;
         int ssid;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /*
          * Only migration can raise populated from zero and we're already
          * holding cgroup_mutex.
          */
         if (cgroup_is_populated(cgrp))
                 return -EBUSY;
 
         /*
          * Make sure there's no live children.  We can't test emptiness of
          * ->self.children as dead children linger on it while being
          * drained; otherwise, "rmdir parent/child parent" may fail.
          */
         if (css_has_online_children(&cgrp->self))
                 return -EBUSY;
 
         /*
          * Mark @cgrp and the associated csets dead.  The former prevents
          * further task migration and child creation by disabling
          * cgroup_kn_lock_live().  The latter makes the csets ignored by
          * the migration path.
          */
         cgrp->self.flags &= ~CSS_ONLINE;
 
         spin_lock_irq(&css_set_lock);
         list_for_each_entry(link, &cgrp->cset_links, cset_link)
                 link->cset->dead = true;
         spin_unlock_irq(&css_set_lock);
 
         /* initiate massacre of all css's */
         for_each_css(css, ssid, cgrp)
                 kill_css(css);
 
         /* clear and remove @cgrp dir, @cgrp has an extra ref on its kn */
         css_clear_dir(&cgrp->self);
         kernfs_remove(cgrp->kn);
 
         if (cgroup_is_threaded(cgrp))
                 parent->nr_threaded_children--;
 
         spin_lock_irq(&css_set_lock);
         for (tcgrp = parent; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
                 tcgrp->nr_descendants--;
                 tcgrp->nr_dying_descendants++;
                 /*
                  * If the dying cgroup is frozen, decrease frozen descendants
                  * counters of ancestor cgroups.
                  */
                 if (test_bit(CGRP_FROZEN, &cgrp->flags))
                         tcgrp->freezer.nr_frozen_descendants--;
         }
         spin_unlock_irq(&css_set_lock);
 
         cgroup1_check_for_release(parent);
 
         cgroup_bpf_offline(cgrp);
 
         /* put the base reference */
         percpu_ref_kill(&cgrp->self.refcnt);
 
         return 0;
 };
 
 int cgroup_rmdir(struct kernfs_node *kn)
 {
         struct cgroup *cgrp;
         int ret = 0;
 
         cgrp = cgroup_kn_lock_live(kn, false);
         if (!cgrp)
                 return 0;
 
         ret = cgroup_destroy_locked(cgrp);
         if (!ret)
                 TRACE_CGROUP_PATH(rmdir, cgrp);
 
         cgroup_kn_unlock(kn);
         return ret;
 }
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
         .show_options           = cgroup_show_options,
         .mkdir                  = cgroup_mkdir,
         .rmdir                  = cgroup_rmdir,
         .show_path              = cgroup_show_path,
 };
 
 static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 {
         struct cgroup_subsys_state *css;
 
         pr_debug("Initializing cgroup subsys %s\n", ss->name);
 
         cgroup_lock();
 
         idr_init(&ss->css_idr);
         INIT_LIST_HEAD(&ss->cfts);
 
         /* Create the root cgroup state for this subsystem */
         ss->root = &cgrp_dfl_root;
         css = ss->css_alloc(NULL);
         /* We don't handle early failures gracefully */
         BUG_ON(IS_ERR(css));
         init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
 
         /*
          * Root csses are never destroyed and we can't initialize
          * percpu_ref during early init.  Disable refcnting.
          */
         css->flags |= CSS_NO_REF;
 
         if (early) {
                 /* allocation can't be done safely during early init */
                 css->id = 1;
         } else {
                 css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
                 BUG_ON(css->id < 0);
         }
 
         /* Update the init_css_set to contain a subsys
          * pointer to this state - since the subsystem is
          * newly registered, all tasks and hence the
          * init_css_set is in the subsystem's root cgroup. */
         init_css_set.subsys[ss->id] = css;
 
         have_fork_callback |= (bool)ss->fork << ss->id;
         have_exit_callback |= (bool)ss->exit << ss->id;
         have_release_callback |= (bool)ss->release << ss->id;
         have_canfork_callback |= (bool)ss->can_fork << ss->id;
 
         /* At system boot, before all subsystems have been
          * registered, no tasks have been forked, so we don't
          * need to invoke fork callbacks here. */
         BUG_ON(!list_empty(&init_task.tasks));
 
         BUG_ON(online_css(css));
 
         cgroup_unlock();
 }
 
 /**
  * cgroup_init_early - cgroup initialization at system boot
  *
  * Initialize cgroups at system boot, and initialize any
  * subsystems that request early init.
  */
 int __init cgroup_init_early(void)
 {
         static struct cgroup_fs_context __initdata ctx;
         struct cgroup_subsys *ss;
         int i;
 
         ctx.root = &cgrp_dfl_root;
         init_cgroup_root(&ctx);
         cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;
 
         RCU_INIT_POINTER(init_task.cgroups, &init_css_set);
 
         for_each_subsys(ss, i) {
                 WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
                      "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p id:name=%d:%s\n",
                      i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
                      ss->id, ss->name);
                 WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
                      "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);
 
                 ss->id = i;
                 ss->name = cgroup_subsys_name[i];
                 if (!ss->legacy_name)
                         ss->legacy_name = cgroup_subsys_name[i];
 
                 if (ss->early_init)
                         cgroup_init_subsys(ss, true);
         }
         return 0;
 }
 
 /**
  * cgroup_init - cgroup initialization
  *
  * Register cgroup filesystem and /proc file, and initialize
  * any subsystems that didn't request early init.
  */
 int __init cgroup_init(void)
 {
         struct cgroup_subsys *ss;
         int ssid;
 
         BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
         BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
         BUG_ON(cgroup_init_cftypes(NULL, cgroup_psi_files));
         BUG_ON(cgroup_init_cftypes(NULL, cgroup1_base_files));
 
         cgroup_rstat_boot();
 
         get_user_ns(init_cgroup_ns.user_ns);
 
         cgroup_lock();
 
         /*
          * Add init_css_set to the hash table so that dfl_root can link to
          * it during init.
          */
         hash_add(css_set_table, &init_css_set.hlist,
                  css_set_hash(init_css_set.subsys));
 
         BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));
 
         cgroup_unlock();
 
         for_each_subsys(ss, ssid) {
                 if (ss->early_init) {
                         struct cgroup_subsys_state *css =
                                 init_css_set.subsys[ss->id];
 
                         css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
                                                    GFP_KERNEL);
                         BUG_ON(css->id < 0);
                 } else {
                         cgroup_init_subsys(ss, false);
                 }
 
                 list_add_tail(&init_css_set.e_cset_node[ssid],
                               &cgrp_dfl_root.cgrp.e_csets[ssid]);
 
                 /*
                  * Setting dfl_root subsys_mask needs to consider the
                  * disabled flag and cftype registration needs kmalloc,
                  * both of which aren't available during early_init.
                  */
                 if (!cgroup_ssid_enabled(ssid))
                         continue;
 
                 if (cgroup1_ssid_disabled(ssid))
                         pr_info("Disabling %s control group subsystem in v1 mounts\n",
                                 ss->legacy_name);
 
                 cgrp_dfl_root.subsys_mask |= 1 << ss->id;
 
                 /* implicit controllers must be threaded too */
                 WARN_ON(ss->implicit_on_dfl && !ss->threaded);
 
                 if (ss->implicit_on_dfl)
                         cgrp_dfl_implicit_ss_mask |= 1 << ss->id;
                 else if (!ss->dfl_cftypes)
                         cgrp_dfl_inhibit_ss_mask |= 1 << ss->id;
 
                 if (ss->threaded)
                         cgrp_dfl_threaded_ss_mask |= 1 << ss->id;
 
                 if (ss->dfl_cftypes == ss->legacy_cftypes) {
                         WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
                 } else {
                         WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
                         WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
                 }
 
                 if (ss->bind)
                         ss->bind(init_css_set.subsys[ssid]);
 
                 cgroup_lock();
                 css_populate_dir(init_css_set.subsys[ssid]);
                 cgroup_unlock();
         }
 
         /* init_css_set.subsys[] has been updated, re-hash */
         hash_del(&init_css_set.hlist);
         hash_add(css_set_table, &init_css_set.hlist,
                  css_set_hash(init_css_set.subsys));
 
         WARN_ON(sysfs_create_mount_point(fs_kobj, "cgroup"));
         WARN_ON(register_filesystem(&cgroup_fs_type));
         WARN_ON(register_filesystem(&cgroup2_fs_type));
         WARN_ON(!proc_create_single("cgroups", 0, NULL, proc_cgroupstats_show));
 #ifdef CONFIG_CPUSETS
         WARN_ON(register_filesystem(&cpuset_fs_type));
 #endif
 
         return 0;
 }
 
 static int __init cgroup_wq_init(void)
 {
         /*
          * There isn't much point in executing destruction path in
          * parallel.  Good chunk is serialized with cgroup_mutex anyway.
          * Use 1 for @max_active.
          *
          * We would prefer to do this in cgroup_init() above, but that
          * is called before init_workqueues(): so leave this until after.
          */
         cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
         BUG_ON(!cgroup_destroy_wq);
         return 0;
 }
 core_initcall(cgroup_wq_init);
 
 void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
 {
         struct kernfs_node *kn;
 
         kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
         if (!kn)
                 return;
         kernfs_path(kn, buf, buflen);
         kernfs_put(kn);
 }
 
 /*
  * cgroup_get_from_id : get the cgroup associated with cgroup id
  * @id: cgroup id
  * On success return the cgrp or ERR_PTR on failure
  * Only cgroups within current task's cgroup NS are valid.
  */
 struct cgroup *cgroup_get_from_id(u64 id)
 {
         struct kernfs_node *kn;
         struct cgroup *cgrp, *root_cgrp;
 
         kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
         if (!kn)
                 return ERR_PTR(-ENOENT);
 
         if (kernfs_type(kn) != KERNFS_DIR) {
                 kernfs_put(kn);
                 return ERR_PTR(-ENOENT);
         }
 
         rcu_read_lock();
 
         cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
         if (cgrp && !cgroup_tryget(cgrp))
                 cgrp = NULL;
 
         rcu_read_unlock();
         kernfs_put(kn);
 
         if (!cgrp)
                 return ERR_PTR(-ENOENT);
 
         root_cgrp = current_cgns_cgroup_dfl();
         if (!cgroup_is_descendant(cgrp, root_cgrp)) {
                 cgroup_put(cgrp);
                 return ERR_PTR(-ENOENT);
         }
 
         return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_id);
 
 /*
  * proc_cgroup_show()
  *  - Print task's cgroup paths into seq_file, one line for each hierarchy
  *  - Used for /proc/<pid>/cgroup.
  */
 int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
                      struct pid *pid, struct task_struct *tsk)
 {
         char *buf;
         int retval;
         struct cgroup_root *root;
 
         retval = -ENOMEM;
         buf = kmalloc(PATH_MAX, GFP_KERNEL);
         if (!buf)
                 goto out;
 
         rcu_read_lock();
         spin_lock_irq(&css_set_lock);
 
         for_each_root(root) {
                 struct cgroup_subsys *ss;
                 struct cgroup *cgrp;
                 int ssid, count = 0;
 
                 if (root == &cgrp_dfl_root && !READ_ONCE(cgrp_dfl_visible))
                         continue;
 
                 cgrp = task_cgroup_from_root(tsk, root);
                 /* The root has already been unmounted. */
                 if (!cgrp)
                         continue;
 
                 seq_printf(m, "%d:", root->hierarchy_id);
                 if (root != &cgrp_dfl_root)
                         for_each_subsys(ss, ssid)
                                 if (root->subsys_mask & (1 << ssid))
                                         seq_printf(m, "%s%s", count++ ? "," : "",
                                                    ss->legacy_name);
                 if (strlen(root->name))
                         seq_printf(m, "%sname=%s", count ? "," : "",
                                    root->name);
                 seq_putc(m, ':');
                 /*
                  * On traditional hierarchies, all zombie tasks show up as
                  * belonging to the root cgroup.  On the default hierarchy,
                  * while a zombie doesn't show up in "cgroup.procs" and
                  * thus can't be migrated, its /proc/PID/cgroup keeps
                  * reporting the cgroup it belonged to before exiting.  If
                  * the cgroup is removed before the zombie is reaped,
                  * " (deleted)" is appended to the cgroup path.
                  */
                 if (cgroup_on_dfl(cgrp) || !(tsk->flags & PF_EXITING)) {
                         retval = cgroup_path_ns_locked(cgrp, buf, PATH_MAX,
                                                 current->nsproxy->cgroup_ns);
                         if (retval == -E2BIG)
                                 retval = -ENAMETOOLONG;
                         if (retval < 0)
                                 goto out_unlock;
 
                         seq_puts(m, buf);
                 } else {
                         seq_puts(m, "/");
                 }
 
                 if (cgroup_on_dfl(cgrp) && cgroup_is_dead(cgrp))
                         seq_puts(m, " (deleted)\n");
                 else
                         seq_putc(m, '\n');
         }
 
         retval = 0;
 out_unlock:
         spin_unlock_irq(&css_set_lock);
         rcu_read_unlock();
         kfree(buf);
 out:
         return retval;
 }
 
 /**
  * cgroup_fork - initialize cgroup related fields during copy_process()
  * @child: pointer to task_struct of forking parent process.
  *
  * A task is associated with the init_css_set until cgroup_post_fork()
  * attaches it to the target css_set.
  */
 void cgroup_fork(struct task_struct *child)
 {
         RCU_INIT_POINTER(child->cgroups, &init_css_set);
         INIT_LIST_HEAD(&child->cg_list);
 }
 
 /**
  * cgroup_v1v2_get_from_file - get a cgroup pointer from a file pointer
  * @f: file corresponding to cgroup_dir
  *
  * Find the cgroup from a file pointer associated with a cgroup directory.
  * Returns a pointer to the cgroup on success. ERR_PTR is returned if the
  * cgroup cannot be found.
  */
 static struct cgroup *cgroup_v1v2_get_from_file(struct file *f)
 {
         struct cgroup_subsys_state *css;
 
         css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
         if (IS_ERR(css))
                 return ERR_CAST(css);
 
         return css->cgroup;
 }
 
 /**
  * cgroup_get_from_file - same as cgroup_v1v2_get_from_file, but only supports
  * cgroup2.
  * @f: file corresponding to cgroup2_dir
  */
 static struct cgroup *cgroup_get_from_file(struct file *f)
 {
         struct cgroup *cgrp = cgroup_v1v2_get_from_file(f);
 
         if (IS_ERR(cgrp))
                 return ERR_CAST(cgrp);
 
         if (!cgroup_on_dfl(cgrp)) {
                 cgroup_put(cgrp);
                 return ERR_PTR(-EBADF);
         }
 
         return cgrp;
 }
 
 /**
  * cgroup_css_set_fork - find or create a css_set for a child process
  * @kargs: the arguments passed to create the child process
  *
  * This functions finds or creates a new css_set which the child
  * process will be attached to in cgroup_post_fork(). By default,
  * the child process will be given the same css_set as its parent.
  *
  * If CLONE_INTO_CGROUP is specified this function will try to find an
  * existing css_set which includes the requested cgroup and if not create
  * a new css_set that the child will be attached to later. If this function
  * succeeds it will hold cgroup_threadgroup_rwsem on return. If
  * CLONE_INTO_CGROUP is requested this function will grab cgroup mutex
  * before grabbing cgroup_threadgroup_rwsem and will hold a reference
  * to the target cgroup.
  */
 static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
         __acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
 {
         int ret;
         struct cgroup *dst_cgrp = NULL;
         struct css_set *cset;
         struct super_block *sb;
         struct file *f;
 
         if (kargs->flags & CLONE_INTO_CGROUP)
                 cgroup_lock();
 
         cgroup_threadgroup_change_begin(current);
 
         spin_lock_irq(&css_set_lock);
         cset = task_css_set(current);
         get_css_set(cset);
         spin_unlock_irq(&css_set_lock);
 
         if (!(kargs->flags & CLONE_INTO_CGROUP)) {
                 kargs->cset = cset;
                 return 0;
         }
 
         f = fget_raw(kargs->cgroup);
         if (!f) {
                 ret = -EBADF;
                 goto err;
         }
         sb = f->f_path.dentry->d_sb;
 
         dst_cgrp = cgroup_get_from_file(f);
         if (IS_ERR(dst_cgrp)) {
                 ret = PTR_ERR(dst_cgrp);
                 dst_cgrp = NULL;
                 goto err;
         }
 
         if (cgroup_is_dead(dst_cgrp)) {
                 ret = -ENODEV;
                 goto err;
         }
 
         /*
          * Verify that we the target cgroup is writable for us. This is
          * usually done by the vfs layer but since we're not going through
          * the vfs layer here we need to do it "manually".
          */
         ret = cgroup_may_write(dst_cgrp, sb);
         if (ret)
                 goto err;
 
         /*
          * Spawning a task directly into a cgroup works by passing a file
          * descriptor to the target cgroup directory. This can even be an O_PATH
          * file descriptor. But it can never be a cgroup.procs file descriptor.
          * This was done on purpose so spawning into a cgroup could be
          * conceptualized as an atomic
          *
          *   fd = openat(dfd_cgroup, "cgroup.procs", ...);
          *   write(fd, <child-pid>, ...);
          *
          * sequence, i.e. it's a shorthand for the caller opening and writing
          * cgroup.procs of the cgroup indicated by @dfd_cgroup. This allows us
          * to always use the caller's credentials.
          */
         ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
                                         !(kargs->flags & CLONE_THREAD),
                                         current->nsproxy->cgroup_ns);
         if (ret)
                 goto err;
 
         kargs->cset = find_css_set(cset, dst_cgrp);
         if (!kargs->cset) {
                 ret = -ENOMEM;
                 goto err;
         }
 
         put_css_set(cset);
         fput(f);
         kargs->cgrp = dst_cgrp;
         return ret;
 
 err:
         cgroup_threadgroup_change_end(current);
         cgroup_unlock();
         if (f)
                 fput(f);
         if (dst_cgrp)
                 cgroup_put(dst_cgrp);
         put_css_set(cset);
         if (kargs->cset)
                 put_css_set(kargs->cset);
         return ret;
 }
 
 /**
  * cgroup_css_set_put_fork - drop references we took during fork
  * @kargs: the arguments passed to create the child process
  *
  * Drop references to the prepared css_set and target cgroup if
  * CLONE_INTO_CGROUP was requested.
  */
 static void cgroup_css_set_put_fork(struct kernel_clone_args *kargs)
         __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
         struct cgroup *cgrp = kargs->cgrp;
         struct css_set *cset = kargs->cset;
 
         cgroup_threadgroup_change_end(current);
 
         if (cset) {
                 put_css_set(cset);
                 kargs->cset = NULL;
         }
 
         if (kargs->flags & CLONE_INTO_CGROUP) {
                 cgroup_unlock();
                 if (cgrp) {
                         cgroup_put(cgrp);
                         kargs->cgrp = NULL;
                 }
         }
 }
 
 /**
  * cgroup_can_fork - called on a new task before the process is exposed
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * This prepares a new css_set for the child process which the child will
  * be attached to in cgroup_post_fork().
  * This calls the subsystem can_fork() callbacks. If the cgroup_can_fork()
  * callback returns an error, the fork aborts with that error code. This
  * allows for a cgroup subsystem to conditionally allow or deny new forks.
  */
 int cgroup_can_fork(struct task_struct *child, struct kernel_clone_args *kargs)
 {
         struct cgroup_subsys *ss;
         int i, j, ret;
 
         ret = cgroup_css_set_fork(kargs);
         if (ret)
                 return ret;
 
         do_each_subsys_mask(ss, i, have_canfork_callback) {
                 ret = ss->can_fork(child, kargs->cset);
                 if (ret)
                         goto out_revert;
         } while_each_subsys_mask();
 
         return 0;
 
 out_revert:
         for_each_subsys(ss, j) {
                 if (j >= i)
                         break;
                 if (ss->cancel_fork)
                         ss->cancel_fork(child, kargs->cset);
         }
 
         cgroup_css_set_put_fork(kargs);
 
         return ret;
 }
 
 /**
  * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * This calls the cancel_fork() callbacks if a fork failed *after*
  * cgroup_can_fork() succeeded and cleans up references we took to
  * prepare a new css_set for the child process in cgroup_can_fork().
  */
 void cgroup_cancel_fork(struct task_struct *child,
                         struct kernel_clone_args *kargs)
 {
         struct cgroup_subsys *ss;
         int i;
 
         for_each_subsys(ss, i)
                 if (ss->cancel_fork)
                         ss->cancel_fork(child, kargs->cset);
 
         cgroup_css_set_put_fork(kargs);
 }
 
 /**
  * cgroup_post_fork - finalize cgroup setup for the child process
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * Attach the child process to its css_set calling the subsystem fork()
  * callbacks.
  */
 void cgroup_post_fork(struct task_struct *child,
                       struct kernel_clone_args *kargs)
         __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
         unsigned long cgrp_flags = 0;
         bool kill = false;
         struct cgroup_subsys *ss;
         struct css_set *cset;
         int i;
 
         cset = kargs->cset;
         kargs->cset = NULL;
 
         spin_lock_irq(&css_set_lock);
 
         /* init tasks are special, only link regular threads */
         if (likely(child->pid)) {
                 if (kargs->cgrp)
                         cgrp_flags = kargs->cgrp->flags;
                 else
                         cgrp_flags = cset->dfl_cgrp->flags;
 
                 WARN_ON_ONCE(!list_empty(&child->cg_list));
                 cset->nr_tasks++;
                 css_set_move_task(child, NULL, cset, false);
         } else {
                 put_css_set(cset);
                 cset = NULL;
         }
 
         if (!(child->flags & PF_KTHREAD)) {
                 if (unlikely(test_bit(CGRP_FREEZE, &cgrp_flags))) {
                         /*
                          * If the cgroup has to be frozen, the new task has
                          * too. Let's set the JOBCTL_TRAP_FREEZE jobctl bit to
                          * get the task into the frozen state.
                          */
                         spin_lock(&child->sighand->siglock);
                         WARN_ON_ONCE(child->frozen);
                         child->jobctl |= JOBCTL_TRAP_FREEZE;
                         spin_unlock(&child->sighand->siglock);
 
                         /*
                          * Calling cgroup_update_frozen() isn't required here,
                          * because it will be called anyway a bit later from
                          * do_freezer_trap(). So we avoid cgroup's transient
                          * switch from the frozen state and back.
                          */
                 }
 
                 /*
                  * If the cgroup is to be killed notice it now and take the
                  * child down right after we finished preparing it for
                  * userspace.
                  */
                 kill = test_bit(CGRP_KILL, &cgrp_flags);
         }
 
         spin_unlock_irq(&css_set_lock);
 
         /*
          * Call ss->fork().  This must happen after @child is linked on
          * css_set; otherwise, @child might change state between ->fork()
          * and addition to css_set.
          */
         do_each_subsys_mask(ss, i, have_fork_callback) {
                 ss->fork(child);
         } while_each_subsys_mask();
 
         /* Make the new cset the root_cset of the new cgroup namespace. */
         if (kargs->flags & CLONE_NEWCGROUP) {
                 struct css_set *rcset = child->nsproxy->cgroup_ns->root_cset;
 
                 get_css_set(cset);
                 child->nsproxy->cgroup_ns->root_cset = cset;
                 put_css_set(rcset);
         }
 
         /* Cgroup has to be killed so take down child immediately. */
         if (unlikely(kill))
                 do_send_sig_info(SIGKILL, SEND_SIG_NOINFO, child, PIDTYPE_TGID);
 
         cgroup_css_set_put_fork(kargs);
 }
 
 /**
  * cgroup_exit - detach cgroup from exiting task
  * @tsk: pointer to task_struct of exiting process
  *
  * Description: Detach cgroup from @tsk.
  *
  */
 void cgroup_exit(struct task_struct *tsk)
 {
         struct cgroup_subsys *ss;
         struct css_set *cset;
         int i;
 
         spin_lock_irq(&css_set_lock);
 
         WARN_ON_ONCE(list_empty(&tsk->cg_list));
         cset = task_css_set(tsk);
         css_set_move_task(tsk, cset, NULL, false);
         cset->nr_tasks--;
         /* matches the signal->live check in css_task_iter_advance() */
         if (thread_group_leader(tsk) && atomic_read(&tsk->signal->live))
                 list_add_tail(&tsk->cg_list, &cset->dying_tasks);
 
         if (dl_task(tsk))
                 dec_dl_tasks_cs(tsk);
 
         WARN_ON_ONCE(cgroup_task_frozen(tsk));
         if (unlikely(!(tsk->flags & PF_KTHREAD) &&
                      test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))
                 cgroup_update_frozen(task_dfl_cgroup(tsk));
 
         spin_unlock_irq(&css_set_lock);
 
         /* see cgroup_post_fork() for details */
         do_each_subsys_mask(ss, i, have_exit_callback) {
                 ss->exit(tsk);
         } while_each_subsys_mask();
 }
 
 void cgroup_release(struct task_struct *task)
 {
         struct cgroup_subsys *ss;
         int ssid;
 
         do_each_subsys_mask(ss, ssid, have_release_callback) {
                 ss->release(task);
         } while_each_subsys_mask();
 
         if (!list_empty(&task->cg_list)) {
                 spin_lock_irq(&css_set_lock);
                 css_set_skip_task_iters(task_css_set(task), task);
                 list_del_init(&task->cg_list);
                 spin_unlock_irq(&css_set_lock);
         }
 }
 
 void cgroup_free(struct task_struct *task)
 {
         struct css_set *cset = task_css_set(task);
         put_css_set(cset);
 }
 
 static int __init cgroup_disable(char *str)
 {
         struct cgroup_subsys *ss;
         char *token;
         int i;
 
         while ((token = strsep(&str, ",")) != NULL) {
                 if (!*token)
                         continue;
 
                 for_each_subsys(ss, i) {
                         if (strcmp(token, ss->name) &&
                             strcmp(token, ss->legacy_name))
                                 continue;
 
                         static_branch_disable(cgroup_subsys_enabled_key[i]);
                         pr_info("Disabling %s control group subsystem\n",
                                 ss->name);
                 }
 
                 for (i = 0; i < OPT_FEATURE_COUNT; i++) {
                         if (strcmp(token, cgroup_opt_feature_names[i]))
                                 continue;
                         cgroup_feature_disable_mask |= 1 << i;
                         pr_info("Disabling %s control group feature\n",
                                 cgroup_opt_feature_names[i]);
                         break;
                 }
         }
         return 1;
 }
 __setup("cgroup_disable=", cgroup_disable);
 
 void __init __weak enable_debug_cgroup(void) { }
 
 static int __init enable_cgroup_debug(char *str)
 {
         cgroup_debug = true;
         enable_debug_cgroup();
         return 1;
 }
 __setup("cgroup_debug", enable_cgroup_debug);
 
 static int __init cgroup_favordynmods_setup(char *str)
 {
         return (kstrtobool(str, &have_favordynmods) == 0);
 }
 __setup("cgroup_favordynmods=", cgroup_favordynmods_setup);
 
 /**
  * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
  * @dentry: directory dentry of interest
  * @ss: subsystem of interest
  *
  * If @dentry is a directory for a cgroup which has @ss enabled on it, try
  * to get the corresponding css and return it.  If such css doesn't exist
  * or can't be pinned, an ERR_PTR value is returned.
  */
 struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
                                                        struct cgroup_subsys *ss)
 {
         struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
         struct file_system_type *s_type = dentry->d_sb->s_type;
         struct cgroup_subsys_state *css = NULL;
         struct cgroup *cgrp;
 
         /* is @dentry a cgroup dir? */
         if ((s_type != &cgroup_fs_type && s_type != &cgroup2_fs_type) ||
             !kn || kernfs_type(kn) != KERNFS_DIR)
                 return ERR_PTR(-EBADF);
 
         rcu_read_lock();
 
         /*
          * This path doesn't originate from kernfs and @kn could already
          * have been or be removed at any point.  @kn->priv is RCU
          * protected for this access.  See css_release_work_fn() for details.
          */
         cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
         if (cgrp)
                 css = cgroup_css(cgrp, ss);
 
         if (!css || !css_tryget_online(css))
                 css = ERR_PTR(-ENOENT);
 
         rcu_read_unlock();
         return css;
 }
 
 /**
  * css_from_id - lookup css by id
  * @id: the cgroup id
  * @ss: cgroup subsys to be looked into
  *
  * Returns the css if there's valid one with @id, otherwise returns NULL.
  * Should be called under rcu_read_lock().
  */
 struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
 {
         WARN_ON_ONCE(!rcu_read_lock_held());
         return idr_find(&ss->css_idr, id);
 }
 
 /**
  * cgroup_get_from_path - lookup and get a cgroup from its default hierarchy path
  * @path: path on the default hierarchy
  *
  * Find the cgroup at @path on the default hierarchy, increment its
  * reference count and return it.  Returns pointer to the found cgroup on
  * success, ERR_PTR(-ENOENT) if @path doesn't exist or if the cgroup has already
  * been released and ERR_PTR(-ENOTDIR) if @path points to a non-directory.
  */
 struct cgroup *cgroup_get_from_path(const char *path)
 {
         struct kernfs_node *kn;
         struct cgroup *cgrp = ERR_PTR(-ENOENT);
         struct cgroup *root_cgrp;
 
         root_cgrp = current_cgns_cgroup_dfl();
         kn = kernfs_walk_and_get(root_cgrp->kn, path);
         if (!kn)
                 goto out;
 
         if (kernfs_type(kn) != KERNFS_DIR) {
                 cgrp = ERR_PTR(-ENOTDIR);
                 goto out_kernfs;
         }
 
         rcu_read_lock();
 
         cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
         if (!cgrp || !cgroup_tryget(cgrp))
                 cgrp = ERR_PTR(-ENOENT);
 
         rcu_read_unlock();
 
 out_kernfs:
         kernfs_put(kn);
 out:
         return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_path);
 
 /**
  * cgroup_v1v2_get_from_fd - get a cgroup pointer from a fd
  * @fd: fd obtained by open(cgroup_dir)
  *
  * Find the cgroup from a fd which should be obtained
  * by opening a cgroup directory.  Returns a pointer to the
  * cgroup on success. ERR_PTR is returned if the cgroup
  * cannot be found.
  */
 struct cgroup *cgroup_v1v2_get_from_fd(int fd)
 {
         struct cgroup *cgrp;
         struct fd f = fdget_raw(fd);
         if (!f.file)
                 return ERR_PTR(-EBADF);
 
         cgrp = cgroup_v1v2_get_from_file(f.file);
         fdput(f);
         return cgrp;
 }
 
 /**
  * cgroup_get_from_fd - same as cgroup_v1v2_get_from_fd, but only supports
  * cgroup2.
  * @fd: fd obtained by open(cgroup2_dir)
  */
 struct cgroup *cgroup_get_from_fd(int fd)
 {
         struct cgroup *cgrp = cgroup_v1v2_get_from_fd(fd);
 
         if (IS_ERR(cgrp))
                 return ERR_CAST(cgrp);
 
         if (!cgroup_on_dfl(cgrp)) {
                 cgroup_put(cgrp);
                 return ERR_PTR(-EBADF);
         }
         return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
 
 static u64 power_of_ten(int power)
 {
         u64 v = 1;
         while (power--)
                 v *= 10;
         return v;
 }
 
 /**
  * cgroup_parse_float - parse a floating number
  * @input: input string
  * @dec_shift: number of decimal digits to shift
  * @v: output
  *
  * Parse a decimal floating point number in @input and store the result in
  * @v with decimal point right shifted @dec_shift times.  For example, if
  * @input is "12.3456" and @dec_shift is 3, *@v will be set to 12345.
  * Returns 0 on success, -errno otherwise.
  *
  * There's nothing cgroup specific about this function except that it's
  * currently the only user.
  */
 int cgroup_parse_float(const char *input, unsigned dec_shift, s64 *v)
 {
         s64 whole, frac = 0;
         int fstart = 0, fend = 0, flen;
 
         if (!sscanf(input, "%lld.%n%lld%n", &whole, &fstart, &frac, &fend))
                 return -EINVAL;
         if (frac < 0)
                 return -EINVAL;
 
         flen = fend > fstart ? fend - fstart : 0;
         if (flen < dec_shift)
                 frac *= power_of_ten(dec_shift - flen);
         else
                 frac = DIV_ROUND_CLOSEST_ULL(frac, power_of_ten(flen - dec_shift));
 
         *v = whole * power_of_ten(dec_shift) + frac;
         return 0;
 }
 
 /*
  * sock->sk_cgrp_data handling.  For more info, see sock_cgroup_data
  * definition in cgroup-defs.h.
  */
 #ifdef CONFIG_SOCK_CGROUP_DATA
 
 void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
 {
         struct cgroup *cgroup;
 
         rcu_read_lock();
         /* Don't associate the sock with unrelated interrupted task's cgroup. */
         if (in_interrupt()) {
                 cgroup = &cgrp_dfl_root.cgrp;
                 cgroup_get(cgroup);
                 goto out;
         }
 
         while (true) {
                 struct css_set *cset;
 
                 cset = task_css_set(current);
                 if (likely(cgroup_tryget(cset->dfl_cgrp))) {
                         cgroup = cset->dfl_cgrp;
                         break;
                 }
                 cpu_relax();
         }
 out:
         skcd->cgroup = cgroup;
         cgroup_bpf_get(cgroup);
         rcu_read_unlock();
 }
 
 void cgroup_sk_clone(struct sock_cgroup_data *skcd)
 {
         struct cgroup *cgrp = sock_cgroup_ptr(skcd);
 
         /*
          * We might be cloning a socket which is left in an empty
          * cgroup and the cgroup might have already been rmdir'd.
          * Don't use cgroup_get_live().
          */
         cgroup_get(cgrp);
         cgroup_bpf_get(cgrp);
 }
 
 void cgroup_sk_free(struct sock_cgroup_data *skcd)
 {
         struct cgroup *cgrp = sock_cgroup_ptr(skcd);
 
         cgroup_bpf_put(cgrp);
         cgroup_put(cgrp);
 }
 
 #endif  /* CONFIG_SOCK_CGROUP_DATA */
 
 #ifdef CONFIG_SYSFS
 static ssize_t show_delegatable_files(struct cftype *files, char *buf,
                                       ssize_t size, const char *prefix)
 {
         struct cftype *cft;
         ssize_t ret = 0;
 
         for (cft = files; cft && cft->name[0] != '\0'; cft++) {
                 if (!(cft->flags & CFTYPE_NS_DELEGATABLE))
                         continue;
 
                 if (prefix)
                         ret += snprintf(buf + ret, size - ret, "%s.", prefix);
 
                 ret += snprintf(buf + ret, size - ret, "%s\n", cft->name);
 
                 if (WARN_ON(ret >= size))
                         break;
         }
 
         return ret;
 }
 
 static ssize_t delegate_show(struct kobject *kobj, struct kobj_attribute *attr,
                               char *buf)
 {
         struct cgroup_subsys *ss;
         int ssid;
         ssize_t ret = 0;
 
         ret = show_delegatable_files(cgroup_base_files, buf + ret,
                                      PAGE_SIZE - ret, NULL);
         if (cgroup_psi_enabled())
                 ret += show_delegatable_files(cgroup_psi_files, buf + ret,
                                               PAGE_SIZE - ret, NULL);
 
         for_each_subsys(ss, ssid)
                 ret += show_delegatable_files(ss->dfl_cftypes, buf + ret,
                                               PAGE_SIZE - ret,
                                               cgroup_subsys_name[ssid]);
 
         return ret;
 }
 static struct kobj_attribute cgroup_delegate_attr = __ATTR_RO(delegate);
 
 static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr,
                              char *buf)
 {
         return snprintf(buf, PAGE_SIZE,
                         "nsdelegate\n"
                         "favordynmods\n"
                         "memory_localevents\n"
                         "memory_recursiveprot\n"
                         "memory_hugetlb_accounting\n"
                         "pids_localevents\n");
 }
 static struct kobj_attribute cgroup_features_attr = __ATTR_RO(features);
 
 static struct attribute *cgroup_sysfs_attrs[] = {
         &cgroup_delegate_attr.attr,
         &cgroup_features_attr.attr,
         NULL,
 };
 
 static const struct attribute_group cgroup_sysfs_attr_group = {
         .attrs = cgroup_sysfs_attrs,
         .name = "cgroup",
 };
 
 static int __init cgroup_sysfs_init(void)
 {
         return sysfs_create_group(kernel_kobj, &cgroup_sysfs_attr_group);
 }
 subsys_initcall(cgroup_sysfs_init);
 
 #endif /* CONFIG_SYSFS */