TOMOYO Linux Cross Reference
Linux/kernel/rcu/tree.c

   // SPDX-License-Identifier: GPL-2.0+
   /*
    * Read-Copy Update mechanism for mutual exclusion (tree-based version)
    *
    * Copyright IBM Corporation, 2008
    *
    * Authors: Dipankar Sarma <dipankar@in.ibm.com>
    *          Manfred Spraul <manfred@colorfullife.com>
    *          Paul E. McKenney <paulmck@linux.ibm.com>
   *
   * Based on the original work by Paul McKenney <paulmck@linux.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   *
   * For detailed explanation of Read-Copy Update mechanism see -
   *      Documentation/RCU
   */
  
  #define pr_fmt(fmt) "rcu: " fmt
  
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/spinlock.h>
  #include <linux/smp.h>
  #include <linux/rcupdate_wait.h>
  #include <linux/interrupt.h>
  #include <linux/sched.h>
  #include <linux/sched/debug.h>
  #include <linux/nmi.h>
  #include <linux/atomic.h>
  #include <linux/bitops.h>
  #include <linux/export.h>
  #include <linux/completion.h>
  #include <linux/kmemleak.h>
  #include <linux/moduleparam.h>
  #include <linux/panic.h>
  #include <linux/panic_notifier.h>
  #include <linux/percpu.h>
  #include <linux/notifier.h>
  #include <linux/cpu.h>
  #include <linux/mutex.h>
  #include <linux/time.h>
  #include <linux/kernel_stat.h>
  #include <linux/wait.h>
  #include <linux/kthread.h>
  #include <uapi/linux/sched/types.h>
  #include <linux/prefetch.h>
  #include <linux/delay.h>
  #include <linux/random.h>
  #include <linux/trace_events.h>
  #include <linux/suspend.h>
  #include <linux/ftrace.h>
  #include <linux/tick.h>
  #include <linux/sysrq.h>
  #include <linux/kprobes.h>
  #include <linux/gfp.h>
  #include <linux/oom.h>
  #include <linux/smpboot.h>
  #include <linux/jiffies.h>
  #include <linux/slab.h>
  #include <linux/sched/isolation.h>
  #include <linux/sched/clock.h>
  #include <linux/vmalloc.h>
  #include <linux/mm.h>
  #include <linux/kasan.h>
  #include <linux/context_tracking.h>
  #include "../time/tick-internal.h"
  
  #include "tree.h"
  #include "rcu.h"
  
  #ifdef MODULE_PARAM_PREFIX
  #undef MODULE_PARAM_PREFIX
  #endif
  #define MODULE_PARAM_PREFIX "rcutree."
  
  /* Data structures. */
  static void rcu_sr_normal_gp_cleanup_work(struct work_struct *);
  
  static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
          .gpwrap = true,
  #ifdef CONFIG_RCU_NOCB_CPU
          .cblist.flags = SEGCBLIST_RCU_CORE,
  #endif
  };
  static struct rcu_state rcu_state = {
          .level = { &rcu_state.node[0] },
          .gp_state = RCU_GP_IDLE,
          .gp_seq = (0UL - 300UL) << RCU_SEQ_CTR_SHIFT,
          .barrier_mutex = __MUTEX_INITIALIZER(rcu_state.barrier_mutex),
          .barrier_lock = __RAW_SPIN_LOCK_UNLOCKED(rcu_state.barrier_lock),
          .name = RCU_NAME,
          .abbr = RCU_ABBR,
          .exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex),
          .exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex),
          .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
          .srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
                  rcu_sr_normal_gp_cleanup_work),
          .srs_cleanups_pending = ATOMIC_INIT(0),
 };
 
 /* Dump rcu_node combining tree at boot to verify correct setup. */
 static bool dump_tree;
 module_param(dump_tree, bool, 0444);
 /* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
 static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
 #ifndef CONFIG_PREEMPT_RT
 module_param(use_softirq, bool, 0444);
 #endif
 /* Control rcu_node-tree auto-balancing at boot time. */
 static bool rcu_fanout_exact;
 module_param(rcu_fanout_exact, bool, 0444);
 /* Increase (but not decrease) the RCU_FANOUT_LEAF at boot time. */
 static int rcu_fanout_leaf = RCU_FANOUT_LEAF;
 module_param(rcu_fanout_leaf, int, 0444);
 int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
 /* Number of rcu_nodes at specified level. */
 int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
 int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
 
 /*
  * The rcu_scheduler_active variable is initialized to the value
  * RCU_SCHEDULER_INACTIVE and transitions RCU_SCHEDULER_INIT just before the
  * first task is spawned.  So when this variable is RCU_SCHEDULER_INACTIVE,
  * RCU can assume that there is but one task, allowing RCU to (for example)
  * optimize synchronize_rcu() to a simple barrier().  When this variable
  * is RCU_SCHEDULER_INIT, RCU must actually do all the hard work required
  * to detect real grace periods.  This variable is also used to suppress
  * boot-time false positives from lockdep-RCU error checking.  Finally, it
  * transitions from RCU_SCHEDULER_INIT to RCU_SCHEDULER_RUNNING after RCU
  * is fully initialized, including all of its kthreads having been spawned.
  */
 int rcu_scheduler_active __read_mostly;
 EXPORT_SYMBOL_GPL(rcu_scheduler_active);
 
 /*
  * The rcu_scheduler_fully_active variable transitions from zero to one
  * during the early_initcall() processing, which is after the scheduler
  * is capable of creating new tasks.  So RCU processing (for example,
  * creating tasks for RCU priority boosting) must be delayed until after
  * rcu_scheduler_fully_active transitions from zero to one.  We also
  * currently delay invocation of any RCU callbacks until after this point.
  *
  * It might later prove better for people registering RCU callbacks during
  * early boot to take responsibility for these callbacks, but one step at
  * a time.
  */
 static int rcu_scheduler_fully_active __read_mostly;
 
 static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
                               unsigned long gps, unsigned long flags);
 static struct task_struct *rcu_boost_task(struct rcu_node *rnp);
 static void invoke_rcu_core(void);
 static void rcu_report_exp_rdp(struct rcu_data *rdp);
 static void sync_sched_exp_online_cleanup(int cpu);
 static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
 static bool rcu_rdp_cpu_online(struct rcu_data *rdp);
 static bool rcu_init_invoked(void);
 static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf);
 static void rcu_init_new_rnp(struct rcu_node *rnp_leaf);
 
 /*
  * rcuc/rcub/rcuop kthread realtime priority. The "rcuop"
  * real-time priority(enabling/disabling) is controlled by
  * the extra CONFIG_RCU_NOCB_CPU_CB_BOOST configuration.
  */
 static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
 module_param(kthread_prio, int, 0444);
 
 /* Delay in jiffies for grace-period initialization delays, debug only. */
 
 static int gp_preinit_delay;
 module_param(gp_preinit_delay, int, 0444);
 static int gp_init_delay;
 module_param(gp_init_delay, int, 0444);
 static int gp_cleanup_delay;
 module_param(gp_cleanup_delay, int, 0444);
 static int nohz_full_patience_delay;
 module_param(nohz_full_patience_delay, int, 0444);
 static int nohz_full_patience_delay_jiffies;
 
 // Add delay to rcu_read_unlock() for strict grace periods.
 static int rcu_unlock_delay;
 #ifdef CONFIG_RCU_STRICT_GRACE_PERIOD
 module_param(rcu_unlock_delay, int, 0444);
 #endif
 
 /*
  * This rcu parameter is runtime-read-only. It reflects
  * a minimum allowed number of objects which can be cached
  * per-CPU. Object size is equal to one page. This value
  * can be changed at boot time.
  */
 static int rcu_min_cached_objs = 5;
 module_param(rcu_min_cached_objs, int, 0444);
 
 // A page shrinker can ask for pages to be freed to make them
 // available for other parts of the system. This usually happens
 // under low memory conditions, and in that case we should also
 // defer page-cache filling for a short time period.
 //
 // The default value is 5 seconds, which is long enough to reduce
 // interference with the shrinker while it asks other systems to
 // drain their caches.
 static int rcu_delay_page_cache_fill_msec = 5000;
 module_param(rcu_delay_page_cache_fill_msec, int, 0444);
 
 /* Retrieve RCU kthreads priority for rcutorture */
 int rcu_get_gp_kthreads_prio(void)
 {
         return kthread_prio;
 }
 EXPORT_SYMBOL_GPL(rcu_get_gp_kthreads_prio);
 
 /*
  * Number of grace periods between delays, normalized by the duration of
  * the delay.  The longer the delay, the more the grace periods between
  * each delay.  The reason for this normalization is that it means that,
  * for non-zero delays, the overall slowdown of grace periods is constant
  * regardless of the duration of the delay.  This arrangement balances
  * the need for long delays to increase some race probabilities with the
  * need for fast grace periods to increase other race probabilities.
  */
 #define PER_RCU_NODE_PERIOD 3   /* Number of grace periods between delays for debugging. */
 
 /*
  * Return true if an RCU grace period is in progress.  The READ_ONCE()s
  * permit this function to be invoked without holding the root rcu_node
  * structure's ->lock, but of course results can be subject to change.
  */
 static int rcu_gp_in_progress(void)
 {
         return rcu_seq_state(rcu_seq_current(&rcu_state.gp_seq));
 }
 
 /*
  * Return the number of callbacks queued on the specified CPU.
  * Handles both the nocbs and normal cases.
  */
 static long rcu_get_n_cbs_cpu(int cpu)
 {
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
         if (rcu_segcblist_is_enabled(&rdp->cblist))
                 return rcu_segcblist_n_cbs(&rdp->cblist);
         return 0;
 }
 
 /**
  * rcu_softirq_qs - Provide a set of RCU quiescent states in softirq processing
  *
  * Mark a quiescent state for RCU, Tasks RCU, and Tasks Trace RCU.
  * This is a special-purpose function to be used in the softirq
  * infrastructure and perhaps the occasional long-running softirq
  * handler.
  *
  * Note that from RCU's viewpoint, a call to rcu_softirq_qs() is
  * equivalent to momentarily completely enabling preemption.  For
  * example, given this code::
  *
  *      local_bh_disable();
  *      do_something();
  *      rcu_softirq_qs();  // A
  *      do_something_else();
  *      local_bh_enable();  // B
  *
  * A call to synchronize_rcu() that began concurrently with the
  * call to do_something() would be guaranteed to wait only until
  * execution reached statement A.  Without that rcu_softirq_qs(),
  * that same synchronize_rcu() would instead be guaranteed to wait
  * until execution reached statement B.
  */
 void rcu_softirq_qs(void)
 {
         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
                          lock_is_held(&rcu_lock_map) ||
                          lock_is_held(&rcu_sched_lock_map),
                          "Illegal rcu_softirq_qs() in RCU read-side critical section");
         rcu_qs();
         rcu_preempt_deferred_qs(current);
         rcu_tasks_qs(current, false);
 }
 
 /*
  * Reset the current CPU's ->dynticks counter to indicate that the
  * newly onlined CPU is no longer in an extended quiescent state.
  * This will either leave the counter unchanged, or increment it
  * to the next non-quiescent value.
  *
  * The non-atomic test/increment sequence works because the upper bits
  * of the ->dynticks counter are manipulated only by the corresponding CPU,
  * or when the corresponding CPU is offline.
  */
 static void rcu_dynticks_eqs_online(void)
 {
         if (ct_dynticks() & RCU_DYNTICKS_IDX)
                 return;
         ct_state_inc(RCU_DYNTICKS_IDX);
 }
 
 /*
  * Return true if the snapshot returned from rcu_dynticks_snap()
  * indicates that RCU is in an extended quiescent state.
  */
 static bool rcu_dynticks_in_eqs(int snap)
 {
         return !(snap & RCU_DYNTICKS_IDX);
 }
 
 /*
  * Return true if the CPU corresponding to the specified rcu_data
  * structure has spent some time in an extended quiescent state since
  * rcu_dynticks_snap() returned the specified snapshot.
  */
 static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
 {
         /*
          * The first failing snapshot is already ordered against the accesses
          * performed by the remote CPU after it exits idle.
          *
          * The second snapshot therefore only needs to order against accesses
          * performed by the remote CPU prior to entering idle and therefore can
          * rely solely on acquire semantics.
          */
         return snap != ct_dynticks_cpu_acquire(rdp->cpu);
 }
 
 /*
  * Return true if the referenced integer is zero while the specified
  * CPU remains within a single extended quiescent state.
  */
 bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
 {
         int snap;
 
         // If not quiescent, force back to earlier extended quiescent state.
         snap = ct_dynticks_cpu(cpu) & ~RCU_DYNTICKS_IDX;
         smp_rmb(); // Order ->dynticks and *vp reads.
         if (READ_ONCE(*vp))
                 return false;  // Non-zero, so report failure;
         smp_rmb(); // Order *vp read and ->dynticks re-read.
 
         // If still in the same extended quiescent state, we are good!
         return snap == ct_dynticks_cpu(cpu);
 }
 
 /*
  * Let the RCU core know that this CPU has gone through the scheduler,
  * which is a quiescent state.  This is called when the need for a
  * quiescent state is urgent, so we burn an atomic operation and full
  * memory barriers to let the RCU core know about it, regardless of what
  * this CPU might (or might not) do in the near future.
  *
  * We inform the RCU core by emulating a zero-duration dyntick-idle period.
  *
  * The caller must have disabled interrupts and must not be idle.
  */
 notrace void rcu_momentary_dyntick_idle(void)
 {
         int seq;
 
         raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
         seq = ct_state_inc(2 * RCU_DYNTICKS_IDX);
         /* It is illegal to call this from idle state. */
         WARN_ON_ONCE(!(seq & RCU_DYNTICKS_IDX));
         rcu_preempt_deferred_qs(current);
 }
 EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
 
 /**
  * rcu_is_cpu_rrupt_from_idle - see if 'interrupted' from idle
  *
  * If the current CPU is idle and running at a first-level (not nested)
  * interrupt, or directly, from idle, return true.
  *
  * The caller must have at least disabled IRQs.
  */
 static int rcu_is_cpu_rrupt_from_idle(void)
 {
         long nesting;
 
         /*
          * Usually called from the tick; but also used from smp_function_call()
          * for expedited grace periods. This latter can result in running from
          * the idle task, instead of an actual IPI.
          */
         lockdep_assert_irqs_disabled();
 
         /* Check for counter underflows */
         RCU_LOCKDEP_WARN(ct_dynticks_nesting() < 0,
                          "RCU dynticks_nesting counter underflow!");
         RCU_LOCKDEP_WARN(ct_dynticks_nmi_nesting() <= 0,
                          "RCU dynticks_nmi_nesting counter underflow/zero!");
 
         /* Are we at first interrupt nesting level? */
         nesting = ct_dynticks_nmi_nesting();
         if (nesting > 1)
                 return false;
 
         /*
          * If we're not in an interrupt, we must be in the idle task!
          */
         WARN_ON_ONCE(!nesting && !is_idle_task(current));
 
         /* Does CPU appear to be idle from an RCU standpoint? */
         return ct_dynticks_nesting() == 0;
 }
 
 #define DEFAULT_RCU_BLIMIT (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ? 1000 : 10)
                                 // Maximum callbacks per rcu_do_batch ...
 #define DEFAULT_MAX_RCU_BLIMIT 10000 // ... even during callback flood.
 static long blimit = DEFAULT_RCU_BLIMIT;
 #define DEFAULT_RCU_QHIMARK 10000 // If this many pending, ignore blimit.
 static long qhimark = DEFAULT_RCU_QHIMARK;
 #define DEFAULT_RCU_QLOMARK 100   // Once only this many pending, use blimit.
 static long qlowmark = DEFAULT_RCU_QLOMARK;
 #define DEFAULT_RCU_QOVLD_MULT 2
 #define DEFAULT_RCU_QOVLD (DEFAULT_RCU_QOVLD_MULT * DEFAULT_RCU_QHIMARK)
 static long qovld = DEFAULT_RCU_QOVLD; // If this many pending, hammer QS.
 static long qovld_calc = -1;      // No pre-initialization lock acquisitions!
 
 module_param(blimit, long, 0444);
 module_param(qhimark, long, 0444);
 module_param(qlowmark, long, 0444);
 module_param(qovld, long, 0444);
 
 static ulong jiffies_till_first_fqs = IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ? 0 : ULONG_MAX;
 static ulong jiffies_till_next_fqs = ULONG_MAX;
 static bool rcu_kick_kthreads;
 static int rcu_divisor = 7;
 module_param(rcu_divisor, int, 0644);
 
 /* Force an exit from rcu_do_batch() after 3 milliseconds. */
 static long rcu_resched_ns = 3 * NSEC_PER_MSEC;
 module_param(rcu_resched_ns, long, 0644);
 
 /*
  * How long the grace period must be before we start recruiting
  * quiescent-state help from rcu_note_context_switch().
  */
 static ulong jiffies_till_sched_qs = ULONG_MAX;
 module_param(jiffies_till_sched_qs, ulong, 0444);
 static ulong jiffies_to_sched_qs; /* See adjust_jiffies_till_sched_qs(). */
 module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */
 
 /*
  * Make sure that we give the grace-period kthread time to detect any
  * idle CPUs before taking active measures to force quiescent states.
  * However, don't go below 100 milliseconds, adjusted upwards for really
  * large systems.
  */
 static void adjust_jiffies_till_sched_qs(void)
 {
         unsigned long j;
 
         /* If jiffies_till_sched_qs was specified, respect the request. */
         if (jiffies_till_sched_qs != ULONG_MAX) {
                 WRITE_ONCE(jiffies_to_sched_qs, jiffies_till_sched_qs);
                 return;
         }
         /* Otherwise, set to third fqs scan, but bound below on large system. */
         j = READ_ONCE(jiffies_till_first_fqs) +
                       2 * READ_ONCE(jiffies_till_next_fqs);
         if (j < HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV)
                 j = HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
         pr_info("RCU calculated value of scheduler-enlistment delay is %ld jiffies.\n", j);
         WRITE_ONCE(jiffies_to_sched_qs, j);
 }
 
 static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
 {
         ulong j;
         int ret = kstrtoul(val, 0, &j);
 
         if (!ret) {
                 WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : j);
                 adjust_jiffies_till_sched_qs();
         }
         return ret;
 }
 
 static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param *kp)
 {
         ulong j;
         int ret = kstrtoul(val, 0, &j);
 
         if (!ret) {
                 WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : (j ?: 1));
                 adjust_jiffies_till_sched_qs();
         }
         return ret;
 }
 
 static const struct kernel_param_ops first_fqs_jiffies_ops = {
         .set = param_set_first_fqs_jiffies,
         .get = param_get_ulong,
 };
 
 static const struct kernel_param_ops next_fqs_jiffies_ops = {
         .set = param_set_next_fqs_jiffies,
         .get = param_get_ulong,
 };
 
 module_param_cb(jiffies_till_first_fqs, &first_fqs_jiffies_ops, &jiffies_till_first_fqs, 0644);
 module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next_fqs, 0644);
 module_param(rcu_kick_kthreads, bool, 0644);
 
 static void force_qs_rnp(int (*f)(struct rcu_data *rdp));
 static int rcu_pending(int user);
 
 /*
  * Return the number of RCU GPs completed thus far for debug & stats.
  */
 unsigned long rcu_get_gp_seq(void)
 {
         return READ_ONCE(rcu_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcu_get_gp_seq);
 
 /*
  * Return the number of RCU expedited batches completed thus far for
  * debug & stats.  Odd numbers mean that a batch is in progress, even
  * numbers mean idle.  The value returned will thus be roughly double
  * the cumulative batches since boot.
  */
 unsigned long rcu_exp_batches_completed(void)
 {
         return rcu_state.expedited_sequence;
 }
 EXPORT_SYMBOL_GPL(rcu_exp_batches_completed);
 
 /*
  * Return the root node of the rcu_state structure.
  */
 static struct rcu_node *rcu_get_root(void)
 {
         return &rcu_state.node[0];
 }
 
 /*
  * Send along grace-period-related data for rcutorture diagnostics.
  */
 void rcutorture_get_gp_data(int *flags, unsigned long *gp_seq)
 {
         *flags = READ_ONCE(rcu_state.gp_flags);
         *gp_seq = rcu_seq_current(&rcu_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
 
 #if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK))
 /*
  * An empty function that will trigger a reschedule on
  * IRQ tail once IRQs get re-enabled on userspace/guest resume.
  */
 static void late_wakeup_func(struct irq_work *work)
 {
 }
 
 static DEFINE_PER_CPU(struct irq_work, late_wakeup_work) =
         IRQ_WORK_INIT(late_wakeup_func);
 
 /*
  * If either:
  *
  * 1) the task is about to enter in guest mode and $ARCH doesn't support KVM generic work
  * 2) the task is about to enter in user mode and $ARCH doesn't support generic entry.
  *
  * In these cases the late RCU wake ups aren't supported in the resched loops and our
  * last resort is to fire a local irq_work that will trigger a reschedule once IRQs
  * get re-enabled again.
  */
 noinstr void rcu_irq_work_resched(void)
 {
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
         if (IS_ENABLED(CONFIG_GENERIC_ENTRY) && !(current->flags & PF_VCPU))
                 return;
 
         if (IS_ENABLED(CONFIG_KVM_XFER_TO_GUEST_WORK) && (current->flags & PF_VCPU))
                 return;
 
         instrumentation_begin();
         if (do_nocb_deferred_wakeup(rdp) && need_resched()) {
                 irq_work_queue(this_cpu_ptr(&late_wakeup_work));
         }
         instrumentation_end();
 }
 #endif /* #if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK)) */
 
 #ifdef CONFIG_PROVE_RCU
 /**
  * rcu_irq_exit_check_preempt - Validate that scheduling is possible
  */
 void rcu_irq_exit_check_preempt(void)
 {
         lockdep_assert_irqs_disabled();
 
         RCU_LOCKDEP_WARN(ct_dynticks_nesting() <= 0,
                          "RCU dynticks_nesting counter underflow/zero!");
         RCU_LOCKDEP_WARN(ct_dynticks_nmi_nesting() !=
                          DYNTICK_IRQ_NONIDLE,
                          "Bad RCU  dynticks_nmi_nesting counter\n");
         RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),
                          "RCU in extended quiescent state!");
 }
 #endif /* #ifdef CONFIG_PROVE_RCU */
 
 #ifdef CONFIG_NO_HZ_FULL
 /**
  * __rcu_irq_enter_check_tick - Enable scheduler tick on CPU if RCU needs it.
  *
  * The scheduler tick is not normally enabled when CPUs enter the kernel
  * from nohz_full userspace execution.  After all, nohz_full userspace
  * execution is an RCU quiescent state and the time executing in the kernel
  * is quite short.  Except of course when it isn't.  And it is not hard to
  * cause a large system to spend tens of seconds or even minutes looping
  * in the kernel, which can cause a number of problems, include RCU CPU
  * stall warnings.
  *
  * Therefore, if a nohz_full CPU fails to report a quiescent state
  * in a timely manner, the RCU grace-period kthread sets that CPU's
  * ->rcu_urgent_qs flag with the expectation that the next interrupt or
  * exception will invoke this function, which will turn on the scheduler
  * tick, which will enable RCU to detect that CPU's quiescent states,
  * for example, due to cond_resched() calls in CONFIG_PREEMPT=n kernels.
  * The tick will be disabled once a quiescent state is reported for
  * this CPU.
  *
  * Of course, in carefully tuned systems, there might never be an
  * interrupt or exception.  In that case, the RCU grace-period kthread
  * will eventually cause one to happen.  However, in less carefully
  * controlled environments, this function allows RCU to get what it
  * needs without creating otherwise useless interruptions.
  */
 void __rcu_irq_enter_check_tick(void)
 {
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
         // If we're here from NMI there's nothing to do.
         if (in_nmi())
                 return;
 
         RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),
                          "Illegal rcu_irq_enter_check_tick() from extended quiescent state");
 
         if (!tick_nohz_full_cpu(rdp->cpu) ||
             !READ_ONCE(rdp->rcu_urgent_qs) ||
             READ_ONCE(rdp->rcu_forced_tick)) {
                 // RCU doesn't need nohz_full help from this CPU, or it is
                 // already getting that help.
                 return;
         }
 
         // We get here only when not in an extended quiescent state and
         // from interrupts (as opposed to NMIs).  Therefore, (1) RCU is
         // already watching and (2) The fact that we are in an interrupt
         // handler and that the rcu_node lock is an irq-disabled lock
         // prevents self-deadlock.  So we can safely recheck under the lock.
         // Note that the nohz_full state currently cannot change.
         raw_spin_lock_rcu_node(rdp->mynode);
         if (READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
                 // A nohz_full CPU is in the kernel and RCU needs a
                 // quiescent state.  Turn on the tick!
                 WRITE_ONCE(rdp->rcu_forced_tick, true);
                 tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
         }
         raw_spin_unlock_rcu_node(rdp->mynode);
 }
 NOKPROBE_SYMBOL(__rcu_irq_enter_check_tick);
 #endif /* CONFIG_NO_HZ_FULL */
 
 /*
  * Check to see if any future non-offloaded RCU-related work will need
  * to be done by the current CPU, even if none need be done immediately,
  * returning 1 if so.  This function is part of the RCU implementation;
  * it is -not- an exported member of the RCU API.  This is used by
  * the idle-entry code to figure out whether it is safe to disable the
  * scheduler-clock interrupt.
  *
  * Just check whether or not this CPU has non-offloaded RCU callbacks
  * queued.
  */
 int rcu_needs_cpu(void)
 {
         return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
                 !rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
 }
 
 /*
  * If any sort of urgency was applied to the current CPU (for example,
  * the scheduler-clock interrupt was enabled on a nohz_full CPU) in order
  * to get to a quiescent state, disable it.
  */
 static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
 {
         raw_lockdep_assert_held_rcu_node(rdp->mynode);
         WRITE_ONCE(rdp->rcu_urgent_qs, false);
         WRITE_ONCE(rdp->rcu_need_heavy_qs, false);
         if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) {
                 tick_dep_clear_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
                 WRITE_ONCE(rdp->rcu_forced_tick, false);
         }
 }
 
 /**
  * rcu_is_watching - RCU read-side critical sections permitted on current CPU?
  *
  * Return @true if RCU is watching the running CPU and @false otherwise.
  * An @true return means that this CPU can safely enter RCU read-side
  * critical sections.
  *
  * Although calls to rcu_is_watching() from most parts of the kernel
  * will return @true, there are important exceptions.  For example, if the
  * current CPU is deep within its idle loop, in kernel entry/exit code,
  * or offline, rcu_is_watching() will return @false.
  *
  * Make notrace because it can be called by the internal functions of
  * ftrace, and making this notrace removes unnecessary recursion calls.
  */
 notrace bool rcu_is_watching(void)
 {
         bool ret;
 
         preempt_disable_notrace();
         ret = !rcu_dynticks_curr_cpu_in_eqs();
         preempt_enable_notrace();
         return ret;
 }
 EXPORT_SYMBOL_GPL(rcu_is_watching);
 
 /*
  * If a holdout task is actually running, request an urgent quiescent
  * state from its CPU.  This is unsynchronized, so migrations can cause
  * the request to go to the wrong CPU.  Which is OK, all that will happen
  * is that the CPU's next context switch will be a bit slower and next
  * time around this task will generate another request.
  */
 void rcu_request_urgent_qs_task(struct task_struct *t)
 {
         int cpu;
 
         barrier();
         cpu = task_cpu(t);
         if (!task_curr(t))
                 return; /* This task is not running on that CPU. */
         smp_store_release(per_cpu_ptr(&rcu_data.rcu_urgent_qs, cpu), true);
 }
 
 /*
  * When trying to report a quiescent state on behalf of some other CPU,
  * it is our responsibility to check for and handle potential overflow
  * of the rcu_node ->gp_seq counter with respect to the rcu_data counters.
  * After all, the CPU might be in deep idle state, and thus executing no
  * code whatsoever.
  */
 static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
 {
         raw_lockdep_assert_held_rcu_node(rnp);
         if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + ULONG_MAX / 4,
                          rnp->gp_seq))
                 WRITE_ONCE(rdp->gpwrap, true);
         if (ULONG_CMP_LT(rdp->rcu_iw_gp_seq + ULONG_MAX / 4, rnp->gp_seq))
                 rdp->rcu_iw_gp_seq = rnp->gp_seq + ULONG_MAX / 4;
 }
 
 /*
  * Snapshot the specified CPU's dynticks counter so that we can later
  * credit them with an implicit quiescent state.  Return 1 if this CPU
  * is in dynticks idle mode, which is an extended quiescent state.
  */
 static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
         /*
          * Full ordering between remote CPU's post idle accesses and updater's
          * accesses prior to current GP (and also the started GP sequence number)
          * is enforced by rcu_seq_start() implicit barrier and even further by
          * smp_mb__after_unlock_lock() barriers chained all the way throughout the
          * rnp locking tree since rcu_gp_init() and up to the current leaf rnp
          * locking.
          *
          * Ordering between remote CPU's pre idle accesses and post grace period
          * updater's accesses is enforced by the below acquire semantic.
          */
         rdp->dynticks_snap = ct_dynticks_cpu_acquire(rdp->cpu);
         if (rcu_dynticks_in_eqs(rdp->dynticks_snap)) {
                 trace_rcu_fqs(rcu_state.name, rdp->gp_seq, rdp->cpu, TPS("dti"));
                 rcu_gpnum_ovf(rdp->mynode, rdp);
                 return 1;
         }
         return 0;
 }
 
 /*
  * Returns positive if the specified CPU has passed through a quiescent state
  * by virtue of being in or having passed through an dynticks idle state since
  * the last call to dyntick_save_progress_counter() for this same CPU, or by
  * virtue of having been offline.
  *
  * Returns negative if the specified CPU needs a force resched.
  *
  * Returns zero otherwise.
  */
 static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 {
         unsigned long jtsq;
         int ret = 0;
         struct rcu_node *rnp = rdp->mynode;
 
         /*
          * If the CPU passed through or entered a dynticks idle phase with
          * no active irq/NMI handlers, then we can safely pretend that the CPU
          * already acknowledged the request to pass through a quiescent
          * state.  Either way, that CPU cannot possibly be in an RCU
          * read-side critical section that started before the beginning
          * of the current RCU grace period.
          */
         if (rcu_dynticks_in_eqs_since(rdp, rdp->dynticks_snap)) {
                 trace_rcu_fqs(rcu_state.name, rdp->gp_seq, rdp->cpu, TPS("dti"));
                 rcu_gpnum_ovf(rnp, rdp);
                 return 1;
         }
 
         /*
          * Complain if a CPU that is considered to be offline from RCU's
          * perspective has not yet reported a quiescent state.  After all,
          * the offline CPU should have reported a quiescent state during
          * the CPU-offline process, or, failing that, by rcu_gp_init()
          * if it ran concurrently with either the CPU going offline or the
          * last task on a leaf rcu_node structure exiting its RCU read-side
          * critical section while all CPUs corresponding to that structure
          * are offline.  This added warning detects bugs in any of these
          * code paths.
          *
          * The rcu_node structure's ->lock is held here, which excludes
          * the relevant portions the CPU-hotplug code, the grace-period
          * initialization code, and the rcu_read_unlock() code paths.
          *
          * For more detail, please refer to the "Hotplug CPU" section
          * of RCU's Requirements documentation.
          */
         if (WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp))) {
                 struct rcu_node *rnp1;
 
                 pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
                         __func__, rnp->grplo, rnp->grphi, rnp->level,
                         (long)rnp->gp_seq, (long)rnp->completedqs);
                 for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
                         pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx ->rcu_gp_init_mask %#lx\n",
                                 __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext, rnp1->rcu_gp_init_mask);
                 pr_info("%s %d: %c online: %ld(%d) offline: %ld(%d)\n",
                         __func__, rdp->cpu, ".o"[rcu_rdp_cpu_online(rdp)],
                         (long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_state,
                         (long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_state);
                 return 1; /* Break things loose after complaining. */
         }
 
         /*
          * A CPU running for an extended time within the kernel can
          * delay RCU grace periods: (1) At age jiffies_to_sched_qs,
          * set .rcu_urgent_qs, (2) At age 2*jiffies_to_sched_qs, set
          * both .rcu_need_heavy_qs and .rcu_urgent_qs.  Note that the
          * unsynchronized assignments to the per-CPU rcu_need_heavy_qs
          * variable are safe because the assignments are repeated if this
          * CPU failed to pass through a quiescent state.  This code
          * also checks .jiffies_resched in case jiffies_to_sched_qs
          * is set way high.
          */
         jtsq = READ_ONCE(jiffies_to_sched_qs);
         if (!READ_ONCE(rdp->rcu_need_heavy_qs) &&
             (time_after(jiffies, rcu_state.gp_start + jtsq * 2) ||
              time_after(jiffies, rcu_state.jiffies_resched) ||
              rcu_state.cbovld)) {
                 WRITE_ONCE(rdp->rcu_need_heavy_qs, true);
                 /* Store rcu_need_heavy_qs before rcu_urgent_qs. */
                 smp_store_release(&rdp->rcu_urgent_qs, true);
         } else if (time_after(jiffies, rcu_state.gp_start + jtsq)) {
                 WRITE_ONCE(rdp->rcu_urgent_qs, true);
         }
 
         /*
          * NO_HZ_FULL CPUs can run in-kernel without rcu_sched_clock_irq!
          * The above code handles this, but only for straight cond_resched().
          * And some in-kernel loops check need_resched() before calling
          * cond_resched(), which defeats the above code for CPUs that are
          * running in-kernel with scheduling-clock interrupts disabled.
          * So hit them over the head with the resched_cpu() hammer!
          */
         if (tick_nohz_full_cpu(rdp->cpu) &&
             (time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3) ||
              rcu_state.cbovld)) {
                 WRITE_ONCE(rdp->rcu_urgent_qs, true);
                 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
                 ret = -1;
         }
 
         /*
          * If more than halfway to RCU CPU stall-warning time, invoke
          * resched_cpu() more frequently to try to loosen things up a bit.
          * Also check to see if the CPU is getting hammered with interrupts,
          * but only once per grace period, just to keep the IPIs down to
          * a dull roar.
          */
         if (time_after(jiffies, rcu_state.jiffies_resched)) {
                 if (time_after(jiffies,
                                READ_ONCE(rdp->last_fqs_resched) + jtsq)) {
                         WRITE_ONCE(rdp->last_fqs_resched, jiffies);
                         ret = -1;
                 }
                 if (IS_ENABLED(CONFIG_IRQ_WORK) &&
                     !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
                     (rnp->ffmask & rdp->grpmask)) {
                         rdp->rcu_iw_pending = true;
                         rdp->rcu_iw_gp_seq = rnp->gp_seq;
                         irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
                 }
 
                 if (rcu_cpu_stall_cputime && rdp->snap_record.gp_seq != rdp->gp_seq) {
                         int cpu = rdp->cpu;
                         struct rcu_snap_record *rsrp;
                         struct kernel_cpustat *kcsp;
 
                         kcsp = &kcpustat_cpu(cpu);
 
                         rsrp = &rdp->snap_record;
                         rsrp->cputime_irq     = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
                         rsrp->cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
                         rsrp->cputime_system  = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);
                         rsrp->nr_hardirqs = kstat_cpu_irqs_sum(rdp->cpu);
                         rsrp->nr_softirqs = kstat_cpu_softirqs_sum(rdp->cpu);
                         rsrp->nr_csw = nr_context_switches_cpu(rdp->cpu);
                         rsrp->jiffies = jiffies;
                         rsrp->gp_seq = rdp->gp_seq;
                 }
         }
 
         return ret;
 }
 
 /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
                               unsigned long gp_seq_req, const char *s)
 {
         trace_rcu_future_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
                                       gp_seq_req, rnp->level,
                                       rnp->grplo, rnp->grphi, s);
 }
 
 /*
  * rcu_start_this_gp - Request the start of a particular grace period
  * @rnp_start: The leaf node of the CPU from which to start.
  * @rdp: The rcu_data corresponding to the CPU from which to start.
  * @gp_seq_req: The gp_seq of the grace period to start.
  *
  * Start the specified grace period, as needed to handle newly arrived
  * callbacks.  The required future grace periods are recorded in each
  * rcu_node structure's ->gp_seq_needed field.  Returns true if there
  * is reason to awaken the grace-period kthread.
  *
  * The caller must hold the specified rcu_node structure's ->lock, which
  * is why the caller is responsible for waking the grace-period kthread.
  *
  * Returns true if the GP thread needs to be awakened else false.
  */
 static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
                               unsigned long gp_seq_req)
 {
         bool ret = false;
         struct rcu_node *rnp;
 
         /*
          * Use funnel locking to either acquire the root rcu_node
          * structure's lock or bail out if the need for this grace period
          * has already been recorded -- or if that grace period has in
          * fact already started.  If there is already a grace period in
          * progress in a non-leaf node, no recording is needed because the
          * end of the grace period will scan the leaf rcu_node structures.
          * Note that rnp_start->lock must not be released.
          */
         raw_lockdep_assert_held_rcu_node(rnp_start);
         trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));
         for (rnp = rnp_start; 1; rnp = rnp->parent) {
                 if (rnp != rnp_start)
                         raw_spin_lock_rcu_node(rnp);
                 if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) ||
                     rcu_seq_started(&rnp->gp_seq, gp_seq_req) ||
                     (rnp != rnp_start &&
                      rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) {
                         trace_rcu_this_gp(rnp, rdp, gp_seq_req,
                                           TPS("Prestarted"));
                         goto unlock_out;
                 }
                 WRITE_ONCE(rnp->gp_seq_needed, gp_seq_req);
                 if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
                         /*
                          * We just marked the leaf or internal node, and a
                          * grace period is in progress, which means that
                          * rcu_gp_cleanup() will see the marking.  Bail to
                          * reduce contention.
                          */
                         trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
                                           TPS("Startedleaf"));
                         goto unlock_out;
                 }
                 if (rnp != rnp_start && rnp->parent != NULL)
                         raw_spin_unlock_rcu_node(rnp);
                 if (!rnp->parent)
                         break;  /* At root, and perhaps also leaf. */
         }
 
         /* If GP already in progress, just leave, otherwise start one. */
         if (rcu_gp_in_progress()) {
                 trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedleafroot"));
                 goto unlock_out;
         }
         trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
         WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
         WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
         if (!READ_ONCE(rcu_state.gp_kthread)) {
                 trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
                 goto unlock_out;
         }
         trace_rcu_grace_period(rcu_state.name, data_race(rcu_state.gp_seq), TPS("newreq"));
         ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
         /* Push furthest requested GP to leaf node and rcu_data structure. */
         if (ULONG_CMP_LT(gp_seq_req, rnp->gp_seq_needed)) {
                 WRITE_ONCE(rnp_start->gp_seq_needed, rnp->gp_seq_needed);
                 WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
         }
         if (rnp != rnp_start)
                 raw_spin_unlock_rcu_node(rnp);
         return ret;
 }
 
 /*
  * Clean up any old requests for the just-ended grace period.  Also return
  * whether any additional grace periods have been requested.
  */
 static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
 {
         bool needmore;
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
         needmore = ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed);
         if (!needmore)
                 rnp->gp_seq_needed = rnp->gp_seq; /* Avoid counter wrap. */
         trace_rcu_this_gp(rnp, rdp, rnp->gp_seq,
                           needmore ? TPS("CleanupMore") : TPS("Cleanup"));
         return needmore;
 }
 
 static void swake_up_one_online_ipi(void *arg)
 {
         struct swait_queue_head *wqh = arg;
 
         swake_up_one(wqh);
 }
 
 static void swake_up_one_online(struct swait_queue_head *wqh)
 {
         int cpu = get_cpu();
 
         /*
          * If called from rcutree_report_cpu_starting(), wake up
          * is dangerous that late in the CPU-down hotplug process. The
          * scheduler might queue an ignored hrtimer. Defer the wake up
          * to an online CPU instead.
          */
         if (unlikely(cpu_is_offline(cpu))) {
                 int target;
 
                 target = cpumask_any_and(housekeeping_cpumask(HK_TYPE_RCU),
                                          cpu_online_mask);
 
                 smp_call_function_single(target, swake_up_one_online_ipi,
                                          wqh, 0);
                 put_cpu();
         } else {
                 put_cpu();
                 swake_up_one(wqh);
         }
 }
 
 /*
  * Awaken the grace-period kthread.  Don't do a self-awaken (unless in an
  * interrupt or softirq handler, in which case we just might immediately
  * sleep upon return, resulting in a grace-period hang), and don't bother
  * awakening when there is nothing for the grace-period kthread to do
  * (as in several CPUs raced to awaken, we lost), and finally don't try
  * to awaken a kthread that has not yet been created.  If all those checks
  * are passed, track some debug information and awaken.
  *
  * So why do the self-wakeup when in an interrupt or softirq handler
  * in the grace-period kthread's context?  Because the kthread might have
  * been interrupted just as it was going to sleep, and just after the final
  * pre-sleep check of the awaken condition.  In this case, a wakeup really
  * is required, and is therefore supplied.
  */
 static void rcu_gp_kthread_wake(void)
 {
         struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
 
         if ((current == t && !in_hardirq() && !in_serving_softirq()) ||
             !READ_ONCE(rcu_state.gp_flags) || !t)
                 return;
         WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
         WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
         swake_up_one_online(&rcu_state.gp_wq);
 }
 
 /*
  * If there is room, assign a ->gp_seq number to any callbacks on this
  * CPU that have not already been assigned.  Also accelerate any callbacks
  * that were previously assigned a ->gp_seq number that has since proven
  * to be too conservative, which can happen if callbacks get assigned a
  * ->gp_seq number while RCU is idle, but with reference to a non-root
  * rcu_node structure.  This function is idempotent, so it does not hurt
  * to call it repeatedly.  Returns an flag saying that we should awaken
  * the RCU grace-period kthread.
  *
  * The caller must hold rnp->lock with interrupts disabled.
  */
 static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 {
         unsigned long gp_seq_req;
         bool ret = false;
 
         rcu_lockdep_assert_cblist_protected(rdp);
         raw_lockdep_assert_held_rcu_node(rnp);
 
         /* If no pending (not yet ready to invoke) callbacks, nothing to do. */
         if (!rcu_segcblist_pend_cbs(&rdp->cblist))
                 return false;
 
         trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPreAcc"));
 
         /*
          * Callbacks are often registered with incomplete grace-period
          * information.  Something about the fact that getting exact
          * information requires acquiring a global lock...  RCU therefore
          * makes a conservative estimate of the grace period number at which
          * a given callback will become ready to invoke.        The following
          * code checks this estimate and improves it when possible, thus
          * accelerating callback invocation to an earlier grace-period
          * number.
          */
         gp_seq_req = rcu_seq_snap(&rcu_state.gp_seq);
         if (rcu_segcblist_accelerate(&rdp->cblist, gp_seq_req))
                 ret = rcu_start_this_gp(rnp, rdp, gp_seq_req);
 
         /* Trace depending on how much we were able to accelerate. */
         if (rcu_segcblist_restempty(&rdp->cblist, RCU_WAIT_TAIL))
                 trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccWaitCB"));
         else
                 trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));
 
         trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPostAcc"));
 
         return ret;
 }
 
 /*
  * Similar to rcu_accelerate_cbs(), but does not require that the leaf
  * rcu_node structure's ->lock be held.  It consults the cached value
  * of ->gp_seq_needed in the rcu_data structure, and if that indicates
  * that a new grace-period request be made, invokes rcu_accelerate_cbs()
  * while holding the leaf rcu_node structure's ->lock.
  */
 static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
                                         struct rcu_data *rdp)
 {
         unsigned long c;
         bool needwake;
 
         rcu_lockdep_assert_cblist_protected(rdp);
         c = rcu_seq_snap(&rcu_state.gp_seq);
         if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
                 /* Old request still live, so mark recent callbacks. */
                 (void)rcu_segcblist_accelerate(&rdp->cblist, c);
                 return;
         }
         raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
         needwake = rcu_accelerate_cbs(rnp, rdp);
         raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
         if (needwake)
                 rcu_gp_kthread_wake();
 }
 
 /*
  * Move any callbacks whose grace period has completed to the
  * RCU_DONE_TAIL sublist, then compact the remaining sublists and
  * assign ->gp_seq numbers to any callbacks in the RCU_NEXT_TAIL
  * sublist.  This function is idempotent, so it does not hurt to
  * invoke it repeatedly.  As long as it is not invoked -too- often...
  * Returns true if the RCU grace-period kthread needs to be awakened.
  *
  * The caller must hold rnp->lock with interrupts disabled.
  */
 static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 {
         rcu_lockdep_assert_cblist_protected(rdp);
         raw_lockdep_assert_held_rcu_node(rnp);
 
         /* If no pending (not yet ready to invoke) callbacks, nothing to do. */
         if (!rcu_segcblist_pend_cbs(&rdp->cblist))
                 return false;
 
         /*
          * Find all callbacks whose ->gp_seq numbers indicate that they
          * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.
          */
         rcu_segcblist_advance(&rdp->cblist, rnp->gp_seq);
 
         /* Classify any remaining callbacks. */
         return rcu_accelerate_cbs(rnp, rdp);
 }
 
 /*
  * Move and classify callbacks, but only if doing so won't require
  * that the RCU grace-period kthread be awakened.
  */
 static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
                                                   struct rcu_data *rdp)
 {
         rcu_lockdep_assert_cblist_protected(rdp);
         if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) || !raw_spin_trylock_rcu_node(rnp))
                 return;
         // The grace period cannot end while we hold the rcu_node lock.
         if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
                 WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
         raw_spin_unlock_rcu_node(rnp);
 }
 
 /*
  * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, attempt to generate a
  * quiescent state.  This is intended to be invoked when the CPU notices
  * a new grace period.
  */
 static void rcu_strict_gp_check_qs(void)
 {
         if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
                 rcu_read_lock();
                 rcu_read_unlock();
         }
 }
 
 /*
  * Update CPU-local rcu_data state to record the beginnings and ends of
  * grace periods.  The caller must hold the ->lock of the leaf rcu_node
  * structure corresponding to the current CPU, and must have irqs disabled.
  * Returns true if the grace-period kthread needs to be awakened.
  */
 static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 {
         bool ret = false;
         bool need_qs;
         const bool offloaded = rcu_rdp_is_offloaded(rdp);
 
         raw_lockdep_assert_held_rcu_node(rnp);
 
         if (rdp->gp_seq == rnp->gp_seq)
                 return false; /* Nothing to do. */
 
         /* Handle the ends of any preceding grace periods first. */
         if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
             unlikely(READ_ONCE(rdp->gpwrap))) {
                 if (!offloaded)
                         ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */
                 rdp->core_needs_qs = false;
                 trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("cpuend"));
         } else {
                 if (!offloaded)
                         ret = rcu_accelerate_cbs(rnp, rdp); /* Recent CBs. */
                 if (rdp->core_needs_qs)
                         rdp->core_needs_qs = !!(rnp->qsmask & rdp->grpmask);
         }
 
         /* Now handle the beginnings of any new-to-this-CPU grace periods. */
         if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
             unlikely(READ_ONCE(rdp->gpwrap))) {
                 /*
                  * If the current grace period is waiting for this CPU,
                  * set up to detect a quiescent state, otherwise don't
                  * go looking for one.
                  */
                 trace_rcu_grace_period(rcu_state.name, rnp->gp_seq, TPS("cpustart"));
                 need_qs = !!(rnp->qsmask & rdp->grpmask);
                 rdp->cpu_no_qs.b.norm = need_qs;
                 rdp->core_needs_qs = need_qs;
                 zero_cpu_stall_ticks(rdp);
         }
         rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
         if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
                 WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
         if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap))
                 WRITE_ONCE(rdp->last_sched_clock, jiffies);
         WRITE_ONCE(rdp->gpwrap, false);
         rcu_gpnum_ovf(rnp, rdp);
         return ret;
 }
 
 static void note_gp_changes(struct rcu_data *rdp)
 {
         unsigned long flags;
         bool needwake;
         struct rcu_node *rnp;
 
         local_irq_save(flags);
         rnp = rdp->mynode;
         if ((rdp->gp_seq == rcu_seq_current(&rnp->gp_seq) &&
              !unlikely(READ_ONCE(rdp->gpwrap))) || /* w/out lock. */
             !raw_spin_trylock_rcu_node(rnp)) { /* irqs already off, so later. */
                 local_irq_restore(flags);
                 return;
         }
         needwake = __note_gp_changes(rnp, rdp);
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         rcu_strict_gp_check_qs();
         if (needwake)
                 rcu_gp_kthread_wake();
 }
 
 static atomic_t *rcu_gp_slow_suppress;
 
 /* Register a counter to suppress debugging grace-period delays. */
 void rcu_gp_slow_register(atomic_t *rgssp)
 {
         WARN_ON_ONCE(rcu_gp_slow_suppress);
 
         WRITE_ONCE(rcu_gp_slow_suppress, rgssp);
 }
 EXPORT_SYMBOL_GPL(rcu_gp_slow_register);
 
 /* Unregister a counter, with NULL for not caring which. */
 void rcu_gp_slow_unregister(atomic_t *rgssp)
 {
         WARN_ON_ONCE(rgssp && rgssp != rcu_gp_slow_suppress && rcu_gp_slow_suppress != NULL);
 
         WRITE_ONCE(rcu_gp_slow_suppress, NULL);
 }
 EXPORT_SYMBOL_GPL(rcu_gp_slow_unregister);
 
 static bool rcu_gp_slow_is_suppressed(void)
 {
         atomic_t *rgssp = READ_ONCE(rcu_gp_slow_suppress);
 
         return rgssp && atomic_read(rgssp);
 }
 
 static void rcu_gp_slow(int delay)
 {
         if (!rcu_gp_slow_is_suppressed() && delay > 0 &&
             !(rcu_seq_ctr(rcu_state.gp_seq) % (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
                 schedule_timeout_idle(delay);
 }
 
 static unsigned long sleep_duration;
 
 /* Allow rcutorture to stall the grace-period kthread. */
 void rcu_gp_set_torture_wait(int duration)
 {
         if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST) && duration > 0)
                 WRITE_ONCE(sleep_duration, duration);
 }
 EXPORT_SYMBOL_GPL(rcu_gp_set_torture_wait);
 
 /* Actually implement the aforementioned wait. */
 static void rcu_gp_torture_wait(void)
 {
         unsigned long duration;
 
         if (!IS_ENABLED(CONFIG_RCU_TORTURE_TEST))
                 return;
         duration = xchg(&sleep_duration, 0UL);
         if (duration > 0) {
                 pr_alert("%s: Waiting %lu jiffies\n", __func__, duration);
                 schedule_timeout_idle(duration);
                 pr_alert("%s: Wait complete\n", __func__);
         }
 }
 
 /*
  * Handler for on_each_cpu() to invoke the target CPU's RCU core
  * processing.
  */
 static void rcu_strict_gp_boundary(void *unused)
 {
         invoke_rcu_core();
 }
 
 // Make the polled API aware of the beginning of a grace period.
 static void rcu_poll_gp_seq_start(unsigned long *snap)
 {
         struct rcu_node *rnp = rcu_get_root();
 
         if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
                 raw_lockdep_assert_held_rcu_node(rnp);
 
         // If RCU was idle, note beginning of GP.
         if (!rcu_seq_state(rcu_state.gp_seq_polled))
                 rcu_seq_start(&rcu_state.gp_seq_polled);
 
         // Either way, record current state.
         *snap = rcu_state.gp_seq_polled;
 }
 
 // Make the polled API aware of the end of a grace period.
 static void rcu_poll_gp_seq_end(unsigned long *snap)
 {
         struct rcu_node *rnp = rcu_get_root();
 
         if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
                 raw_lockdep_assert_held_rcu_node(rnp);
 
         // If the previously noted GP is still in effect, record the
         // end of that GP.  Either way, zero counter to avoid counter-wrap
         // problems.
         if (*snap && *snap == rcu_state.gp_seq_polled) {
                 rcu_seq_end(&rcu_state.gp_seq_polled);
                 rcu_state.gp_seq_polled_snap = 0;
                 rcu_state.gp_seq_polled_exp_snap = 0;
         } else {
                 *snap = 0;
         }
 }
 
 // Make the polled API aware of the beginning of a grace period, but
 // where caller does not hold the root rcu_node structure's lock.
 static void rcu_poll_gp_seq_start_unlocked(unsigned long *snap)
 {
         unsigned long flags;
         struct rcu_node *rnp = rcu_get_root();
 
         if (rcu_init_invoked()) {
                 if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
                         lockdep_assert_irqs_enabled();
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
         }
         rcu_poll_gp_seq_start(snap);
         if (rcu_init_invoked())
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
 // Make the polled API aware of the end of a grace period, but where
 // caller does not hold the root rcu_node structure's lock.
 static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
 {
         unsigned long flags;
         struct rcu_node *rnp = rcu_get_root();
 
         if (rcu_init_invoked()) {
                 if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE)
                         lockdep_assert_irqs_enabled();
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
         }
         rcu_poll_gp_seq_end(snap);
         if (rcu_init_invoked())
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
 /*
  * There is a single llist, which is used for handling
  * synchronize_rcu() users' enqueued rcu_synchronize nodes.
  * Within this llist, there are two tail pointers:
  *
  * wait tail: Tracks the set of nodes, which need to
  *            wait for the current GP to complete.
  * done tail: Tracks the set of nodes, for which grace
  *            period has elapsed. These nodes processing
  *            will be done as part of the cleanup work
  *            execution by a kworker.
  *
  * At every grace period init, a new wait node is added
  * to the llist. This wait node is used as wait tail
  * for this new grace period. Given that there are a fixed
  * number of wait nodes, if all wait nodes are in use
  * (which can happen when kworker callback processing
  * is delayed) and additional grace period is requested.
  * This means, a system is slow in processing callbacks.
  *
  * TODO: If a slow processing is detected, a first node
  * in the llist should be used as a wait-tail for this
  * grace period, therefore users which should wait due
  * to a slow process are handled by _this_ grace period
  * and not next.
  *
  * Below is an illustration of how the done and wait
  * tail pointers move from one set of rcu_synchronize nodes
  * to the other, as grace periods start and finish and
  * nodes are processed by kworker.
  *
  *
  * a. Initial llist callbacks list:
  *
  * +----------+           +--------+          +-------+
  * |          |           |        |          |       |
  * |   head   |---------> |   cb2  |--------->| cb1   |
  * |          |           |        |          |       |
  * +----------+           +--------+          +-------+
  *
  *
  *
  * b. New GP1 Start:
  *
  *                    WAIT TAIL
  *                      |
  *                      |
  *                      v
  * +----------+     +--------+      +--------+        +-------+
  * |          |     |        |      |        |        |       |
  * |   head   ------> wait   |------>   cb2  |------> |  cb1  |
  * |          |     | head1  |      |        |        |       |
  * +----------+     +--------+      +--------+        +-------+
  *
  *
  *
  * c. GP completion:
  *
  * WAIT_TAIL == DONE_TAIL
  *
  *                   DONE TAIL
  *                     |
  *                     |
  *                     v
  * +----------+     +--------+      +--------+        +-------+
  * |          |     |        |      |        |        |       |
  * |   head   ------> wait   |------>   cb2  |------> |  cb1  |
  * |          |     | head1  |      |        |        |       |
  * +----------+     +--------+      +--------+        +-------+
  *
  *
  *
  * d. New callbacks and GP2 start:
  *
  *                    WAIT TAIL                          DONE TAIL
  *                      |                                 |
  *                      |                                 |
  *                      v                                 v
  * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
  * |          |     |      |    |      |    |      |    |     |    |     |    |     |
  * |   head   ------> wait |--->|  cb4 |--->| cb3  |--->|wait |--->| cb2 |--->| cb1 |
  * |          |     | head2|    |      |    |      |    |head1|    |     |    |     |
  * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
  *
  *
  *
  * e. GP2 completion:
  *
  * WAIT_TAIL == DONE_TAIL
  *                   DONE TAIL
  *                      |
  *                      |
  *                      v
  * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
  * |          |     |      |    |      |    |      |    |     |    |     |    |     |
  * |   head   ------> wait |--->|  cb4 |--->| cb3  |--->|wait |--->| cb2 |--->| cb1 |
  * |          |     | head2|    |      |    |      |    |head1|    |     |    |     |
  * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
  *
  *
  * While the llist state transitions from d to e, a kworker
  * can start executing rcu_sr_normal_gp_cleanup_work() and
  * can observe either the old done tail (@c) or the new
  * done tail (@e). So, done tail updates and reads need
  * to use the rel-acq semantics. If the concurrent kworker
  * observes the old done tail, the newly queued work
  * execution will process the updated done tail. If the
  * concurrent kworker observes the new done tail, then
  * the newly queued work will skip processing the done
  * tail, as workqueue semantics guarantees that the new
  * work is executed only after the previous one completes.
  *
  * f. kworker callbacks processing complete:
  *
  *
  *                   DONE TAIL
  *                     |
  *                     |
  *                     v
  * +----------+     +--------+
  * |          |     |        |
  * |   head   ------> wait   |
  * |          |     | head2  |
  * +----------+     +--------+
  *
  */
 static bool rcu_sr_is_wait_head(struct llist_node *node)
 {
         return &(rcu_state.srs_wait_nodes)[0].node <= node &&
                 node <= &(rcu_state.srs_wait_nodes)[SR_NORMAL_GP_WAIT_HEAD_MAX - 1].node;
 }
 
 static struct llist_node *rcu_sr_get_wait_head(void)
 {
         struct sr_wait_node *sr_wn;
         int i;
 
         for (i = 0; i < SR_NORMAL_GP_WAIT_HEAD_MAX; i++) {
                 sr_wn = &(rcu_state.srs_wait_nodes)[i];
 
                 if (!atomic_cmpxchg_acquire(&sr_wn->inuse, 0, 1))
                         return &sr_wn->node;
         }
 
         return NULL;
 }
 
 static void rcu_sr_put_wait_head(struct llist_node *node)
 {
         struct sr_wait_node *sr_wn = container_of(node, struct sr_wait_node, node);
 
         atomic_set_release(&sr_wn->inuse, 0);
 }
 
 /* Disabled by default. */
 static int rcu_normal_wake_from_gp;
 module_param(rcu_normal_wake_from_gp, int, 0644);
 static struct workqueue_struct *sync_wq;
 
 static void rcu_sr_normal_complete(struct llist_node *node)
 {
         struct rcu_synchronize *rs = container_of(
                 (struct rcu_head *) node, struct rcu_synchronize, head);
         unsigned long oldstate = (unsigned long) rs->head.func;
 
         WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
                 !poll_state_synchronize_rcu(oldstate),
                 "A full grace period is not passed yet: %lu",
                 rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
 
         /* Finally. */
         complete(&rs->completion);
 }
 
 static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
 {
         struct llist_node *done, *rcu, *next, *head;
 
         /*
          * This work execution can potentially execute
          * while a new done tail is being updated by
          * grace period kthread in rcu_sr_normal_gp_cleanup().
          * So, read and updates of done tail need to
          * follow acq-rel semantics.
          *
          * Given that wq semantics guarantees that a single work
          * cannot execute concurrently by multiple kworkers,
          * the done tail list manipulations are protected here.
          */
         done = smp_load_acquire(&rcu_state.srs_done_tail);
         if (!done)
                 return;
 
         WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
         head = done->next;
         done->next = NULL;
 
         /*
          * The dummy node, which is pointed to by the
          * done tail which is acq-read above is not removed
          * here.  This allows lockless additions of new
          * rcu_synchronize nodes in rcu_sr_normal_add_req(),
          * while the cleanup work executes. The dummy
          * nodes is removed, in next round of cleanup
          * work execution.
          */
         llist_for_each_safe(rcu, next, head) {
                 if (!rcu_sr_is_wait_head(rcu)) {
                         rcu_sr_normal_complete(rcu);
                         continue;
                 }
 
                 rcu_sr_put_wait_head(rcu);
         }
 
         /* Order list manipulations with atomic access. */
         atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
 }
 
 /*
  * Helper function for rcu_gp_cleanup().
  */
 static void rcu_sr_normal_gp_cleanup(void)
 {
         struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
         int done = 0;
 
         wait_tail = rcu_state.srs_wait_tail;
         if (wait_tail == NULL)
                 return;
 
         rcu_state.srs_wait_tail = NULL;
         ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_wait_tail);
         WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
 
         /*
          * Process (a) and (d) cases. See an illustration.
          */
         llist_for_each_safe(rcu, next, wait_tail->next) {
                 if (rcu_sr_is_wait_head(rcu))
                         break;
 
                 rcu_sr_normal_complete(rcu);
                 // It can be last, update a next on this step.
                 wait_tail->next = next;
 
                 if (++done == SR_MAX_USERS_WAKE_FROM_GP)
                         break;
         }
 
         /*
          * Fast path, no more users to process except putting the second last
          * wait head if no inflight-workers. If there are in-flight workers,
          * they will remove the last wait head.
          *
          * Note that the ACQUIRE orders atomic access with list manipulation.
          */
         if (wait_tail->next && wait_tail->next->next == NULL &&
             rcu_sr_is_wait_head(wait_tail->next) &&
             !atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
                 rcu_sr_put_wait_head(wait_tail->next);
                 wait_tail->next = NULL;
         }
 
         /* Concurrent sr_normal_gp_cleanup work might observe this update. */
         ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
         smp_store_release(&rcu_state.srs_done_tail, wait_tail);
 
         /*
          * We schedule a work in order to perform a final processing
          * of outstanding users(if still left) and releasing wait-heads
          * added by rcu_sr_normal_gp_init() call.
          */
         if (wait_tail->next) {
                 atomic_inc(&rcu_state.srs_cleanups_pending);
                 if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work))
                         atomic_dec(&rcu_state.srs_cleanups_pending);
         }
 }
 
 /*
  * Helper function for rcu_gp_init().
  */
 static bool rcu_sr_normal_gp_init(void)
 {
         struct llist_node *first;
         struct llist_node *wait_head;
         bool start_new_poll = false;
 
         first = READ_ONCE(rcu_state.srs_next.first);
         if (!first || rcu_sr_is_wait_head(first))
                 return start_new_poll;
 
         wait_head = rcu_sr_get_wait_head();
         if (!wait_head) {
                 // Kick another GP to retry.
                 start_new_poll = true;
                 return start_new_poll;
         }
 
         /* Inject a wait-dummy-node. */
         llist_add(wait_head, &rcu_state.srs_next);
 
         /*
          * A waiting list of rcu_synchronize nodes should be empty on
          * this step, since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
          * rolls it over. If not, it is a BUG, warn a user.
          */
         WARN_ON_ONCE(rcu_state.srs_wait_tail != NULL);
         rcu_state.srs_wait_tail = wait_head;
         ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_wait_tail);
 
         return start_new_poll;
 }
 
 static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
 {
         llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
 }
 
 /*
  * Initialize a new grace period.  Return false if no grace period required.
  */
 static noinline_for_stack bool rcu_gp_init(void)
 {
         unsigned long flags;
         unsigned long oldmask;
         unsigned long mask;
         struct rcu_data *rdp;
         struct rcu_node *rnp = rcu_get_root();
         bool start_new_poll;
 
         WRITE_ONCE(rcu_state.gp_activity, jiffies);
         raw_spin_lock_irq_rcu_node(rnp);
         if (!rcu_state.gp_flags) {
                 /* Spurious wakeup, tell caller to go back to sleep.  */
                 raw_spin_unlock_irq_rcu_node(rnp);
                 return false;
         }
         WRITE_ONCE(rcu_state.gp_flags, 0); /* Clear all flags: New GP. */
 
         if (WARN_ON_ONCE(rcu_gp_in_progress())) {
                 /*
                  * Grace period already in progress, don't start another.
                  * Not supposed to be able to happen.
                  */
                 raw_spin_unlock_irq_rcu_node(rnp);
                 return false;
         }
 
         /* Advance to a new grace period and initialize state. */
         record_gp_stall_check_time();
         /* Record GP times before starting GP, hence rcu_seq_start(). */
         rcu_seq_start(&rcu_state.gp_seq);
         ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
         start_new_poll = rcu_sr_normal_gp_init();
         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
         rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
         raw_spin_unlock_irq_rcu_node(rnp);
 
         /*
          * The "start_new_poll" is set to true, only when this GP is not able
          * to handle anything and there are outstanding users. It happens when
          * the rcu_sr_normal_gp_init() function was not able to insert a dummy
          * separator to the llist, because there were no left any dummy-nodes.
          *
          * Number of dummy-nodes is fixed, it could be that we are run out of
          * them, if so we start a new pool request to repeat a try. It is rare
          * and it means that a system is doing a slow processing of callbacks.
          */
         if (start_new_poll)
                 (void) start_poll_synchronize_rcu();
 
         /*
          * Apply per-leaf buffered online and offline operations to
          * the rcu_node tree. Note that this new grace period need not
          * wait for subsequent online CPUs, and that RCU hooks in the CPU
          * offlining path, when combined with checks in this function,
          * will handle CPUs that are currently going offline or that will
          * go offline later.  Please also refer to "Hotplug CPU" section
          * of RCU's Requirements documentation.
          */
         WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF);
         /* Exclude CPU hotplug operations. */
         rcu_for_each_leaf_node(rnp) {
                 local_irq_disable();
                 arch_spin_lock(&rcu_state.ofl_lock);
                 raw_spin_lock_rcu_node(rnp);
                 if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
                     !rnp->wait_blkd_tasks) {
                         /* Nothing to do on this leaf rcu_node structure. */
                         raw_spin_unlock_rcu_node(rnp);
                         arch_spin_unlock(&rcu_state.ofl_lock);
                         local_irq_enable();
                         continue;
                 }
 
                 /* Record old state, apply changes to ->qsmaskinit field. */
                 oldmask = rnp->qsmaskinit;
                 rnp->qsmaskinit = rnp->qsmaskinitnext;
 
                 /* If zero-ness of ->qsmaskinit changed, propagate up tree. */
                 if (!oldmask != !rnp->qsmaskinit) {
                         if (!oldmask) { /* First online CPU for rcu_node. */
                                 if (!rnp->wait_blkd_tasks) /* Ever offline? */
                                         rcu_init_new_rnp(rnp);
                         } else if (rcu_preempt_has_tasks(rnp)) {
                                 rnp->wait_blkd_tasks = true; /* blocked tasks */
                         } else { /* Last offline CPU and can propagate. */
                                 rcu_cleanup_dead_rnp(rnp);
                         }
                 }
 
                 /*
                  * If all waited-on tasks from prior grace period are
                  * done, and if all this rcu_node structure's CPUs are
                  * still offline, propagate up the rcu_node tree and
                  * clear ->wait_blkd_tasks.  Otherwise, if one of this
                  * rcu_node structure's CPUs has since come back online,
                  * simply clear ->wait_blkd_tasks.
                  */
                 if (rnp->wait_blkd_tasks &&
                     (!rcu_preempt_has_tasks(rnp) || rnp->qsmaskinit)) {
                         rnp->wait_blkd_tasks = false;
                         if (!rnp->qsmaskinit)
                                 rcu_cleanup_dead_rnp(rnp);
                 }
 
                 raw_spin_unlock_rcu_node(rnp);
                 arch_spin_unlock(&rcu_state.ofl_lock);
                 local_irq_enable();
         }
         rcu_gp_slow(gp_preinit_delay); /* Races with CPU hotplug. */
 
         /*
          * Set the quiescent-state-needed bits in all the rcu_node
          * structures for all currently online CPUs in breadth-first
          * order, starting from the root rcu_node structure, relying on the
          * layout of the tree within the rcu_state.node[] array.  Note that
          * other CPUs will access only the leaves of the hierarchy, thus
          * seeing that no grace period is in progress, at least until the
          * corresponding leaf node has been initialized.
          *
          * The grace period cannot complete until the initialization
          * process finishes, because this kthread handles both.
          */
         WRITE_ONCE(rcu_state.gp_state, RCU_GP_INIT);
         rcu_for_each_node_breadth_first(rnp) {
                 rcu_gp_slow(gp_init_delay);
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
                 rdp = this_cpu_ptr(&rcu_data);
                 rcu_preempt_check_blocked_tasks(rnp);
                 rnp->qsmask = rnp->qsmaskinit;
                 WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq);
                 if (rnp == rdp->mynode)
                         (void)__note_gp_changes(rnp, rdp);
                 rcu_preempt_boost_start_gp(rnp);
                 trace_rcu_grace_period_init(rcu_state.name, rnp->gp_seq,
                                             rnp->level, rnp->grplo,
                                             rnp->grphi, rnp->qsmask);
                 /* Quiescent states for tasks on any now-offline CPUs. */
                 mask = rnp->qsmask & ~rnp->qsmaskinitnext;
                 rnp->rcu_gp_init_mask = mask;
                 if ((mask || rnp->wait_blkd_tasks) && rcu_is_leaf_node(rnp))
                         rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                 else
                         raw_spin_unlock_irq_rcu_node(rnp);
                 cond_resched_tasks_rcu_qs();
                 WRITE_ONCE(rcu_state.gp_activity, jiffies);
         }
 
         // If strict, make all CPUs aware of new grace period.
         if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
                 on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
 
         return true;
 }
 
 /*
  * Helper function for swait_event_idle_exclusive() wakeup at force-quiescent-state
  * time.
  */
 static bool rcu_gp_fqs_check_wake(int *gfp)
 {
         struct rcu_node *rnp = rcu_get_root();
 
         // If under overload conditions, force an immediate FQS scan.
         if (*gfp & RCU_GP_FLAG_OVLD)
                 return true;
 
         // Someone like call_rcu() requested a force-quiescent-state scan.
         *gfp = READ_ONCE(rcu_state.gp_flags);
         if (*gfp & RCU_GP_FLAG_FQS)
                 return true;
 
         // The current grace period has completed.
         if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp))
                 return true;
 
         return false;
 }
 
 /*
  * Do one round of quiescent-state forcing.
  */
 static void rcu_gp_fqs(bool first_time)
 {
         int nr_fqs = READ_ONCE(rcu_state.nr_fqs_jiffies_stall);
         struct rcu_node *rnp = rcu_get_root();
 
         WRITE_ONCE(rcu_state.gp_activity, jiffies);
         WRITE_ONCE(rcu_state.n_force_qs, rcu_state.n_force_qs + 1);
 
         WARN_ON_ONCE(nr_fqs > 3);
         /* Only countdown nr_fqs for stall purposes if jiffies moves. */
         if (nr_fqs) {
                 if (nr_fqs == 1) {
                         WRITE_ONCE(rcu_state.jiffies_stall,
                                    jiffies + rcu_jiffies_till_stall_check());
                 }
                 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, --nr_fqs);
         }
 
         if (first_time) {
                 /* Collect dyntick-idle snapshots. */
                 force_qs_rnp(dyntick_save_progress_counter);
         } else {
                 /* Handle dyntick-idle and offline CPUs. */
                 force_qs_rnp(rcu_implicit_dynticks_qs);
         }
         /* Clear flag to prevent immediate re-entry. */
         if (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) {
                 raw_spin_lock_irq_rcu_node(rnp);
                 WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags & ~RCU_GP_FLAG_FQS);
                 raw_spin_unlock_irq_rcu_node(rnp);
         }
 }
 
 /*
  * Loop doing repeated quiescent-state forcing until the grace period ends.
  */
 static noinline_for_stack void rcu_gp_fqs_loop(void)
 {
         bool first_gp_fqs = true;
         int gf = 0;
         unsigned long j;
         int ret;
         struct rcu_node *rnp = rcu_get_root();
 
         j = READ_ONCE(jiffies_till_first_fqs);
         if (rcu_state.cbovld)
                 gf = RCU_GP_FLAG_OVLD;
         ret = 0;
         for (;;) {
                 if (rcu_state.cbovld) {
                         j = (j + 2) / 3;
                         if (j <= 0)
                                 j = 1;
                 }
                 if (!ret || time_before(jiffies + j, rcu_state.jiffies_force_qs)) {
                         WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j);
                         /*
                          * jiffies_force_qs before RCU_GP_WAIT_FQS state
                          * update; required for stall checks.
                          */
                         smp_wmb();
                         WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
                                    jiffies + (j ? 3 * j : 2));
                 }
                 trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                        TPS("fqswait"));
                 WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_FQS);
                 (void)swait_event_idle_timeout_exclusive(rcu_state.gp_wq,
                                  rcu_gp_fqs_check_wake(&gf), j);
                 rcu_gp_torture_wait();
                 WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
                 /* Locking provides needed memory barriers. */
                 /*
                  * Exit the loop if the root rcu_node structure indicates that the grace period
                  * has ended, leave the loop.  The rcu_preempt_blocked_readers_cgp(rnp) check
                  * is required only for single-node rcu_node trees because readers blocking
                  * the current grace period are queued only on leaf rcu_node structures.
                  * For multi-node trees, checking the root node's ->qsmask suffices, because a
                  * given root node's ->qsmask bit is cleared only when all CPUs and tasks from
                  * the corresponding leaf nodes have passed through their quiescent state.
                  */
                 if (!READ_ONCE(rnp->qsmask) &&
                     !rcu_preempt_blocked_readers_cgp(rnp))
                         break;
                 /* If time for quiescent-state forcing, do it. */
                 if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
                     (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
                         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                                TPS("fqsstart"));
                         rcu_gp_fqs(first_gp_fqs);
                         gf = 0;
                         if (first_gp_fqs) {
                                 first_gp_fqs = false;
                                 gf = rcu_state.cbovld ? RCU_GP_FLAG_OVLD : 0;
                         }
                         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                                TPS("fqsend"));
                         cond_resched_tasks_rcu_qs();
                         WRITE_ONCE(rcu_state.gp_activity, jiffies);
                         ret = 0; /* Force full wait till next FQS. */
                         j = READ_ONCE(jiffies_till_next_fqs);
                 } else {
                         /* Deal with stray signal. */
                         cond_resched_tasks_rcu_qs();
                         WRITE_ONCE(rcu_state.gp_activity, jiffies);
                         WARN_ON(signal_pending(current));
                         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                                TPS("fqswaitsig"));
                         ret = 1; /* Keep old FQS timing. */
                         j = jiffies;
                         if (time_after(jiffies, rcu_state.jiffies_force_qs))
                                 j = 1;
                         else
                                 j = rcu_state.jiffies_force_qs - j;
                         gf = 0;
                 }
         }
 }
 
 /*
  * Clean up after the old grace period.
  */
 static noinline void rcu_gp_cleanup(void)
 {
         int cpu;
         bool needgp = false;
         unsigned long gp_duration;
         unsigned long new_gp_seq;
         bool offloaded;
         struct rcu_data *rdp;
         struct rcu_node *rnp = rcu_get_root();
         struct swait_queue_head *sq;
 
         WRITE_ONCE(rcu_state.gp_activity, jiffies);
         raw_spin_lock_irq_rcu_node(rnp);
         rcu_state.gp_end = jiffies;
         gp_duration = rcu_state.gp_end - rcu_state.gp_start;
         if (gp_duration > rcu_state.gp_max)
                 rcu_state.gp_max = gp_duration;
 
         /*
          * We know the grace period is complete, but to everyone else
          * it appears to still be ongoing.  But it is also the case
          * that to everyone else it looks like there is nothing that
          * they can do to advance the grace period.  It is therefore
          * safe for us to drop the lock in order to mark the grace
          * period as completed in all of the rcu_node structures.
          */
         rcu_poll_gp_seq_end(&rcu_state.gp_seq_polled_snap);
         raw_spin_unlock_irq_rcu_node(rnp);
 
         /*
          * Propagate new ->gp_seq value to rcu_node structures so that
          * other CPUs don't have to wait until the start of the next grace
          * period to process their callbacks.  This also avoids some nasty
          * RCU grace-period initialization races by forcing the end of
          * the current grace period to be completely recorded in all of
          * the rcu_node structures before the beginning of the next grace
          * period is recorded in any of the rcu_node structures.
          */
         new_gp_seq = rcu_state.gp_seq;
         rcu_seq_end(&new_gp_seq);
         rcu_for_each_node_breadth_first(rnp) {
                 raw_spin_lock_irq_rcu_node(rnp);
                 if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
                         dump_blkd_tasks(rnp, 10);
                 WARN_ON_ONCE(rnp->qsmask);
                 WRITE_ONCE(rnp->gp_seq, new_gp_seq);
                 if (!rnp->parent)
                         smp_mb(); // Order against failing poll_state_synchronize_rcu_full().
                 rdp = this_cpu_ptr(&rcu_data);
                 if (rnp == rdp->mynode)
                         needgp = __note_gp_changes(rnp, rdp) || needgp;
                 /* smp_mb() provided by prior unlock-lock pair. */
                 needgp = rcu_future_gp_cleanup(rnp) || needgp;
                 // Reset overload indication for CPUs no longer overloaded
                 if (rcu_is_leaf_node(rnp))
                         for_each_leaf_node_cpu_mask(rnp, cpu, rnp->cbovldmask) {
                                 rdp = per_cpu_ptr(&rcu_data, cpu);
                                 check_cb_ovld_locked(rdp, rnp);
                         }
                 sq = rcu_nocb_gp_get(rnp);
                 raw_spin_unlock_irq_rcu_node(rnp);
                 rcu_nocb_gp_cleanup(sq);
                 cond_resched_tasks_rcu_qs();
                 WRITE_ONCE(rcu_state.gp_activity, jiffies);
                 rcu_gp_slow(gp_cleanup_delay);
         }
         rnp = rcu_get_root();
         raw_spin_lock_irq_rcu_node(rnp); /* GP before ->gp_seq update. */
 
         /* Declare grace period done, trace first to use old GP number. */
         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("end"));
         rcu_seq_end(&rcu_state.gp_seq);
         ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
         WRITE_ONCE(rcu_state.gp_state, RCU_GP_IDLE);
         /* Check for GP requests since above loop. */
         rdp = this_cpu_ptr(&rcu_data);
         if (!needgp && ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
                 trace_rcu_this_gp(rnp, rdp, rnp->gp_seq_needed,
                                   TPS("CleanupMore"));
                 needgp = true;
         }
         /* Advance CBs to reduce false positives below. */
         offloaded = rcu_rdp_is_offloaded(rdp);
         if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
 
                 // We get here if a grace period was needed (“needgp”)
                 // and the above call to rcu_accelerate_cbs() did not set
                 // the RCU_GP_FLAG_INIT bit in ->gp_state (which records
                 // the need for another grace period).  The purpose
                 // of the “offloaded” check is to avoid invoking
                 // rcu_accelerate_cbs() on an offloaded CPU because we do not
                 // hold the ->nocb_lock needed to safely access an offloaded
                 // ->cblist.  We do not want to acquire that lock because
                 // it can be heavily contended during callback floods.
 
                 WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
                 WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
                 trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("newreq"));
         } else {
 
                 // We get here either if there is no need for an
                 // additional grace period or if rcu_accelerate_cbs() has
                 // already set the RCU_GP_FLAG_INIT bit in ->gp_flags. 
                 // So all we need to do is to clear all of the other
                 // ->gp_flags bits.
 
                 WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags & RCU_GP_FLAG_INIT);
         }
         raw_spin_unlock_irq_rcu_node(rnp);
 
         // Make synchronize_rcu() users aware of the end of old grace period.
         rcu_sr_normal_gp_cleanup();
 
         // If strict, make all CPUs aware of the end of the old grace period.
         if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
                 on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
 }
 
 /*
  * Body of kthread that handles grace periods.
  */
 static int __noreturn rcu_gp_kthread(void *unused)
 {
         rcu_bind_gp_kthread();
         for (;;) {
 
                 /* Handle grace-period start. */
                 for (;;) {
                         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                                TPS("reqwait"));
                         WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_GPS);
                         swait_event_idle_exclusive(rcu_state.gp_wq,
                                          READ_ONCE(rcu_state.gp_flags) &
                                          RCU_GP_FLAG_INIT);
                         rcu_gp_torture_wait();
                         WRITE_ONCE(rcu_state.gp_state, RCU_GP_DONE_GPS);
                         /* Locking provides needed memory barrier. */
                         if (rcu_gp_init())
                                 break;
                         cond_resched_tasks_rcu_qs();
                         WRITE_ONCE(rcu_state.gp_activity, jiffies);
                         WARN_ON(signal_pending(current));
                         trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
                                                TPS("reqwaitsig"));
                 }
 
                 /* Handle quiescent-state forcing. */
                 rcu_gp_fqs_loop();
 
                 /* Handle grace-period end. */
                 WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANUP);
                 rcu_gp_cleanup();
                 WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANED);
         }
 }
 
 /*
  * Report a full set of quiescent states to the rcu_state data structure.
  * Invoke rcu_gp_kthread_wake() to awaken the grace-period kthread if
  * another grace period is required.  Whether we wake the grace-period
  * kthread or it awakens itself for the next round of quiescent-state
  * forcing, that kthread will clean up after the just-completed grace
  * period.  Note that the caller must hold rnp->lock, which is released
  * before return.
  */
 static void rcu_report_qs_rsp(unsigned long flags)
         __releases(rcu_get_root()->lock)
 {
         raw_lockdep_assert_held_rcu_node(rcu_get_root());
         WARN_ON_ONCE(!rcu_gp_in_progress());
         WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
         raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(), flags);
         rcu_gp_kthread_wake();
 }
 
 /*
  * Similar to rcu_report_qs_rdp(), for which it is a helper function.
  * Allows quiescent states for a group of CPUs to be reported at one go
  * to the specified rcu_node structure, though all the CPUs in the group
  * must be represented by the same rcu_node structure (which need not be a
  * leaf rcu_node structure, though it often will be).  The gps parameter
  * is the grace-period snapshot, which means that the quiescent states
  * are valid only if rnp->gp_seq is equal to gps.  That structure's lock
  * must be held upon entry, and it is released before return.
  *
  * As a special case, if mask is zero, the bit-already-cleared check is
  * disabled.  This allows propagating quiescent state due to resumed tasks
  * during grace-period initialization.
  */
 static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
                               unsigned long gps, unsigned long flags)
         __releases(rnp->lock)
 {
         unsigned long oldmask = 0;
         struct rcu_node *rnp_c;
 
         raw_lockdep_assert_held_rcu_node(rnp);
 
         /* Walk up the rcu_node hierarchy. */
         for (;;) {
                 if ((!(rnp->qsmask & mask) && mask) || rnp->gp_seq != gps) {
 
                         /*
                          * Our bit has already been cleared, or the
                          * relevant grace period is already over, so done.
                          */
                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                         return;
                 }
                 WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
                 WARN_ON_ONCE(!rcu_is_leaf_node(rnp) &&
                              rcu_preempt_blocked_readers_cgp(rnp));
                 WRITE_ONCE(rnp->qsmask, rnp->qsmask & ~mask);
                 trace_rcu_quiescent_state_report(rcu_state.name, rnp->gp_seq,
                                                  mask, rnp->qsmask, rnp->level,
                                                  rnp->grplo, rnp->grphi,
                                                  !!rnp->gp_tasks);
                 if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
 
                         /* Other bits still set at this level, so done. */
                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                         return;
                 }
                 rnp->completedqs = rnp->gp_seq;
                 mask = rnp->grpmask;
                 if (rnp->parent == NULL) {
 
                         /* No more levels.  Exit loop holding root lock. */
 
                         break;
                 }
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                 rnp_c = rnp;
                 rnp = rnp->parent;
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
                 oldmask = READ_ONCE(rnp_c->qsmask);
         }
 
         /*
          * Get here if we are the last CPU to pass through a quiescent
          * state for this grace period.  Invoke rcu_report_qs_rsp()
          * to clean up and start the next grace period if one is needed.
          */
         rcu_report_qs_rsp(flags); /* releases rnp->lock. */
 }
 
 /*
  * Record a quiescent state for all tasks that were previously queued
  * on the specified rcu_node structure and that were blocking the current
  * RCU grace period.  The caller must hold the corresponding rnp->lock with
  * irqs disabled, and this lock is released upon return, but irqs remain
  * disabled.
  */
 static void __maybe_unused
 rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
         __releases(rnp->lock)
 {
         unsigned long gps;
         unsigned long mask;
         struct rcu_node *rnp_p;
 
         raw_lockdep_assert_held_rcu_node(rnp);
         if (WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_RCU)) ||
             WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)) ||
             rnp->qsmask != 0) {
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                 return;  /* Still need more quiescent states! */
         }
 
         rnp->completedqs = rnp->gp_seq;
         rnp_p = rnp->parent;
         if (rnp_p == NULL) {
                 /*
                  * Only one rcu_node structure in the tree, so don't
                  * try to report up to its nonexistent parent!
                  */
                 rcu_report_qs_rsp(flags);
                 return;
         }
 
         /* Report up the rest of the hierarchy, tracking current ->gp_seq. */
         gps = rnp->gp_seq;
         mask = rnp->grpmask;
         raw_spin_unlock_rcu_node(rnp);  /* irqs remain disabled. */
         raw_spin_lock_rcu_node(rnp_p);  /* irqs already disabled. */
         rcu_report_qs_rnp(mask, rnp_p, gps, flags);
 }
 
 /*
  * Record a quiescent state for the specified CPU to that CPU's rcu_data
  * structure.  This must be called from the specified CPU.
  */
 static void
 rcu_report_qs_rdp(struct rcu_data *rdp)
 {
         unsigned long flags;
         unsigned long mask;
         bool needacc = false;
         struct rcu_node *rnp;
 
         WARN_ON_ONCE(rdp->cpu != smp_processor_id());
         rnp = rdp->mynode;
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
             rdp->gpwrap) {
 
                 /*
                  * The grace period in which this quiescent state was
                  * recorded has ended, so don't report it upwards.
                  * We will instead need a new quiescent state that lies
                  * within the current grace period.
                  */
                 rdp->cpu_no_qs.b.norm = true;   /* need qs for new gp. */
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                 return;
         }
         mask = rdp->grpmask;
         rdp->core_needs_qs = false;
         if ((rnp->qsmask & mask) == 0) {
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         } else {
                 /*
                  * This GP can't end until cpu checks in, so all of our
                  * callbacks can be processed during the next GP.
                  *
                  * NOCB kthreads have their own way to deal with that...
                  */
                 if (!rcu_rdp_is_offloaded(rdp)) {
                         /*
                          * The current GP has not yet ended, so it
                          * should not be possible for rcu_accelerate_cbs()
                          * to return true.  So complain, but don't awaken.
                          */
                         WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp));
                 } else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
                         /*
                          * ...but NOCB kthreads may miss or delay callbacks acceleration
                          * if in the middle of a (de-)offloading process.
                          */
                         needacc = true;
                 }
 
                 rcu_disable_urgency_upon_qs(rdp);
                 rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                 /* ^^^ Released rnp->lock */
 
                 if (needacc) {
                         rcu_nocb_lock_irqsave(rdp, flags);
                         rcu_accelerate_cbs_unlocked(rnp, rdp);
                         rcu_nocb_unlock_irqrestore(rdp, flags);
                 }
         }
 }
 
 /*
  * Check to see if there is a new grace period of which this CPU
  * is not yet aware, and if so, set up local rcu_data state for it.
  * Otherwise, see if this CPU has just passed through its first
  * quiescent state for this grace period, and record that fact if so.
  */
 static void
 rcu_check_quiescent_state(struct rcu_data *rdp)
 {
         /* Check for grace-period ends and beginnings. */
         note_gp_changes(rdp);
 
         /*
          * Does this CPU still need to do its part for current grace period?
          * If no, return and let the other CPUs do their part as well.
          */
         if (!rdp->core_needs_qs)
                 return;
 
         /*
          * Was there a quiescent state since the beginning of the grace
          * period? If no, then exit and wait for the next call.
          */
         if (rdp->cpu_no_qs.b.norm)
                 return;
 
         /*
          * Tell RCU we are done (but rcu_report_qs_rdp() will be the
          * judge of that).
          */
         rcu_report_qs_rdp(rdp);
 }
 
 /* Return true if callback-invocation time limit exceeded. */
 static bool rcu_do_batch_check_time(long count, long tlimit,
                                     bool jlimit_check, unsigned long jlimit)
 {
         // Invoke local_clock() only once per 32 consecutive callbacks.
         return unlikely(tlimit) &&
                (!likely(count & 31) ||
                 (IS_ENABLED(CONFIG_RCU_DOUBLE_CHECK_CB_TIME) &&
                  jlimit_check && time_after(jiffies, jlimit))) &&
                local_clock() >= tlimit;
 }
 
 /*
  * Invoke any RCU callbacks that have made it to the end of their grace
  * period.  Throttle as specified by rdp->blimit.
  */
 static void rcu_do_batch(struct rcu_data *rdp)
 {
         long bl;
         long count = 0;
         int div;
         bool __maybe_unused empty;
         unsigned long flags;
         unsigned long jlimit;
         bool jlimit_check = false;
         long pending;
         struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
         struct rcu_head *rhp;
         long tlimit = 0;
 
         /* If no callbacks are ready, just return. */
         if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
                 trace_rcu_batch_start(rcu_state.name,
                                       rcu_segcblist_n_cbs(&rdp->cblist), 0);
                 trace_rcu_batch_end(rcu_state.name, 0,
                                     !rcu_segcblist_empty(&rdp->cblist),
                                     need_resched(), is_idle_task(current),
                                     rcu_is_callbacks_kthread(rdp));
                 return;
         }
 
         /*
          * Extract the list of ready callbacks, disabling IRQs to prevent
          * races with call_rcu() from interrupt handlers.  Leave the
          * callback counts, as rcu_barrier() needs to be conservative.
          *
          * Callbacks execution is fully ordered against preceding grace period
          * completion (materialized by rnp->gp_seq update) thanks to the
          * smp_mb__after_unlock_lock() upon node locking required for callbacks
          * advancing. In NOCB mode this ordering is then further relayed through
          * the nocb locking that protects both callbacks advancing and extraction.
          */
         rcu_nocb_lock_irqsave(rdp, flags);
         WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
         pending = rcu_segcblist_get_seglen(&rdp->cblist, RCU_DONE_TAIL);
         div = READ_ONCE(rcu_divisor);
         div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
         bl = max(rdp->blimit, pending >> div);
         if ((in_serving_softirq() || rdp->rcu_cpu_kthread_status == RCU_KTHREAD_RUNNING) &&
             (IS_ENABLED(CONFIG_RCU_DOUBLE_CHECK_CB_TIME) || unlikely(bl > 100))) {
                 const long npj = NSEC_PER_SEC / HZ;
                 long rrn = READ_ONCE(rcu_resched_ns);
 
                 rrn = rrn < NSEC_PER_MSEC ? NSEC_PER_MSEC : rrn > NSEC_PER_SEC ? NSEC_PER_SEC : rrn;
                 tlimit = local_clock() + rrn;
                 jlimit = jiffies + (rrn + npj + 1) / npj;
                 jlimit_check = true;
         }
         trace_rcu_batch_start(rcu_state.name,
                               rcu_segcblist_n_cbs(&rdp->cblist), bl);
         rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
         if (rcu_rdp_is_offloaded(rdp))
                 rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
 
         trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbDequeued"));
         rcu_nocb_unlock_irqrestore(rdp, flags);
 
         /* Invoke callbacks. */
         tick_dep_set_task(current, TICK_DEP_BIT_RCU);
         rhp = rcu_cblist_dequeue(&rcl);
 
         for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
                 rcu_callback_t f;
 
                 count++;
                 debug_rcu_head_unqueue(rhp);
 
                 rcu_lock_acquire(&rcu_callback_map);
                 trace_rcu_invoke_callback(rcu_state.name, rhp);
 
                 f = rhp->func;
                 debug_rcu_head_callback(rhp);
                 WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
                 f(rhp);
 
                 rcu_lock_release(&rcu_callback_map);
 
                 /*
                  * Stop only if limit reached and CPU has something to do.
                  */
                 if (in_serving_softirq()) {
                         if (count >= bl && (need_resched() || !is_idle_task(current)))
                                 break;
                         /*
                          * Make sure we don't spend too much time here and deprive other
                          * softirq vectors of CPU cycles.
                          */
                         if (rcu_do_batch_check_time(count, tlimit, jlimit_check, jlimit))
                                 break;
                 } else {
                         // In rcuc/rcuoc context, so no worries about
                         // depriving other softirq vectors of CPU cycles.
                         local_bh_enable();
                         lockdep_assert_irqs_enabled();
                         cond_resched_tasks_rcu_qs();
                         lockdep_assert_irqs_enabled();
                         local_bh_disable();
                         // But rcuc kthreads can delay quiescent-state
                         // reporting, so check time limits for them.
                         if (rdp->rcu_cpu_kthread_status == RCU_KTHREAD_RUNNING &&
                             rcu_do_batch_check_time(count, tlimit, jlimit_check, jlimit)) {
                                 rdp->rcu_cpu_has_work = 1;
                                 break;
                         }
                 }
         }
 
         rcu_nocb_lock_irqsave(rdp, flags);
         rdp->n_cbs_invoked += count;
         trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(),
                             is_idle_task(current), rcu_is_callbacks_kthread(rdp));
 
         /* Update counts and requeue any remaining callbacks. */
         rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
         rcu_segcblist_add_len(&rdp->cblist, -count);
 
         /* Reinstate batch limit if we have worked down the excess. */
         count = rcu_segcblist_n_cbs(&rdp->cblist);
         if (rdp->blimit >= DEFAULT_MAX_RCU_BLIMIT && count <= qlowmark)
                 rdp->blimit = blimit;
 
         /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
         if (count == 0 && rdp->qlen_last_fqs_check != 0) {
                 rdp->qlen_last_fqs_check = 0;
                 rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
         } else if (count < rdp->qlen_last_fqs_check - qhimark)
                 rdp->qlen_last_fqs_check = count;
 
         /*
          * The following usually indicates a double call_rcu().  To track
          * this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
          */
         empty = rcu_segcblist_empty(&rdp->cblist);
         WARN_ON_ONCE(count == 0 && !empty);
         WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
                      count != 0 && empty);
         WARN_ON_ONCE(count == 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) != 0);
         WARN_ON_ONCE(!empty && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
 
         rcu_nocb_unlock_irqrestore(rdp, flags);
 
         tick_dep_clear_task(current, TICK_DEP_BIT_RCU);
 }
 
 /*
  * This function is invoked from each scheduling-clock interrupt,
  * and checks to see if this CPU is in a non-context-switch quiescent
  * state, for example, user mode or idle loop.  It also schedules RCU
  * core processing.  If the current grace period has gone on too long,
  * it will ask the scheduler to manufacture a context switch for the sole
  * purpose of providing the needed quiescent state.
  */
 void rcu_sched_clock_irq(int user)
 {
         unsigned long j;
 
         if (IS_ENABLED(CONFIG_PROVE_RCU)) {
                 j = jiffies;
                 WARN_ON_ONCE(time_before(j, __this_cpu_read(rcu_data.last_sched_clock)));
                 __this_cpu_write(rcu_data.last_sched_clock, j);
         }
         trace_rcu_utilization(TPS("Start scheduler-tick"));
         lockdep_assert_irqs_disabled();
         raw_cpu_inc(rcu_data.ticks_this_gp);
         /* The load-acquire pairs with the store-release setting to true. */
         if (smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
                 /* Idle and userspace execution already are quiescent states. */
                 if (!rcu_is_cpu_rrupt_from_idle() && !user) {
                         set_tsk_need_resched(current);
                         set_preempt_need_resched();
                 }
                 __this_cpu_write(rcu_data.rcu_urgent_qs, false);
         }
         rcu_flavor_sched_clock_irq(user);
         if (rcu_pending(user))
                 invoke_rcu_core();
         if (user || rcu_is_cpu_rrupt_from_idle())
                 rcu_note_voluntary_context_switch(current);
         lockdep_assert_irqs_disabled();
 
         trace_rcu_utilization(TPS("End scheduler-tick"));
 }
 
 /*
  * Scan the leaf rcu_node structures.  For each structure on which all
  * CPUs have reported a quiescent state and on which there are tasks
  * blocking the current grace period, initiate RCU priority boosting.
  * Otherwise, invoke the specified function to check dyntick state for
  * each CPU that has not yet reported a quiescent state.
  */
 static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 {
         int cpu;
         unsigned long flags;
         struct rcu_node *rnp;
 
         rcu_state.cbovld = rcu_state.cbovldnext;
         rcu_state.cbovldnext = false;
         rcu_for_each_leaf_node(rnp) {
                 unsigned long mask = 0;
                 unsigned long rsmask = 0;
 
                 cond_resched_tasks_rcu_qs();
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
                 rcu_state.cbovldnext |= !!rnp->cbovldmask;
                 if (rnp->qsmask == 0) {
                         if (rcu_preempt_blocked_readers_cgp(rnp)) {
                                 /*
                                  * No point in scanning bits because they
                                  * are all zero.  But we might need to
                                  * priority-boost blocked readers.
                                  */
                                 rcu_initiate_boost(rnp, flags);
                                 /* rcu_initiate_boost() releases rnp->lock */
                                 continue;
                         }
                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                         continue;
                 }
                 for_each_leaf_node_cpu_mask(rnp, cpu, rnp->qsmask) {
                         struct rcu_data *rdp;
                         int ret;
 
                         rdp = per_cpu_ptr(&rcu_data, cpu);
                         ret = f(rdp);
                         if (ret > 0) {
                                 mask |= rdp->grpmask;
                                 rcu_disable_urgency_upon_qs(rdp);
                         }
                         if (ret < 0)
                                 rsmask |= rdp->grpmask;
                 }
                 if (mask != 0) {
                         /* Idle/offline CPUs, report (releases rnp->lock). */
                         rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                 } else {
                         /* Nothing to do here, so just drop the lock. */
                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                 }
 
                 for_each_leaf_node_cpu_mask(rnp, cpu, rsmask)
                         resched_cpu(cpu);
         }
 }
 
 /*
  * Force quiescent states on reluctant CPUs, and also detect which
  * CPUs are in dyntick-idle mode.
  */
 void rcu_force_quiescent_state(void)
 {
         unsigned long flags;
         bool ret;
         struct rcu_node *rnp;
         struct rcu_node *rnp_old = NULL;
 
         if (!rcu_gp_in_progress())
                 return;
         /* Funnel through hierarchy to reduce memory contention. */
         rnp = raw_cpu_read(rcu_data.mynode);
         for (; rnp != NULL; rnp = rnp->parent) {
                 ret = (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) ||
                        !raw_spin_trylock(&rnp->fqslock);
                 if (rnp_old != NULL)
                         raw_spin_unlock(&rnp_old->fqslock);
                 if (ret)
                         return;
                 rnp_old = rnp;
         }
         /* rnp_old == rcu_get_root(), rnp == NULL. */
 
         /* Reached the root of the rcu_node tree, acquire lock. */
         raw_spin_lock_irqsave_rcu_node(rnp_old, flags);
         raw_spin_unlock(&rnp_old->fqslock);
         if (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) {
                 raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
                 return;  /* Someone beat us to it. */
         }
         WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
         raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
         rcu_gp_kthread_wake();
 }
 EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
 
 // Workqueue handler for an RCU reader for kernels enforcing struct RCU
 // grace periods.
 static void strict_work_handler(struct work_struct *work)
 {
         rcu_read_lock();
         rcu_read_unlock();
 }
 
 /* Perform RCU core processing work for the current CPU.  */
 static __latent_entropy void rcu_core(void)
 {
         unsigned long flags;
         struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
         struct rcu_node *rnp = rdp->mynode;
         /*
          * On RT rcu_core() can be preempted when IRQs aren't disabled.
          * Therefore this function can race with concurrent NOCB (de-)offloading
          * on this CPU and the below condition must be considered volatile.
          * However if we race with:
          *
          * _ Offloading:   In the worst case we accelerate or process callbacks
          *                 concurrently with NOCB kthreads. We are guaranteed to
          *                 call rcu_nocb_lock() if that happens.
          *
          * _ Deoffloading: In the worst case we miss callbacks acceleration or
          *                 processing. This is fine because the early stage
          *                 of deoffloading invokes rcu_core() after setting
          *                 SEGCBLIST_RCU_CORE. So we guarantee that we'll process
          *                 what could have been dismissed without the need to wait
          *                 for the next rcu_pending() check in the next jiffy.
          */
         const bool do_batch = !rcu_segcblist_completely_offloaded(&rdp->cblist);
 
         if (cpu_is_offline(smp_processor_id()))
                 return;
         trace_rcu_utilization(TPS("Start RCU core"));
         WARN_ON_ONCE(!rdp->beenonline);
 
         /* Report any deferred quiescent states if preemption enabled. */
         if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && (!(preempt_count() & PREEMPT_MASK))) {
                 rcu_preempt_deferred_qs(current);
         } else if (rcu_preempt_need_deferred_qs(current)) {
                 set_tsk_need_resched(current);
                 set_preempt_need_resched();
         }
 
         /* Update RCU state based on any recent quiescent states. */
         rcu_check_quiescent_state(rdp);
 
         /* No grace period and unregistered callbacks? */
         if (!rcu_gp_in_progress() &&
             rcu_segcblist_is_enabled(&rdp->cblist) && do_batch) {
                 rcu_nocb_lock_irqsave(rdp, flags);
                 if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
                         rcu_accelerate_cbs_unlocked(rnp, rdp);
                 rcu_nocb_unlock_irqrestore(rdp, flags);
         }
 
         rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());
 
         /* If there are callbacks ready, invoke them. */
         if (do_batch && rcu_segcblist_ready_cbs(&rdp->cblist) &&
             likely(READ_ONCE(rcu_scheduler_fully_active))) {
                 rcu_do_batch(rdp);
                 /* Re-invoke RCU core processing if there are callbacks remaining. */
                 if (rcu_segcblist_ready_cbs(&rdp->cblist))
                         invoke_rcu_core();
         }
 
         /* Do any needed deferred wakeups of rcuo kthreads. */
         do_nocb_deferred_wakeup(rdp);
         trace_rcu_utilization(TPS("End RCU core"));
 
         // If strict GPs, schedule an RCU reader in a clean environment.
         if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
                 queue_work_on(rdp->cpu, rcu_gp_wq, &rdp->strict_work);
 }
 
 static void rcu_core_si(struct softirq_action *h)
 {
         rcu_core();
 }
 
 static void rcu_wake_cond(struct task_struct *t, int status)
 {
         /*
          * If the thread is yielding, only wake it when this
          * is invoked from idle
          */
         if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)))
                 wake_up_process(t);
 }
 
 static void invoke_rcu_core_kthread(void)
 {
         struct task_struct *t;
         unsigned long flags;
 
         local_irq_save(flags);
         __this_cpu_write(rcu_data.rcu_cpu_has_work, 1);
         t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task);
         if (t != NULL && t != current)
                 rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status));
         local_irq_restore(flags);
 }
 
 /*
  * Wake up this CPU's rcuc kthread to do RCU core processing.
  */
 static void invoke_rcu_core(void)
 {
         if (!cpu_online(smp_processor_id()))
                 return;
         if (use_softirq)
                 raise_softirq(RCU_SOFTIRQ);
         else
                 invoke_rcu_core_kthread();
 }
 
 static void rcu_cpu_kthread_park(unsigned int cpu)
 {
         per_cpu(rcu_data.rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
 }
 
 static int rcu_cpu_kthread_should_run(unsigned int cpu)
 {
         return __this_cpu_read(rcu_data.rcu_cpu_has_work);
 }
 
 /*
  * Per-CPU kernel thread that invokes RCU callbacks.  This replaces
  * the RCU softirq used in configurations of RCU that do not support RCU
  * priority boosting.
  */
 static void rcu_cpu_kthread(unsigned int cpu)
 {
         unsigned int *statusp = this_cpu_ptr(&rcu_data.rcu_cpu_kthread_status);
         char work, *workp = this_cpu_ptr(&rcu_data.rcu_cpu_has_work);
         unsigned long *j = this_cpu_ptr(&rcu_data.rcuc_activity);
         int spincnt;
 
         trace_rcu_utilization(TPS("Start CPU kthread@rcu_run"));
         for (spincnt = 0; spincnt < 10; spincnt++) {
                 WRITE_ONCE(*j, jiffies);
                 local_bh_disable();
                 *statusp = RCU_KTHREAD_RUNNING;
                 local_irq_disable();
                 work = *workp;
                 WRITE_ONCE(*workp, 0);
                 local_irq_enable();
                 if (work)
                         rcu_core();
                 local_bh_enable();
                 if (!READ_ONCE(*workp)) {
                         trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
                         *statusp = RCU_KTHREAD_WAITING;
                         return;
                 }
         }
         *statusp = RCU_KTHREAD_YIELDING;
         trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
         schedule_timeout_idle(2);
         trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
         *statusp = RCU_KTHREAD_WAITING;
         WRITE_ONCE(*j, jiffies);
 }
 
 static struct smp_hotplug_thread rcu_cpu_thread_spec = {
         .store                  = &rcu_data.rcu_cpu_kthread_task,
         .thread_should_run      = rcu_cpu_kthread_should_run,
         .thread_fn              = rcu_cpu_kthread,
         .thread_comm            = "rcuc/%u",
         .setup                  = rcu_cpu_kthread_setup,
         .park                   = rcu_cpu_kthread_park,
 };
 
 /*
  * Spawn per-CPU RCU core processing kthreads.
  */
 static int __init rcu_spawn_core_kthreads(void)
 {
         int cpu;
 
         for_each_possible_cpu(cpu)
                 per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
         if (use_softirq)
                 return 0;
         WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec),
                   "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__);
         return 0;
 }
 
 static void rcutree_enqueue(struct rcu_data *rdp, struct rcu_head *head, rcu_callback_t func)
 {
         rcu_segcblist_enqueue(&rdp->cblist, head);
         if (__is_kvfree_rcu_offset((unsigned long)func))
                 trace_rcu_kvfree_callback(rcu_state.name, head,
                                          (unsigned long)func,
                                          rcu_segcblist_n_cbs(&rdp->cblist));
         else
                 trace_rcu_callback(rcu_state.name, head,
                                    rcu_segcblist_n_cbs(&rdp->cblist));
         trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
 }
 
 /*
  * Handle any core-RCU processing required by a call_rcu() invocation.
  */
 static void call_rcu_core(struct rcu_data *rdp, struct rcu_head *head,
                           rcu_callback_t func, unsigned long flags)
 {
         rcutree_enqueue(rdp, head, func);
         /*
          * If called from an extended quiescent state, invoke the RCU
          * core in order to force a re-evaluation of RCU's idleness.
          */
         if (!rcu_is_watching())
                 invoke_rcu_core();
 
         /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
         if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
                 return;
 
         /*
          * Force the grace period if too many callbacks or too long waiting.
          * Enforce hysteresis, and don't invoke rcu_force_quiescent_state()
          * if some other CPU has recently done so.  Also, don't bother
          * invoking rcu_force_quiescent_state() if the newly enqueued callback
          * is the only one waiting for a grace period to complete.
          */
         if (unlikely(rcu_segcblist_n_cbs(&rdp->cblist) >
                      rdp->qlen_last_fqs_check + qhimark)) {
 
                 /* Are we ignoring a completed grace period? */
                 note_gp_changes(rdp);
 
                 /* Start a new grace period if one not already started. */
                 if (!rcu_gp_in_progress()) {
                         rcu_accelerate_cbs_unlocked(rdp->mynode, rdp);
                 } else {
                         /* Give the grace period a kick. */
                         rdp->blimit = DEFAULT_MAX_RCU_BLIMIT;
                         if (READ_ONCE(rcu_state.n_force_qs) == rdp->n_force_qs_snap &&
                             rcu_segcblist_first_pend_cb(&rdp->cblist) != head)
                                 rcu_force_quiescent_state();
                         rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
                         rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
                 }
         }
 }
 
 /*
  * RCU callback function to leak a callback.
  */
 static void rcu_leak_callback(struct rcu_head *rhp)
 {
 }
 
 /*
  * Check and if necessary update the leaf rcu_node structure's
  * ->cbovldmask bit corresponding to the current CPU based on that CPU's
  * number of queued RCU callbacks.  The caller must hold the leaf rcu_node
  * structure's ->lock.
  */
 static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp)
 {
         raw_lockdep_assert_held_rcu_node(rnp);
         if (qovld_calc <= 0)
                 return; // Early boot and wildcard value set.
         if (rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc)
                 WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask | rdp->grpmask);
         else
                 WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask & ~rdp->grpmask);
 }
 
 /*
  * Check and if necessary update the leaf rcu_node structure's
  * ->cbovldmask bit corresponding to the current CPU based on that CPU's
  * number of queued RCU callbacks.  No locks need be held, but the
  * caller must have disabled interrupts.
  *
  * Note that this function ignores the possibility that there are a lot
  * of callbacks all of which have already seen the end of their respective
  * grace periods.  This omission is due to the need for no-CBs CPUs to
  * be holding ->nocb_lock to do this check, which is too heavy for a
  * common-case operation.
  */
 static void check_cb_ovld(struct rcu_data *rdp)
 {
         struct rcu_node *const rnp = rdp->mynode;
 
         if (qovld_calc <= 0 ||
             ((rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc) ==
              !!(READ_ONCE(rnp->cbovldmask) & rdp->grpmask)))
                 return; // Early boot wildcard value or already set correctly.
         raw_spin_lock_rcu_node(rnp);
         check_cb_ovld_locked(rdp, rnp);
         raw_spin_unlock_rcu_node(rnp);
 }
 
 static void
 __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
 {
         static atomic_t doublefrees;
         unsigned long flags;
         bool lazy;
         struct rcu_data *rdp;
 
         /* Misaligned rcu_head! */
         WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
 
         if (debug_rcu_head_queue(head)) {
                 /*
                  * Probable double call_rcu(), so leak the callback.
                  * Use rcu:rcu_callback trace event to find the previous
                  * time callback was passed to call_rcu().
                  */
                 if (atomic_inc_return(&doublefrees) < 4) {
                         pr_err("%s(): Double-freed CB %p->%pS()!!!  ", __func__, head, head->func);
                         mem_dump_obj(head);
                 }
                 WRITE_ONCE(head->func, rcu_leak_callback);
                 return;
         }
         head->func = func;
         head->next = NULL;
         kasan_record_aux_stack_noalloc(head);
         local_irq_save(flags);
         rdp = this_cpu_ptr(&rcu_data);
         lazy = lazy_in && !rcu_async_should_hurry();
 
         /* Add the callback to our list. */
         if (unlikely(!rcu_segcblist_is_enabled(&rdp->cblist))) {
                 // This can trigger due to call_rcu() from offline CPU:
                 WARN_ON_ONCE(rcu_scheduler_active != RCU_SCHEDULER_INACTIVE);
                 WARN_ON_ONCE(!rcu_is_watching());
                 // Very early boot, before rcu_init().  Initialize if needed
                 // and then drop through to queue the callback.
                 if (rcu_segcblist_empty(&rdp->cblist))
                         rcu_segcblist_init(&rdp->cblist);
         }
 
         check_cb_ovld(rdp);
 
         if (unlikely(rcu_rdp_is_offloaded(rdp)))
                 call_rcu_nocb(rdp, head, func, flags, lazy);
         else
                 call_rcu_core(rdp, head, func, flags);
         local_irq_restore(flags);
 }
 
 #ifdef CONFIG_RCU_LAZY
 static bool enable_rcu_lazy __read_mostly = !IS_ENABLED(CONFIG_RCU_LAZY_DEFAULT_OFF);
 module_param(enable_rcu_lazy, bool, 0444);
 
 /**
  * call_rcu_hurry() - Queue RCU callback for invocation after grace period, and
  * flush all lazy callbacks (including the new one) to the main ->cblist while
  * doing so.
  *
  * @head: structure to be used for queueing the RCU updates.
  * @func: actual callback function to be invoked after the grace period
  *
  * The callback function will be invoked some time after a full grace
  * period elapses, in other words after all pre-existing RCU read-side
  * critical sections have completed.
  *
  * Use this API instead of call_rcu() if you don't want the callback to be
  * invoked after very long periods of time, which can happen on systems without
  * memory pressure and on systems which are lightly loaded or mostly idle.
  * This function will cause callbacks to be invoked sooner than later at the
  * expense of extra power. Other than that, this function is identical to, and
  * reuses call_rcu()'s logic. Refer to call_rcu() for more details about memory
  * ordering and other functionality.
  */
 void call_rcu_hurry(struct rcu_head *head, rcu_callback_t func)
 {
         __call_rcu_common(head, func, false);
 }
 EXPORT_SYMBOL_GPL(call_rcu_hurry);
 #else
 #define enable_rcu_lazy         false
 #endif
 
 /**
  * call_rcu() - Queue an RCU callback for invocation after a grace period.
  * By default the callbacks are 'lazy' and are kept hidden from the main
  * ->cblist to prevent starting of grace periods too soon.
  * If you desire grace periods to start very soon, use call_rcu_hurry().
  *
  * @head: structure to be used for queueing the RCU updates.
  * @func: actual callback function to be invoked after the grace period
  *
  * The callback function will be invoked some time after a full grace
  * period elapses, in other words after all pre-existing RCU read-side
  * critical sections have completed.  However, the callback function
  * might well execute concurrently with RCU read-side critical sections
  * that started after call_rcu() was invoked.
  *
  * RCU read-side critical sections are delimited by rcu_read_lock()
  * and rcu_read_unlock(), and may be nested.  In addition, but only in
  * v5.0 and later, regions of code across which interrupts, preemption,
  * or softirqs have been disabled also serve as RCU read-side critical
  * sections.  This includes hardware interrupt handlers, softirq handlers,
  * and NMI handlers.
  *
  * Note that all CPUs must agree that the grace period extended beyond
  * all pre-existing RCU read-side critical section.  On systems with more
  * than one CPU, this means that when "func()" is invoked, each CPU is
  * guaranteed to have executed a full memory barrier since the end of its
  * last RCU read-side critical section whose beginning preceded the call
  * to call_rcu().  It also means that each CPU executing an RCU read-side
  * critical section that continues beyond the start of "func()" must have
  * executed a memory barrier after the call_rcu() but before the beginning
  * of that RCU read-side critical section.  Note that these guarantees
  * include CPUs that are offline, idle, or executing in user mode, as
  * well as CPUs that are executing in the kernel.
  *
  * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
  * resulting RCU callback function "func()", then both CPU A and CPU B are
  * guaranteed to execute a full memory barrier during the time interval
  * between the call to call_rcu() and the invocation of "func()" -- even
  * if CPU A and CPU B are the same CPU (but again only if the system has
  * more than one CPU).
  *
  * Implementation of these memory-ordering guarantees is described here:
  * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
  */
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
         __call_rcu_common(head, func, enable_rcu_lazy);
 }
 EXPORT_SYMBOL_GPL(call_rcu);
 
 /* Maximum number of jiffies to wait before draining a batch. */
 #define KFREE_DRAIN_JIFFIES (5 * HZ)
 #define KFREE_N_BATCHES 2
 #define FREE_N_CHANNELS 2
 
 /**
  * struct kvfree_rcu_bulk_data - single block to store kvfree_rcu() pointers
  * @list: List node. All blocks are linked between each other
  * @gp_snap: Snapshot of RCU state for objects placed to this bulk
  * @nr_records: Number of active pointers in the array
  * @records: Array of the kvfree_rcu() pointers
  */
 struct kvfree_rcu_bulk_data {
         struct list_head list;
         struct rcu_gp_oldstate gp_snap;
         unsigned long nr_records;
         void *records[];
 };
 
 /*
  * This macro defines how many entries the "records" array
  * will contain. It is based on the fact that the size of
  * kvfree_rcu_bulk_data structure becomes exactly one page.
  */
 #define KVFREE_BULK_MAX_ENTR \
         ((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *))
 
 /**
  * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
  * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
  * @head_free: List of kfree_rcu() objects waiting for a grace period
  * @head_free_gp_snap: Grace-period snapshot to check for attempted premature frees.
  * @bulk_head_free: Bulk-List of kvfree_rcu() objects waiting for a grace period
  * @krcp: Pointer to @kfree_rcu_cpu structure
  */
 
 struct kfree_rcu_cpu_work {
         struct rcu_work rcu_work;
         struct rcu_head *head_free;
         struct rcu_gp_oldstate head_free_gp_snap;
         struct list_head bulk_head_free[FREE_N_CHANNELS];
         struct kfree_rcu_cpu *krcp;
 };
 
 /**
  * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
  * @head: List of kfree_rcu() objects not yet waiting for a grace period
  * @head_gp_snap: Snapshot of RCU state for objects placed to "@head"
  * @bulk_head: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
  * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
  * @lock: Synchronize access to this structure
  * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
  * @initialized: The @rcu_work fields have been initialized
  * @head_count: Number of objects in rcu_head singular list
  * @bulk_count: Number of objects in bulk-list
  * @bkvcache:
  *      A simple cache list that contains objects for reuse purpose.
  *      In order to save some per-cpu space the list is singular.
  *      Even though it is lockless an access has to be protected by the
  *      per-cpu lock.
  * @page_cache_work: A work to refill the cache when it is empty
  * @backoff_page_cache_fill: Delay cache refills
  * @work_in_progress: Indicates that page_cache_work is running
  * @hrtimer: A hrtimer for scheduling a page_cache_work
  * @nr_bkv_objs: number of allocated objects at @bkvcache.
  *
  * This is a per-CPU structure.  The reason that it is not included in
  * the rcu_data structure is to permit this code to be extracted from
  * the RCU files.  Such extraction could allow further optimization of
  * the interactions with the slab allocators.
  */
 struct kfree_rcu_cpu {
         // Objects queued on a linked list
         // through their rcu_head structures.
         struct rcu_head *head;
         unsigned long head_gp_snap;
         atomic_t head_count;
 
         // Objects queued on a bulk-list.
         struct list_head bulk_head[FREE_N_CHANNELS];
         atomic_t bulk_count[FREE_N_CHANNELS];
 
         struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
         raw_spinlock_t lock;
         struct delayed_work monitor_work;
         bool initialized;
 
         struct delayed_work page_cache_work;
         atomic_t backoff_page_cache_fill;
         atomic_t work_in_progress;
         struct hrtimer hrtimer;
 
         struct llist_head bkvcache;
         int nr_bkv_objs;
 };
 
 static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
         .lock = __RAW_SPIN_LOCK_UNLOCKED(krc.lock),
 };
 
 static __always_inline void
 debug_rcu_bhead_unqueue(struct kvfree_rcu_bulk_data *bhead)
 {
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
         int i;
 
         for (i = 0; i < bhead->nr_records; i++)
                 debug_rcu_head_unqueue((struct rcu_head *)(bhead->records[i]));
 #endif
 }
 
 static inline struct kfree_rcu_cpu *
 krc_this_cpu_lock(unsigned long *flags)
 {
         struct kfree_rcu_cpu *krcp;
 
         local_irq_save(*flags); // For safely calling this_cpu_ptr().
         krcp = this_cpu_ptr(&krc);
         raw_spin_lock(&krcp->lock);
 
         return krcp;
 }
 
 static inline void
 krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
 {
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
 static inline struct kvfree_rcu_bulk_data *
 get_cached_bnode(struct kfree_rcu_cpu *krcp)
 {
         if (!krcp->nr_bkv_objs)
                 return NULL;
 
         WRITE_ONCE(krcp->nr_bkv_objs, krcp->nr_bkv_objs - 1);
         return (struct kvfree_rcu_bulk_data *)
                 llist_del_first(&krcp->bkvcache);
 }
 
 static inline bool
 put_cached_bnode(struct kfree_rcu_cpu *krcp,
         struct kvfree_rcu_bulk_data *bnode)
 {
         // Check the limit.
         if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
                 return false;
 
         llist_add((struct llist_node *) bnode, &krcp->bkvcache);
         WRITE_ONCE(krcp->nr_bkv_objs, krcp->nr_bkv_objs + 1);
         return true;
 }
 
 static int
 drain_page_cache(struct kfree_rcu_cpu *krcp)
 {
         unsigned long flags;
         struct llist_node *page_list, *pos, *n;
         int freed = 0;
 
         if (!rcu_min_cached_objs)
                 return 0;
 
         raw_spin_lock_irqsave(&krcp->lock, flags);
         page_list = llist_del_all(&krcp->bkvcache);
         WRITE_ONCE(krcp->nr_bkv_objs, 0);
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
         llist_for_each_safe(pos, n, page_list) {
                 free_page((unsigned long)pos);
                 freed++;
         }
 
         return freed;
 }
 
 static void
 kvfree_rcu_bulk(struct kfree_rcu_cpu *krcp,
         struct kvfree_rcu_bulk_data *bnode, int idx)
 {
         unsigned long flags;
         int i;
 
         if (!WARN_ON_ONCE(!poll_state_synchronize_rcu_full(&bnode->gp_snap))) {
                 debug_rcu_bhead_unqueue(bnode);
                 rcu_lock_acquire(&rcu_callback_map);
                 if (idx == 0) { // kmalloc() / kfree().
                         trace_rcu_invoke_kfree_bulk_callback(
                                 rcu_state.name, bnode->nr_records,
                                 bnode->records);
 
                         kfree_bulk(bnode->nr_records, bnode->records);
                 } else { // vmalloc() / vfree().
                         for (i = 0; i < bnode->nr_records; i++) {
                                 trace_rcu_invoke_kvfree_callback(
                                         rcu_state.name, bnode->records[i], 0);
 
                                 vfree(bnode->records[i]);
                         }
                 }
                 rcu_lock_release(&rcu_callback_map);
         }
 
         raw_spin_lock_irqsave(&krcp->lock, flags);
         if (put_cached_bnode(krcp, bnode))
                 bnode = NULL;
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
         if (bnode)
                 free_page((unsigned long) bnode);
 
         cond_resched_tasks_rcu_qs();
 }
 
 static void
 kvfree_rcu_list(struct rcu_head *head)
 {
         struct rcu_head *next;
 
         for (; head; head = next) {
                 void *ptr = (void *) head->func;
                 unsigned long offset = (void *) head - ptr;
 
                 next = head->next;
                 debug_rcu_head_unqueue((struct rcu_head *)ptr);
                 rcu_lock_acquire(&rcu_callback_map);
                 trace_rcu_invoke_kvfree_callback(rcu_state.name, head, offset);
 
                 if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset)))
                         kvfree(ptr);
 
                 rcu_lock_release(&rcu_callback_map);
                 cond_resched_tasks_rcu_qs();
         }
 }
 
 /*
  * This function is invoked in workqueue context after a grace period.
  * It frees all the objects queued on ->bulk_head_free or ->head_free.
  */
 static void kfree_rcu_work(struct work_struct *work)
 {
         unsigned long flags;
         struct kvfree_rcu_bulk_data *bnode, *n;
         struct list_head bulk_head[FREE_N_CHANNELS];
         struct rcu_head *head;
         struct kfree_rcu_cpu *krcp;
         struct kfree_rcu_cpu_work *krwp;
         struct rcu_gp_oldstate head_gp_snap;
         int i;
 
         krwp = container_of(to_rcu_work(work),
                 struct kfree_rcu_cpu_work, rcu_work);
         krcp = krwp->krcp;
 
         raw_spin_lock_irqsave(&krcp->lock, flags);
         // Channels 1 and 2.
         for (i = 0; i < FREE_N_CHANNELS; i++)
                 list_replace_init(&krwp->bulk_head_free[i], &bulk_head[i]);
 
         // Channel 3.
         head = krwp->head_free;
         krwp->head_free = NULL;
         head_gp_snap = krwp->head_free_gp_snap;
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
         // Handle the first two channels.
         for (i = 0; i < FREE_N_CHANNELS; i++) {
                 // Start from the tail page, so a GP is likely passed for it.
                 list_for_each_entry_safe(bnode, n, &bulk_head[i], list)
                         kvfree_rcu_bulk(krcp, bnode, i);
         }
 
         /*
          * This is used when the "bulk" path can not be used for the
          * double-argument of kvfree_rcu().  This happens when the
          * page-cache is empty, which means that objects are instead
          * queued on a linked list through their rcu_head structures.
          * This list is named "Channel 3".
          */
         if (head && !WARN_ON_ONCE(!poll_state_synchronize_rcu_full(&head_gp_snap)))
                 kvfree_rcu_list(head);
 }
 
 static bool
 need_offload_krc(struct kfree_rcu_cpu *krcp)
 {
         int i;
 
         for (i = 0; i < FREE_N_CHANNELS; i++)
                 if (!list_empty(&krcp->bulk_head[i]))
                         return true;
 
         return !!READ_ONCE(krcp->head);
 }
 
 static bool
 need_wait_for_krwp_work(struct kfree_rcu_cpu_work *krwp)
 {
         int i;
 
         for (i = 0; i < FREE_N_CHANNELS; i++)
                 if (!list_empty(&krwp->bulk_head_free[i]))
                         return true;
 
         return !!krwp->head_free;
 }
 
 static int krc_count(struct kfree_rcu_cpu *krcp)
 {
         int sum = atomic_read(&krcp->head_count);
         int i;
 
         for (i = 0; i < FREE_N_CHANNELS; i++)
                 sum += atomic_read(&krcp->bulk_count[i]);
 
         return sum;
 }
 
 static void
 schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
 {
         long delay, delay_left;
 
         delay = krc_count(krcp) >= KVFREE_BULK_MAX_ENTR ? 1:KFREE_DRAIN_JIFFIES;
         if (delayed_work_pending(&krcp->monitor_work)) {
                 delay_left = krcp->monitor_work.timer.expires - jiffies;
                 if (delay < delay_left)
                         mod_delayed_work(system_wq, &krcp->monitor_work, delay);
                 return;
         }
         queue_delayed_work(system_wq, &krcp->monitor_work, delay);
 }
 
 static void
 kvfree_rcu_drain_ready(struct kfree_rcu_cpu *krcp)
 {
         struct list_head bulk_ready[FREE_N_CHANNELS];
         struct kvfree_rcu_bulk_data *bnode, *n;
         struct rcu_head *head_ready = NULL;
         unsigned long flags;
         int i;
 
         raw_spin_lock_irqsave(&krcp->lock, flags);
         for (i = 0; i < FREE_N_CHANNELS; i++) {
                 INIT_LIST_HEAD(&bulk_ready[i]);
 
                 list_for_each_entry_safe_reverse(bnode, n, &krcp->bulk_head[i], list) {
                         if (!poll_state_synchronize_rcu_full(&bnode->gp_snap))
                                 break;
 
                         atomic_sub(bnode->nr_records, &krcp->bulk_count[i]);
                         list_move(&bnode->list, &bulk_ready[i]);
                 }
         }
 
         if (krcp->head && poll_state_synchronize_rcu(krcp->head_gp_snap)) {
                 head_ready = krcp->head;
                 atomic_set(&krcp->head_count, 0);
                 WRITE_ONCE(krcp->head, NULL);
         }
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
         for (i = 0; i < FREE_N_CHANNELS; i++) {
                 list_for_each_entry_safe(bnode, n, &bulk_ready[i], list)
                         kvfree_rcu_bulk(krcp, bnode, i);
         }
 
         if (head_ready)
                 kvfree_rcu_list(head_ready);
 }
 
 /*
  * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
  */
 static void kfree_rcu_monitor(struct work_struct *work)
 {
         struct kfree_rcu_cpu *krcp = container_of(work,
                 struct kfree_rcu_cpu, monitor_work.work);
         unsigned long flags;
         int i, j;
 
         // Drain ready for reclaim.
         kvfree_rcu_drain_ready(krcp);
 
         raw_spin_lock_irqsave(&krcp->lock, flags);
 
         // Attempt to start a new batch.
         for (i = 0; i < KFREE_N_BATCHES; i++) {
                 struct kfree_rcu_cpu_work *krwp = &(krcp->krw_arr[i]);
 
                 // Try to detach bulk_head or head and attach it, only when
                 // all channels are free.  Any channel is not free means at krwp
                 // there is on-going rcu work to handle krwp's free business.
                 if (need_wait_for_krwp_work(krwp))
                         continue;
 
                 // kvfree_rcu_drain_ready() might handle this krcp, if so give up.
                 if (need_offload_krc(krcp)) {
                         // Channel 1 corresponds to the SLAB-pointer bulk path.
                         // Channel 2 corresponds to vmalloc-pointer bulk path.
                         for (j = 0; j < FREE_N_CHANNELS; j++) {
                                 if (list_empty(&krwp->bulk_head_free[j])) {
                                         atomic_set(&krcp->bulk_count[j], 0);
                                         list_replace_init(&krcp->bulk_head[j],
                                                 &krwp->bulk_head_free[j]);
                                 }
                         }
 
                         // Channel 3 corresponds to both SLAB and vmalloc
                         // objects queued on the linked list.
                         if (!krwp->head_free) {
                                 krwp->head_free = krcp->head;
                                 get_state_synchronize_rcu_full(&krwp->head_free_gp_snap);
                                 atomic_set(&krcp->head_count, 0);
                                 WRITE_ONCE(krcp->head, NULL);
                         }
 
                         // One work is per one batch, so there are three
                         // "free channels", the batch can handle. It can
                         // be that the work is in the pending state when
                         // channels have been detached following by each
                         // other.
                         queue_rcu_work(system_wq, &krwp->rcu_work);
                 }
         }
 
         raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
         // If there is nothing to detach, it means that our job is
         // successfully done here. In case of having at least one
         // of the channels that is still busy we should rearm the
         // work to repeat an attempt. Because previous batches are
         // still in progress.
         if (need_offload_krc(krcp))
                 schedule_delayed_monitor_work(krcp);
 }
 
 static enum hrtimer_restart
 schedule_page_work_fn(struct hrtimer *t)
 {
         struct kfree_rcu_cpu *krcp =
                 container_of(t, struct kfree_rcu_cpu, hrtimer);
 
         queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, 0);
         return HRTIMER_NORESTART;
 }
 
 static void fill_page_cache_func(struct work_struct *work)
 {
         struct kvfree_rcu_bulk_data *bnode;
         struct kfree_rcu_cpu *krcp =
                 container_of(work, struct kfree_rcu_cpu,
                         page_cache_work.work);
         unsigned long flags;
         int nr_pages;
         bool pushed;
         int i;
 
         nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
                 1 : rcu_min_cached_objs;
 
         for (i = READ_ONCE(krcp->nr_bkv_objs); i < nr_pages; i++) {
                 bnode = (struct kvfree_rcu_bulk_data *)
                         __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
 
                 if (!bnode)
                         break;
 
                 raw_spin_lock_irqsave(&krcp->lock, flags);
                 pushed = put_cached_bnode(krcp, bnode);
                 raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
                 if (!pushed) {
                         free_page((unsigned long) bnode);
                         break;
                 }
         }
 
         atomic_set(&krcp->work_in_progress, 0);
         atomic_set(&krcp->backoff_page_cache_fill, 0);
 }
 
 static void
 run_page_cache_worker(struct kfree_rcu_cpu *krcp)
 {
         // If cache disabled, bail out.
         if (!rcu_min_cached_objs)
                 return;
 
         if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
                         !atomic_xchg(&krcp->work_in_progress, 1)) {
                 if (atomic_read(&krcp->backoff_page_cache_fill)) {
                         queue_delayed_work(system_wq,
                                 &krcp->page_cache_work,
                                         msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
                 } else {
                         hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
                         krcp->hrtimer.function = schedule_page_work_fn;
                         hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
                 }
         }
 }
 
 // Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
 // state specified by flags.  If can_alloc is true, the caller must
 // be schedulable and not be holding any locks or mutexes that might be
 // acquired by the memory allocator or anything that it might invoke.
 // Returns true if ptr was successfully recorded, else the caller must
 // use a fallback.
 static inline bool
 add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
         unsigned long *flags, void *ptr, bool can_alloc)
 {
         struct kvfree_rcu_bulk_data *bnode;
         int idx;
 
         *krcp = krc_this_cpu_lock(flags);
         if (unlikely(!(*krcp)->initialized))
                 return false;
 
         idx = !!is_vmalloc_addr(ptr);
         bnode = list_first_entry_or_null(&(*krcp)->bulk_head[idx],
                 struct kvfree_rcu_bulk_data, list);
 
         /* Check if a new block is required. */
         if (!bnode || bnode->nr_records == KVFREE_BULK_MAX_ENTR) {
                 bnode = get_cached_bnode(*krcp);
                 if (!bnode && can_alloc) {
                         krc_this_cpu_unlock(*krcp, *flags);
 
                         // __GFP_NORETRY - allows a light-weight direct reclaim
                         // what is OK from minimizing of fallback hitting point of
                         // view. Apart of that it forbids any OOM invoking what is
                         // also beneficial since we are about to release memory soon.
                         //
                         // __GFP_NOMEMALLOC - prevents from consuming of all the
                         // memory reserves. Please note we have a fallback path.
                         //
                         // __GFP_NOWARN - it is supposed that an allocation can
                         // be failed under low memory or high memory pressure
                         // scenarios.
                         bnode = (struct kvfree_rcu_bulk_data *)
                                 __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
                         raw_spin_lock_irqsave(&(*krcp)->lock, *flags);
                 }
 
                 if (!bnode)
                         return false;
 
                 // Initialize the new block and attach it.
                 bnode->nr_records = 0;
                 list_add(&bnode->list, &(*krcp)->bulk_head[idx]);
         }
 
         // Finally insert and update the GP for this page.
         bnode->records[bnode->nr_records++] = ptr;
         get_state_synchronize_rcu_full(&bnode->gp_snap);
         atomic_inc(&(*krcp)->bulk_count[idx]);
 
         return true;
 }
 
 /*
  * Queue a request for lazy invocation of the appropriate free routine
  * after a grace period.  Please note that three paths are maintained,
  * two for the common case using arrays of pointers and a third one that
  * is used only when the main paths cannot be used, for example, due to
  * memory pressure.
  *
  * Each kvfree_call_rcu() request is added to a batch. The batch will be drained
  * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch will
  * be free'd in workqueue context. This allows us to: batch requests together to
  * reduce the number of grace periods during heavy kfree_rcu()/kvfree_rcu() load.
  */
 void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 {
         unsigned long flags;
         struct kfree_rcu_cpu *krcp;
         bool success;
 
         /*
          * Please note there is a limitation for the head-less
          * variant, that is why there is a clear rule for such
          * objects: it can be used from might_sleep() context
          * only. For other places please embed an rcu_head to
          * your data.
          */
         if (!head)
                 might_sleep();
 
         // Queue the object but don't yet schedule the batch.
         if (debug_rcu_head_queue(ptr)) {
                 // Probable double kfree_rcu(), just leak.
                 WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
                           __func__, head);
 
                 // Mark as success and leave.
                 return;
         }
 
         kasan_record_aux_stack_noalloc(ptr);
         success = add_ptr_to_bulk_krc_lock(&krcp, &flags, ptr, !head);
         if (!success) {
                 run_page_cache_worker(krcp);
 
                 if (head == NULL)
                         // Inline if kvfree_rcu(one_arg) call.
                         goto unlock_return;
 
                 head->func = ptr;
                 head->next = krcp->head;
                 WRITE_ONCE(krcp->head, head);
                 atomic_inc(&krcp->head_count);
 
                 // Take a snapshot for this krcp.
                 krcp->head_gp_snap = get_state_synchronize_rcu();
                 success = true;
         }
 
         /*
          * The kvfree_rcu() caller considers the pointer freed at this point
          * and likely removes any references to it. Since the actual slab
          * freeing (and kmemleak_free()) is deferred, tell kmemleak to ignore
          * this object (no scanning or false positives reporting).
          */
         kmemleak_ignore(ptr);
 
         // Set timer to drain after KFREE_DRAIN_JIFFIES.
         if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
                 schedule_delayed_monitor_work(krcp);
 
 unlock_return:
         krc_this_cpu_unlock(krcp, flags);
 
         /*
          * Inline kvfree() after synchronize_rcu(). We can do
          * it from might_sleep() context only, so the current
          * CPU can pass the QS state.
          */
         if (!success) {
                 debug_rcu_head_unqueue((struct rcu_head *) ptr);
                 synchronize_rcu();
                 kvfree(ptr);
         }
 }
 EXPORT_SYMBOL_GPL(kvfree_call_rcu);
 
 static unsigned long
 kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
         int cpu;
         unsigned long count = 0;
 
         /* Snapshot count of all CPUs */
         for_each_possible_cpu(cpu) {
                 struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
                 count += krc_count(krcp);
                 count += READ_ONCE(krcp->nr_bkv_objs);
                 atomic_set(&krcp->backoff_page_cache_fill, 1);
         }
 
         return count == 0 ? SHRINK_EMPTY : count;
 }
 
 static unsigned long
 kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 {
         int cpu, freed = 0;
 
         for_each_possible_cpu(cpu) {
                 int count;
                 struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
                 count = krc_count(krcp);
                 count += drain_page_cache(krcp);
                 kfree_rcu_monitor(&krcp->monitor_work.work);
 
                 sc->nr_to_scan -= count;
                 freed += count;
 
                 if (sc->nr_to_scan <= 0)
                         break;
         }
 
         return freed == 0 ? SHRINK_STOP : freed;
 }
 
 void __init kfree_rcu_scheduler_running(void)
 {
         int cpu;
 
         for_each_possible_cpu(cpu) {
                 struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
                 if (need_offload_krc(krcp))
                         schedule_delayed_monitor_work(krcp);
         }
 }
 
 /*
  * During early boot, any blocking grace-period wait automatically
  * implies a grace period.
  *
  * Later on, this could in theory be the case for kernels built with
  * CONFIG_SMP=y && CONFIG_PREEMPTION=y running on a single CPU, but this
  * is not a common case.  Furthermore, this optimization would cause
  * the rcu_gp_oldstate structure to expand by 50%, so this potential
  * grace-period optimization is ignored once the scheduler is running.
  */
 static int rcu_blocking_is_gp(void)
 {
         if (rcu_scheduler_active != RCU_SCHEDULER_INACTIVE) {
                 might_sleep();
                 return false;
         }
         return true;
 }
 
 /*
  * Helper function for the synchronize_rcu() API.
  */
 static void synchronize_rcu_normal(void)
 {
         struct rcu_synchronize rs;
 
         trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("request"));
 
         if (!READ_ONCE(rcu_normal_wake_from_gp)) {
                 wait_rcu_gp(call_rcu_hurry);
                 goto trace_complete_out;
         }
 
         init_rcu_head_on_stack(&rs.head);
         init_completion(&rs.completion);
 
         /*
          * This code might be preempted, therefore take a GP
          * snapshot before adding a request.
          */
         if (IS_ENABLED(CONFIG_PROVE_RCU))
                 rs.head.func = (void *) get_state_synchronize_rcu();
 
         rcu_sr_normal_add_req(&rs);
 
         /* Kick a GP and start waiting. */
         (void) start_poll_synchronize_rcu();
 
         /* Now we can wait. */
         wait_for_completion(&rs.completion);
         destroy_rcu_head_on_stack(&rs.head);
 
 trace_complete_out:
         trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("complete"));
 }
 
 /**
  * synchronize_rcu - wait until a grace period has elapsed.
  *
  * Control will return to the caller some time after a full grace
  * period has elapsed, in other words after all currently executing RCU
  * read-side critical sections have completed.  Note, however, that
  * upon return from synchronize_rcu(), the caller might well be executing
  * concurrently with new RCU read-side critical sections that began while
  * synchronize_rcu() was waiting.
  *
  * RCU read-side critical sections are delimited by rcu_read_lock()
  * and rcu_read_unlock(), and may be nested.  In addition, but only in
  * v5.0 and later, regions of code across which interrupts, preemption,
  * or softirqs have been disabled also serve as RCU read-side critical
  * sections.  This includes hardware interrupt handlers, softirq handlers,
  * and NMI handlers.
  *
  * Note that this guarantee implies further memory-ordering guarantees.
  * On systems with more than one CPU, when synchronize_rcu() returns,
  * each CPU is guaranteed to have executed a full memory barrier since
  * the end of its last RCU read-side critical section whose beginning
  * preceded the call to synchronize_rcu().  In addition, each CPU having
  * an RCU read-side critical section that extends beyond the return from
  * synchronize_rcu() is guaranteed to have executed a full memory barrier
  * after the beginning of synchronize_rcu() and before the beginning of
  * that RCU read-side critical section.  Note that these guarantees include
  * CPUs that are offline, idle, or executing in user mode, as well as CPUs
  * that are executing in the kernel.
  *
  * Furthermore, if CPU A invoked synchronize_rcu(), which returned
  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
  * to have executed a full memory barrier during the execution of
  * synchronize_rcu() -- even if CPU A and CPU B are the same CPU (but
  * again only if the system has more than one CPU).
  *
  * Implementation of these memory-ordering guarantees is described here:
  * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
  */
 void synchronize_rcu(void)
 {
         unsigned long flags;
         struct rcu_node *rnp;
 
         RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
                          lock_is_held(&rcu_lock_map) ||
                          lock_is_held(&rcu_sched_lock_map),
                          "Illegal synchronize_rcu() in RCU read-side critical section");
         if (!rcu_blocking_is_gp()) {
                 if (rcu_gp_is_expedited())
                         synchronize_rcu_expedited();
                 else
                         synchronize_rcu_normal();
                 return;
         }
 
         // Context allows vacuous grace periods.
         // Note well that this code runs with !PREEMPT && !SMP.
         // In addition, all code that advances grace periods runs at
         // process level.  Therefore, this normal GP overlaps with other
         // normal GPs only by being fully nested within them, which allows
         // reuse of ->gp_seq_polled_snap.
         rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_snap);
         rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_snap);
 
         // Update the normal grace-period counters to record
         // this grace period, but only those used by the boot CPU.
         // The rcu_scheduler_starting() will take care of the rest of
         // these counters.
         local_irq_save(flags);
         WARN_ON_ONCE(num_online_cpus() > 1);
         rcu_state.gp_seq += (1 << RCU_SEQ_CTR_SHIFT);
         for (rnp = this_cpu_ptr(&rcu_data)->mynode; rnp; rnp = rnp->parent)
                 rnp->gp_seq_needed = rnp->gp_seq = rcu_state.gp_seq;
         local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
 /**
  * get_completed_synchronize_rcu_full - Return a full pre-completed polled state cookie
  * @rgosp: Place to put state cookie
  *
  * Stores into @rgosp a value that will always be treated by functions
  * like poll_state_synchronize_rcu_full() as a cookie whose grace period
  * has already completed.
  */
 void get_completed_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 {
         rgosp->rgos_norm = RCU_GET_STATE_COMPLETED;
         rgosp->rgos_exp = RCU_GET_STATE_COMPLETED;
 }
 EXPORT_SYMBOL_GPL(get_completed_synchronize_rcu_full);
 
 /**
  * get_state_synchronize_rcu - Snapshot current RCU state
  *
  * Returns a cookie that is used by a later call to cond_synchronize_rcu()
  * or poll_state_synchronize_rcu() to determine whether or not a full
  * grace period has elapsed in the meantime.
  */
 unsigned long get_state_synchronize_rcu(void)
 {
         /*
          * Any prior manipulation of RCU-protected data must happen
          * before the load from ->gp_seq.
          */
         smp_mb();  /* ^^^ */
         return rcu_seq_snap(&rcu_state.gp_seq_polled);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
 
 /**
  * get_state_synchronize_rcu_full - Snapshot RCU state, both normal and expedited
  * @rgosp: location to place combined normal/expedited grace-period state
  *
  * Places the normal and expedited grace-period states in @rgosp.  This
  * state value can be passed to a later call to cond_synchronize_rcu_full()
  * or poll_state_synchronize_rcu_full() to determine whether or not a
  * grace period (whether normal or expedited) has elapsed in the meantime.
  * The rcu_gp_oldstate structure takes up twice the memory of an unsigned
  * long, but is guaranteed to see all grace periods.  In contrast, the
  * combined state occupies less memory, but can sometimes fail to take
  * grace periods into account.
  *
  * This does not guarantee that the needed grace period will actually
  * start.
  */
 void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 {
         struct rcu_node *rnp = rcu_get_root();
 
         /*
          * Any prior manipulation of RCU-protected data must happen
          * before the loads from ->gp_seq and ->expedited_sequence.
          */
         smp_mb();  /* ^^^ */
         rgosp->rgos_norm = rcu_seq_snap(&rnp->gp_seq);
         rgosp->rgos_exp = rcu_seq_snap(&rcu_state.expedited_sequence);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_rcu_full);
 
 /*
  * Helper function for start_poll_synchronize_rcu() and
  * start_poll_synchronize_rcu_full().
  */
 static void start_poll_synchronize_rcu_common(void)
 {
         unsigned long flags;
         bool needwake;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
 
         lockdep_assert_irqs_enabled();
         local_irq_save(flags);
         rdp = this_cpu_ptr(&rcu_data);
         rnp = rdp->mynode;
         raw_spin_lock_rcu_node(rnp); // irqs already disabled.
         // Note it is possible for a grace period to have elapsed between
         // the above call to get_state_synchronize_rcu() and the below call
         // to rcu_seq_snap.  This is OK, the worst that happens is that we
         // get a grace period that no one needed.  These accesses are ordered
         // by smp_mb(), and we are accessing them in the opposite order
         // from which they are updated at grace-period start, as required.
         needwake = rcu_start_this_gp(rnp, rdp, rcu_seq_snap(&rcu_state.gp_seq));
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         if (needwake)
                 rcu_gp_kthread_wake();
 }
 
 /**
  * start_poll_synchronize_rcu - Snapshot and start RCU grace period
  *
  * Returns a cookie that is used by a later call to cond_synchronize_rcu()
  * or poll_state_synchronize_rcu() to determine whether or not a full
  * grace period has elapsed in the meantime.  If the needed grace period
  * is not already slated to start, notifies RCU core of the need for that
  * grace period.
  *
  * Interrupts must be enabled for the case where it is necessary to awaken
  * the grace-period kthread.
  */
 unsigned long start_poll_synchronize_rcu(void)
 {
         unsigned long gp_seq = get_state_synchronize_rcu();
 
         start_poll_synchronize_rcu_common();
         return gp_seq;
 }
 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);
 
 /**
  * start_poll_synchronize_rcu_full - Take a full snapshot and start RCU grace period
  * @rgosp: value from get_state_synchronize_rcu_full() or start_poll_synchronize_rcu_full()
  *
  * Places the normal and expedited grace-period states in *@rgos.  This
  * state value can be passed to a later call to cond_synchronize_rcu_full()
  * or poll_state_synchronize_rcu_full() to determine whether or not a
  * grace period (whether normal or expedited) has elapsed in the meantime.
  * If the needed grace period is not already slated to start, notifies
  * RCU core of the need for that grace period.
  *
  * Interrupts must be enabled for the case where it is necessary to awaken
  * the grace-period kthread.
  */
 void start_poll_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 {
         get_state_synchronize_rcu_full(rgosp);
 
         start_poll_synchronize_rcu_common();
 }
 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
 
 /**
  * poll_state_synchronize_rcu - Has the specified RCU grace period completed?
  * @oldstate: value from get_state_synchronize_rcu() or start_poll_synchronize_rcu()
  *
  * If a full RCU grace period has elapsed since the earlier call from
  * which @oldstate was obtained, return @true, otherwise return @false.
  * If @false is returned, it is the caller's responsibility to invoke this
  * function later on until it does return @true.  Alternatively, the caller
  * can explicitly wait for a grace period, for example, by passing @oldstate
  * to either cond_synchronize_rcu() or cond_synchronize_rcu_expedited()
  * on the one hand or by directly invoking either synchronize_rcu() or
  * synchronize_rcu_expedited() on the other.
  *
  * Yes, this function does not take counter wrap into account.
  * But counter wrap is harmless.  If the counter wraps, we have waited for
  * more than a billion grace periods (and way more on a 64-bit system!).
  * Those needing to keep old state values for very long time periods
  * (many hours even on 32-bit systems) should check them occasionally and
  * either refresh them or set a flag indicating that the grace period has
  * completed.  Alternatively, they can use get_completed_synchronize_rcu()
  * to get a guaranteed-completed grace-period state.
  *
  * In addition, because oldstate compresses the grace-period state for
  * both normal and expedited grace periods into a single unsigned long,
  * it can miss a grace period when synchronize_rcu() runs concurrently
  * with synchronize_rcu_expedited().  If this is unacceptable, please
  * instead use the _full() variant of these polling APIs.
  *
  * This function provides the same memory-ordering guarantees that
  * would be provided by a synchronize_rcu() that was invoked at the call
  * to the function that provided @oldstate, and that returned at the end
  * of this function.
  */
 bool poll_state_synchronize_rcu(unsigned long oldstate)
 {
         if (oldstate == RCU_GET_STATE_COMPLETED ||
             rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
                 smp_mb(); /* Ensure GP ends before subsequent accesses. */
                 return true;
         }
         return false;
 }
 EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
 
 /**
  * poll_state_synchronize_rcu_full - Has the specified RCU grace period completed?
  * @rgosp: value from get_state_synchronize_rcu_full() or start_poll_synchronize_rcu_full()
  *
  * If a full RCU grace period has elapsed since the earlier call from
  * which *rgosp was obtained, return @true, otherwise return @false.
  * If @false is returned, it is the caller's responsibility to invoke this
  * function later on until it does return @true.  Alternatively, the caller
  * can explicitly wait for a grace period, for example, by passing @rgosp
  * to cond_synchronize_rcu() or by directly invoking synchronize_rcu().
  *
  * Yes, this function does not take counter wrap into account.
  * But counter wrap is harmless.  If the counter wraps, we have waited
  * for more than a billion grace periods (and way more on a 64-bit
  * system!).  Those needing to keep rcu_gp_oldstate values for very
  * long time periods (many hours even on 32-bit systems) should check
  * them occasionally and either refresh them or set a flag indicating
  * that the grace period has completed.  Alternatively, they can use
  * get_completed_synchronize_rcu_full() to get a guaranteed-completed
  * grace-period state.
  *
  * This function provides the same memory-ordering guarantees that would
  * be provided by a synchronize_rcu() that was invoked at the call to
  * the function that provided @rgosp, and that returned at the end of this
  * function.  And this guarantee requires that the root rcu_node structure's
  * ->gp_seq field be checked instead of that of the rcu_state structure.
  * The problem is that the just-ending grace-period's callbacks can be
  * invoked between the time that the root rcu_node structure's ->gp_seq
  * field is updated and the time that the rcu_state structure's ->gp_seq
  * field is updated.  Therefore, if a single synchronize_rcu() is to
  * cause a subsequent poll_state_synchronize_rcu_full() to return @true,
  * then the root rcu_node structure is the one that needs to be polled.
  */
 bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 {
         struct rcu_node *rnp = rcu_get_root();
 
         smp_mb(); // Order against root rcu_node structure grace-period cleanup.
         if (rgosp->rgos_norm == RCU_GET_STATE_COMPLETED ||
             rcu_seq_done_exact(&rnp->gp_seq, rgosp->rgos_norm) ||
             rgosp->rgos_exp == RCU_GET_STATE_COMPLETED ||
             rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
                 smp_mb(); /* Ensure GP ends before subsequent accesses. */
                 return true;
         }
         return false;
 }
 EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu_full);
 
 /**
  * cond_synchronize_rcu - Conditionally wait for an RCU grace period
  * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
  *
  * If a full RCU grace period has elapsed since the earlier call to
  * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return.
  * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
  *
  * Yes, this function does not take counter wrap into account.
  * But counter wrap is harmless.  If the counter wraps, we have waited for
  * more than 2 billion grace periods (and way more on a 64-bit system!),
  * so waiting for a couple of additional grace periods should be just fine.
  *
  * This function provides the same memory-ordering guarantees that
  * would be provided by a synchronize_rcu() that was invoked at the call
  * to the function that provided @oldstate and that returned at the end
  * of this function.
  */
 void cond_synchronize_rcu(unsigned long oldstate)
 {
         if (!poll_state_synchronize_rcu(oldstate))
                 synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
 
 /**
  * cond_synchronize_rcu_full - Conditionally wait for an RCU grace period
  * @rgosp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
  *
  * If a full RCU grace period has elapsed since the call to
  * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
  * or start_poll_synchronize_rcu_expedited_full() from which @rgosp was
  * obtained, just return.  Otherwise, invoke synchronize_rcu() to wait
  * for a full grace period.
  *
  * Yes, this function does not take counter wrap into account.
  * But counter wrap is harmless.  If the counter wraps, we have waited for
  * more than 2 billion grace periods (and way more on a 64-bit system!),
  * so waiting for a couple of additional grace periods should be just fine.
  *
  * This function provides the same memory-ordering guarantees that
  * would be provided by a synchronize_rcu() that was invoked at the call
  * to the function that provided @rgosp and that returned at the end of
  * this function.
  */
 void cond_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 {
         if (!poll_state_synchronize_rcu_full(rgosp))
                 synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu_full);
 
 /*
  * Check to see if there is any immediate RCU-related work to be done by
  * the current CPU, returning 1 if so and zero otherwise.  The checks are
  * in order of increasing expense: checks that can be carried out against
  * CPU-local state are performed first.  However, we must check for CPU
  * stalls first, else we might not get a chance.
  */
 static int rcu_pending(int user)
 {
         bool gp_in_progress;
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
         struct rcu_node *rnp = rdp->mynode;
 
         lockdep_assert_irqs_disabled();
 
         /* Check for CPU stalls, if enabled. */
         check_cpu_stall(rdp);
 
         /* Does this CPU need a deferred NOCB wakeup? */
         if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE))
                 return 1;
 
         /* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
         gp_in_progress = rcu_gp_in_progress();
         if ((user || rcu_is_cpu_rrupt_from_idle() ||
              (gp_in_progress &&
               time_before(jiffies, READ_ONCE(rcu_state.gp_start) +
                           nohz_full_patience_delay_jiffies))) &&
             rcu_nohz_full_cpu())
                 return 0;
 
         /* Is the RCU core waiting for a quiescent state from this CPU? */
         if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm && gp_in_progress)
                 return 1;
 
         /* Does this CPU have callbacks ready to invoke? */
         if (!rcu_rdp_is_offloaded(rdp) &&
             rcu_segcblist_ready_cbs(&rdp->cblist))
                 return 1;
 
         /* Has RCU gone idle with this CPU needing another grace period? */
         if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
             !rcu_rdp_is_offloaded(rdp) &&
             !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
                 return 1;
 
         /* Have RCU grace period completed or started?  */
         if (rcu_seq_current(&rnp->gp_seq) != rdp->gp_seq ||
             unlikely(READ_ONCE(rdp->gpwrap))) /* outside lock */
                 return 1;
 
         /* nothing to do */
         return 0;
 }
 
 /*
  * Helper function for rcu_barrier() tracing.  If tracing is disabled,
  * the compiler is expected to optimize this away.
  */
 static void rcu_barrier_trace(const char *s, int cpu, unsigned long done)
 {
         trace_rcu_barrier(rcu_state.name, s, cpu,
                           atomic_read(&rcu_state.barrier_cpu_count), done);
 }
 
 /*
  * RCU callback function for rcu_barrier().  If we are last, wake
  * up the task executing rcu_barrier().
  *
  * Note that the value of rcu_state.barrier_sequence must be captured
  * before the atomic_dec_and_test().  Otherwise, if this CPU is not last,
  * other CPUs might count the value down to zero before this CPU gets
  * around to invoking rcu_barrier_trace(), which might result in bogus
  * data from the next instance of rcu_barrier().
  */
 static void rcu_barrier_callback(struct rcu_head *rhp)
 {
         unsigned long __maybe_unused s = rcu_state.barrier_sequence;
 
         if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
                 rcu_barrier_trace(TPS("LastCB"), -1, s);
                 complete(&rcu_state.barrier_completion);
         } else {
                 rcu_barrier_trace(TPS("CB"), -1, s);
         }
 }
 
 /*
  * If needed, entrain an rcu_barrier() callback on rdp->cblist.
  */
 static void rcu_barrier_entrain(struct rcu_data *rdp)
 {
         unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
         unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
         bool wake_nocb = false;
         bool was_alldone = false;
 
         lockdep_assert_held(&rcu_state.barrier_lock);
         if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
                 return;
         rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
         rdp->barrier_head.func = rcu_barrier_callback;
         debug_rcu_head_queue(&rdp->barrier_head);
         rcu_nocb_lock(rdp);
         /*
          * Flush bypass and wakeup rcuog if we add callbacks to an empty regular
          * queue. This way we don't wait for bypass timer that can reach seconds
          * if it's fully lazy.
          */
         was_alldone = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist);
         WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
         wake_nocb = was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist);
         if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
                 atomic_inc(&rcu_state.barrier_cpu_count);
         } else {
                 debug_rcu_head_unqueue(&rdp->barrier_head);
                 rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
         }
         rcu_nocb_unlock(rdp);
         if (wake_nocb)
                 wake_nocb_gp(rdp, false);
         smp_store_release(&rdp->barrier_seq_snap, gseq);
 }
 
 /*
  * Called with preemption disabled, and from cross-cpu IRQ context.
  */
 static void rcu_barrier_handler(void *cpu_in)
 {
         uintptr_t cpu = (uintptr_t)cpu_in;
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
         lockdep_assert_irqs_disabled();
         WARN_ON_ONCE(cpu != rdp->cpu);
         WARN_ON_ONCE(cpu != smp_processor_id());
         raw_spin_lock(&rcu_state.barrier_lock);
         rcu_barrier_entrain(rdp);
         raw_spin_unlock(&rcu_state.barrier_lock);
 }
 
 /**
  * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
  *
  * Note that this primitive does not necessarily wait for an RCU grace period
  * to complete.  For example, if there are no RCU callbacks queued anywhere
  * in the system, then rcu_barrier() is within its rights to return
  * immediately, without waiting for anything, much less an RCU grace period.
  */
 void rcu_barrier(void)
 {
         uintptr_t cpu;
         unsigned long flags;
         unsigned long gseq;
         struct rcu_data *rdp;
         unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
 
         rcu_barrier_trace(TPS("Begin"), -1, s);
 
         /* Take mutex to serialize concurrent rcu_barrier() requests. */
         mutex_lock(&rcu_state.barrier_mutex);
 
         /* Did someone else do our work for us? */
         if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
                 rcu_barrier_trace(TPS("EarlyExit"), -1, rcu_state.barrier_sequence);
                 smp_mb(); /* caller's subsequent code after above check. */
                 mutex_unlock(&rcu_state.barrier_mutex);
                 return;
         }
 
         /* Mark the start of the barrier operation. */
         raw_spin_lock_irqsave(&rcu_state.barrier_lock, flags);
         rcu_seq_start(&rcu_state.barrier_sequence);
         gseq = rcu_state.barrier_sequence;
         rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
 
         /*
          * Initialize the count to two rather than to zero in order
          * to avoid a too-soon return to zero in case of an immediate
          * invocation of the just-enqueued callback (or preemption of
          * this task).  Exclude CPU-hotplug operations to ensure that no
          * offline non-offloaded CPU has callbacks queued.
          */
         init_completion(&rcu_state.barrier_completion);
         atomic_set(&rcu_state.barrier_cpu_count, 2);
         raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
 
         /*
          * Force each CPU with callbacks to register a new callback.
          * When that callback is invoked, we will know that all of the
          * corresponding CPU's preceding callbacks have been invoked.
          */
         for_each_possible_cpu(cpu) {
                 rdp = per_cpu_ptr(&rcu_data, cpu);
 retry:
                 if (smp_load_acquire(&rdp->barrier_seq_snap) == gseq)
                         continue;
                 raw_spin_lock_irqsave(&rcu_state.barrier_lock, flags);
                 if (!rcu_segcblist_n_cbs(&rdp->cblist)) {
                         WRITE_ONCE(rdp->barrier_seq_snap, gseq);
                         raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
                         rcu_barrier_trace(TPS("NQ"), cpu, rcu_state.barrier_sequence);
                         continue;
                 }
                 if (!rcu_rdp_cpu_online(rdp)) {
                         rcu_barrier_entrain(rdp);
                         WARN_ON_ONCE(READ_ONCE(rdp->barrier_seq_snap) != gseq);
                         raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
                         rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu, rcu_state.barrier_sequence);
                         continue;
                 }
                 raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
                 if (smp_call_function_single(cpu, rcu_barrier_handler, (void *)cpu, 1)) {
                         schedule_timeout_uninterruptible(1);
                         goto retry;
                 }
                 WARN_ON_ONCE(READ_ONCE(rdp->barrier_seq_snap) != gseq);
                 rcu_barrier_trace(TPS("OnlineQ"), cpu, rcu_state.barrier_sequence);
         }
 
         /*
          * Now that we have an rcu_barrier_callback() callback on each
          * CPU, and thus each counted, remove the initial count.
          */
         if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
                 complete(&rcu_state.barrier_completion);
 
         /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
         wait_for_completion(&rcu_state.barrier_completion);
 
         /* Mark the end of the barrier operation. */
         rcu_barrier_trace(TPS("Inc2"), -1, rcu_state.barrier_sequence);
         rcu_seq_end(&rcu_state.barrier_sequence);
         gseq = rcu_state.barrier_sequence;
         for_each_possible_cpu(cpu) {
                 rdp = per_cpu_ptr(&rcu_data, cpu);
 
                 WRITE_ONCE(rdp->barrier_seq_snap, gseq);
         }
 
         /* Other rcu_barrier() invocations can now safely proceed. */
         mutex_unlock(&rcu_state.barrier_mutex);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier);
 
 static unsigned long rcu_barrier_last_throttle;
 
 /**
  * rcu_barrier_throttled - Do rcu_barrier(), but limit to one per second
  *
  * This can be thought of as guard rails around rcu_barrier() that
  * permits unrestricted userspace use, at least assuming the hardware's
  * try_cmpxchg() is robust.  There will be at most one call per second to
  * rcu_barrier() system-wide from use of this function, which means that
  * callers might needlessly wait a second or three.
  *
  * This is intended for use by test suites to avoid OOM by flushing RCU
  * callbacks from the previous test before starting the next.  See the
  * rcutree.do_rcu_barrier module parameter for more information.
  *
  * Why not simply make rcu_barrier() more scalable?  That might be
  * the eventual endpoint, but let's keep it simple for the time being.
  * Note that the module parameter infrastructure serializes calls to a
  * given .set() function, but should concurrent .set() invocation ever be
  * possible, we are ready!
  */
 static void rcu_barrier_throttled(void)
 {
         unsigned long j = jiffies;
         unsigned long old = READ_ONCE(rcu_barrier_last_throttle);
         unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
 
         while (time_in_range(j, old, old + HZ / 16) ||
                !try_cmpxchg(&rcu_barrier_last_throttle, &old, j)) {
                 schedule_timeout_idle(HZ / 16);
                 if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
                         smp_mb(); /* caller's subsequent code after above check. */
                         return;
                 }
                 j = jiffies;
                 old = READ_ONCE(rcu_barrier_last_throttle);
         }
         rcu_barrier();
 }
 
 /*
  * Invoke rcu_barrier_throttled() when a rcutree.do_rcu_barrier
  * request arrives.  We insist on a true value to allow for possible
  * future expansion.
  */
 static int param_set_do_rcu_barrier(const char *val, const struct kernel_param *kp)
 {
         bool b;
         int ret;
 
         if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING)
                 return -EAGAIN;
         ret = kstrtobool(val, &b);
         if (!ret && b) {
                 atomic_inc((atomic_t *)kp->arg);
                 rcu_barrier_throttled();
                 atomic_dec((atomic_t *)kp->arg);
         }
         return ret;
 }
 
 /*
  * Output the number of outstanding rcutree.do_rcu_barrier requests.
  */
 static int param_get_do_rcu_barrier(char *buffer, const struct kernel_param *kp)
 {
         return sprintf(buffer, "%d\n", atomic_read((atomic_t *)kp->arg));
 }
 
 static const struct kernel_param_ops do_rcu_barrier_ops = {
         .set = param_set_do_rcu_barrier,
         .get = param_get_do_rcu_barrier,
 };
 static atomic_t do_rcu_barrier;
 module_param_cb(do_rcu_barrier, &do_rcu_barrier_ops, &do_rcu_barrier, 0644);
 
 /*
  * Compute the mask of online CPUs for the specified rcu_node structure.
  * This will not be stable unless the rcu_node structure's ->lock is
  * held, but the bit corresponding to the current CPU will be stable
  * in most contexts.
  */
 static unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp)
 {
         return READ_ONCE(rnp->qsmaskinitnext);
 }
 
 /*
  * Is the CPU corresponding to the specified rcu_data structure online
  * from RCU's perspective?  This perspective is given by that structure's
  * ->qsmaskinitnext field rather than by the global cpu_online_mask.
  */
 static bool rcu_rdp_cpu_online(struct rcu_data *rdp)
 {
         return !!(rdp->grpmask & rcu_rnp_online_cpus(rdp->mynode));
 }
 
 bool rcu_cpu_online(int cpu)
 {
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
         return rcu_rdp_cpu_online(rdp);
 }
 
 #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
 
 /*
  * Is the current CPU online as far as RCU is concerned?
  *
  * Disable preemption to avoid false positives that could otherwise
  * happen due to the current CPU number being sampled, this task being
  * preempted, its old CPU being taken offline, resuming on some other CPU,
  * then determining that its old CPU is now offline.
  *
  * Disable checking if in an NMI handler because we cannot safely
  * report errors from NMI handlers anyway.  In addition, it is OK to use
  * RCU on an offline processor during initial boot, hence the check for
  * rcu_scheduler_fully_active.
  */
 bool rcu_lockdep_current_cpu_online(void)
 {
         struct rcu_data *rdp;
         bool ret = false;
 
         if (in_nmi() || !rcu_scheduler_fully_active)
                 return true;
         preempt_disable_notrace();
         rdp = this_cpu_ptr(&rcu_data);
         /*
          * Strictly, we care here about the case where the current CPU is
          * in rcutree_report_cpu_starting() and thus has an excuse for rdp->grpmask
          * not being up to date. So arch_spin_is_locked() might have a
          * false positive if it's held by some *other* CPU, but that's
          * OK because that just means a false *negative* on the warning.
          */
         if (rcu_rdp_cpu_online(rdp) || arch_spin_is_locked(&rcu_state.ofl_lock))
                 ret = true;
         preempt_enable_notrace();
         return ret;
 }
 EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
 
 #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
 
 // Has rcu_init() been invoked?  This is used (for example) to determine
 // whether spinlocks may be acquired safely.
 static bool rcu_init_invoked(void)
 {
         return !!READ_ONCE(rcu_state.n_online_cpus);
 }
 
 /*
  * All CPUs for the specified rcu_node structure have gone offline,
  * and all tasks that were preempted within an RCU read-side critical
  * section while running on one of those CPUs have since exited their RCU
  * read-side critical section.  Some other CPU is reporting this fact with
  * the specified rcu_node structure's ->lock held and interrupts disabled.
  * This function therefore goes up the tree of rcu_node structures,
  * clearing the corresponding bits in the ->qsmaskinit fields.  Note that
  * the leaf rcu_node structure's ->qsmaskinit field has already been
  * updated.
  *
  * This function does check that the specified rcu_node structure has
  * all CPUs offline and no blocked tasks, so it is OK to invoke it
  * prematurely.  That said, invoking it after the fact will cost you
  * a needless lock acquisition.  So once it has done its work, don't
  * invoke it again.
  */
 static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
 {
         long mask;
         struct rcu_node *rnp = rnp_leaf;
 
         raw_lockdep_assert_held_rcu_node(rnp_leaf);
         if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) ||
             WARN_ON_ONCE(rnp_leaf->qsmaskinit) ||
             WARN_ON_ONCE(rcu_preempt_has_tasks(rnp_leaf)))
                 return;
         for (;;) {
                 mask = rnp->grpmask;
                 rnp = rnp->parent;
                 if (!rnp)
                         break;
                 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
                 rnp->qsmaskinit &= ~mask;
                 /* Between grace periods, so better already be zero! */
                 WARN_ON_ONCE(rnp->qsmask);
                 if (rnp->qsmaskinit) {
                         raw_spin_unlock_rcu_node(rnp);
                         /* irqs remain disabled. */
                         return;
                 }
                 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
         }
 }
 
 /*
  * Propagate ->qsinitmask bits up the rcu_node tree to account for the
  * first CPU in a given leaf rcu_node structure coming online.  The caller
  * must hold the corresponding leaf rcu_node ->lock with interrupts
  * disabled.
  */
 static void rcu_init_new_rnp(struct rcu_node *rnp_leaf)
 {
         long mask;
         long oldmask;
         struct rcu_node *rnp = rnp_leaf;
 
         raw_lockdep_assert_held_rcu_node(rnp_leaf);
         WARN_ON_ONCE(rnp->wait_blkd_tasks);
         for (;;) {
                 mask = rnp->grpmask;
                 rnp = rnp->parent;
                 if (rnp == NULL)
                         return;
                 raw_spin_lock_rcu_node(rnp); /* Interrupts already disabled. */
                 oldmask = rnp->qsmaskinit;
                 rnp->qsmaskinit |= mask;
                 raw_spin_unlock_rcu_node(rnp); /* Interrupts remain disabled. */
                 if (oldmask)
                         return;
         }
 }
 
 /*
  * Do boot-time initialization of a CPU's per-CPU RCU data.
  */
 static void __init
 rcu_boot_init_percpu_data(int cpu)
 {
         struct context_tracking *ct = this_cpu_ptr(&context_tracking);
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
         /* Set up local state, ensuring consistent view of global state. */
         rdp->grpmask = leaf_node_cpu_bit(rdp->mynode, cpu);
         INIT_WORK(&rdp->strict_work, strict_work_handler);
         WARN_ON_ONCE(ct->dynticks_nesting != 1);
         WARN_ON_ONCE(rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu)));
         rdp->barrier_seq_snap = rcu_state.barrier_sequence;
         rdp->rcu_ofl_gp_seq = rcu_state.gp_seq;
         rdp->rcu_ofl_gp_state = RCU_GP_CLEANED;
         rdp->rcu_onl_gp_seq = rcu_state.gp_seq;
         rdp->rcu_onl_gp_state = RCU_GP_CLEANED;
         rdp->last_sched_clock = jiffies;
         rdp->cpu = cpu;
         rcu_boot_init_nocb_percpu_data(rdp);
 }
 
 struct kthread_worker *rcu_exp_gp_kworker;
 
 static void rcu_spawn_exp_par_gp_kworker(struct rcu_node *rnp)
 {
         struct kthread_worker *kworker;
         const char *name = "rcu_exp_par_gp_kthread_worker/%d";
         struct sched_param param = { .sched_priority = kthread_prio };
         int rnp_index = rnp - rcu_get_root();
 
         if (rnp->exp_kworker)
                 return;
 
         kworker = kthread_create_worker(0, name, rnp_index);
         if (IS_ERR_OR_NULL(kworker)) {
                 pr_err("Failed to create par gp kworker on %d/%d\n",
                        rnp->grplo, rnp->grphi);
                 return;
         }
         WRITE_ONCE(rnp->exp_kworker, kworker);
 
         if (IS_ENABLED(CONFIG_RCU_EXP_KTHREAD))
                 sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param);
 }
 
 static struct task_struct *rcu_exp_par_gp_task(struct rcu_node *rnp)
 {
         struct kthread_worker *kworker = READ_ONCE(rnp->exp_kworker);
 
         if (!kworker)
                 return NULL;
 
         return kworker->task;
 }
 
 static void __init rcu_start_exp_gp_kworker(void)
 {
         const char *name = "rcu_exp_gp_kthread_worker";
         struct sched_param param = { .sched_priority = kthread_prio };
 
         rcu_exp_gp_kworker = kthread_create_worker(0, name);
         if (IS_ERR_OR_NULL(rcu_exp_gp_kworker)) {
                 pr_err("Failed to create %s!\n", name);
                 rcu_exp_gp_kworker = NULL;
                 return;
         }
 
         if (IS_ENABLED(CONFIG_RCU_EXP_KTHREAD))
                 sched_setscheduler_nocheck(rcu_exp_gp_kworker->task, SCHED_FIFO, &param);
 }
 
 static void rcu_spawn_rnp_kthreads(struct rcu_node *rnp)
 {
         if (rcu_scheduler_fully_active) {
                 mutex_lock(&rnp->kthread_mutex);
                 rcu_spawn_one_boost_kthread(rnp);
                 rcu_spawn_exp_par_gp_kworker(rnp);
                 mutex_unlock(&rnp->kthread_mutex);
         }
 }
 
 /*
  * Invoked early in the CPU-online process, when pretty much all services
  * are available.  The incoming CPU is not present.
  *
  * Initializes a CPU's per-CPU RCU data.  Note that only one online or
  * offline event can be happening at a given time.  Note also that we can
  * accept some slop in the rsp->gp_seq access due to the fact that this
  * CPU cannot possibly have any non-offloaded RCU callbacks in flight yet.
  * And any offloaded callbacks are being numbered elsewhere.
  */
 int rcutree_prepare_cpu(unsigned int cpu)
 {
         unsigned long flags;
         struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
         struct rcu_node *rnp = rcu_get_root();
 
         /* Set up local state, ensuring consistent view of global state. */
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         rdp->qlen_last_fqs_check = 0;
         rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
         rdp->blimit = blimit;
         ct->dynticks_nesting = 1;       /* CPU not up, no tearing. */
         raw_spin_unlock_rcu_node(rnp);          /* irqs remain disabled. */
 
         /*
          * Only non-NOCB CPUs that didn't have early-boot callbacks need to be
          * (re-)initialized.
          */
         if (!rcu_segcblist_is_enabled(&rdp->cblist))
                 rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
 
         /*
          * Add CPU to leaf rcu_node pending-online bitmask.  Any needed
          * propagation up the rcu_node tree will happen at the beginning
          * of the next grace period.
          */
         rnp = rdp->mynode;
         raw_spin_lock_rcu_node(rnp);            /* irqs already disabled. */
         rdp->gp_seq = READ_ONCE(rnp->gp_seq);
         rdp->gp_seq_needed = rdp->gp_seq;
         rdp->cpu_no_qs.b.norm = true;
         rdp->core_needs_qs = false;
         rdp->rcu_iw_pending = false;
         rdp->rcu_iw = IRQ_WORK_INIT_HARD(rcu_iw_handler);
         rdp->rcu_iw_gp_seq = rdp->gp_seq - 1;
         trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("cpuonl"));
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         rcu_spawn_rnp_kthreads(rnp);
         rcu_spawn_cpu_nocb_kthread(cpu);
         ASSERT_EXCLUSIVE_WRITER(rcu_state.n_online_cpus);
         WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
 
         return 0;
 }
 
 /*
  * Update kthreads affinity during CPU-hotplug changes.
  *
  * Set the per-rcu_node kthread's affinity to cover all CPUs that are
  * served by the rcu_node in question.  The CPU hotplug lock is still
  * held, so the value of rnp->qsmaskinit will be stable.
  *
  * We don't include outgoingcpu in the affinity set, use -1 if there is
  * no outgoing CPU.  If there are no CPUs left in the affinity set,
  * this function allows the kthread to execute on any CPU.
  *
  * Any future concurrent calls are serialized via ->kthread_mutex.
  */
 static void rcutree_affinity_setting(unsigned int cpu, int outgoingcpu)
 {
         cpumask_var_t cm;
         unsigned long mask;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
         struct task_struct *task_boost, *task_exp;
 
         rdp = per_cpu_ptr(&rcu_data, cpu);
         rnp = rdp->mynode;
 
         task_boost = rcu_boost_task(rnp);
         task_exp = rcu_exp_par_gp_task(rnp);
 
         /*
          * If CPU is the boot one, those tasks are created later from early
          * initcall since kthreadd must be created first.
          */
         if (!task_boost && !task_exp)
                 return;
 
         if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
                 return;
 
         mutex_lock(&rnp->kthread_mutex);
         mask = rcu_rnp_online_cpus(rnp);
         for_each_leaf_node_possible_cpu(rnp, cpu)
                 if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
                     cpu != outgoingcpu)
                         cpumask_set_cpu(cpu, cm);
         cpumask_and(cm, cm, housekeeping_cpumask(HK_TYPE_RCU));
         if (cpumask_empty(cm)) {
                 cpumask_copy(cm, housekeeping_cpumask(HK_TYPE_RCU));
                 if (outgoingcpu >= 0)
                         cpumask_clear_cpu(outgoingcpu, cm);
         }
 
         if (task_exp)
                 set_cpus_allowed_ptr(task_exp, cm);
 
         if (task_boost)
                 set_cpus_allowed_ptr(task_boost, cm);
 
         mutex_unlock(&rnp->kthread_mutex);
 
         free_cpumask_var(cm);
 }
 
 /*
  * Has the specified (known valid) CPU ever been fully online?
  */
 bool rcu_cpu_beenfullyonline(int cpu)
 {
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
         return smp_load_acquire(&rdp->beenonline);
 }
 
 /*
  * Near the end of the CPU-online process.  Pretty much all services
  * enabled, and the CPU is now very much alive.
  */
 int rcutree_online_cpu(unsigned int cpu)
 {
         unsigned long flags;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
 
         rdp = per_cpu_ptr(&rcu_data, cpu);
         rnp = rdp->mynode;
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         rnp->ffmask |= rdp->grpmask;
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
                 return 0; /* Too early in boot for scheduler work. */
         sync_sched_exp_online_cleanup(cpu);
         rcutree_affinity_setting(cpu, -1);
 
         // Stop-machine done, so allow nohz_full to disable tick.
         tick_dep_clear(TICK_DEP_BIT_RCU);
         return 0;
 }
 
 /*
  * Mark the specified CPU as being online so that subsequent grace periods
  * (both expedited and normal) will wait on it.  Note that this means that
  * incoming CPUs are not allowed to use RCU read-side critical sections
  * until this function is called.  Failing to observe this restriction
  * will result in lockdep splats.
  *
  * Note that this function is special in that it is invoked directly
  * from the incoming CPU rather than from the cpuhp_step mechanism.
  * This is because this function must be invoked at a precise location.
  * This incoming CPU must not have enabled interrupts yet.
  *
  * This mirrors the effects of rcutree_report_cpu_dead().
  */
 void rcutree_report_cpu_starting(unsigned int cpu)
 {
         unsigned long mask;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
         bool newcpu;
 
         lockdep_assert_irqs_disabled();
         rdp = per_cpu_ptr(&rcu_data, cpu);
         if (rdp->cpu_started)
                 return;
         rdp->cpu_started = true;
 
         rnp = rdp->mynode;
         mask = rdp->grpmask;
         arch_spin_lock(&rcu_state.ofl_lock);
         rcu_dynticks_eqs_online();
         raw_spin_lock(&rcu_state.barrier_lock);
         raw_spin_lock_rcu_node(rnp);
         WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
         raw_spin_unlock(&rcu_state.barrier_lock);
         newcpu = !(rnp->expmaskinitnext & mask);
         rnp->expmaskinitnext |= mask;
         /* Allow lockless access for expedited grace periods. */
         smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + newcpu); /* ^^^ */
         ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
         rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
         rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
         rdp->rcu_onl_gp_state = READ_ONCE(rcu_state.gp_state);
 
         /* An incoming CPU should never be blocking a grace period. */
         if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */
                 /* rcu_report_qs_rnp() *really* wants some flags to restore */
                 unsigned long flags;
 
                 local_irq_save(flags);
                 rcu_disable_urgency_upon_qs(rdp);
                 /* Report QS -after- changing ->qsmaskinitnext! */
                 rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
         } else {
                 raw_spin_unlock_rcu_node(rnp);
         }
         arch_spin_unlock(&rcu_state.ofl_lock);
         smp_store_release(&rdp->beenonline, true);
         smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
 }
 
 /*
  * The outgoing function has no further need of RCU, so remove it from
  * the rcu_node tree's ->qsmaskinitnext bit masks.
  *
  * Note that this function is special in that it is invoked directly
  * from the outgoing CPU rather than from the cpuhp_step mechanism.
  * This is because this function must be invoked at a precise location.
  *
  * This mirrors the effect of rcutree_report_cpu_starting().
  */
 void rcutree_report_cpu_dead(void)
 {
         unsigned long flags;
         unsigned long mask;
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
         struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
         /*
          * IRQS must be disabled from now on and until the CPU dies, or an interrupt
          * may introduce a new READ-side while it is actually off the QS masks.
          */
         lockdep_assert_irqs_disabled();
         // Do any dangling deferred wakeups.
         do_nocb_deferred_wakeup(rdp);
 
         rcu_preempt_deferred_qs(current);
 
         /* Remove outgoing CPU from mask in the leaf rcu_node structure. */
         mask = rdp->grpmask;
         arch_spin_lock(&rcu_state.ofl_lock);
         raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
         rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq);
         rdp->rcu_ofl_gp_state = READ_ONCE(rcu_state.gp_state);
         if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
                 /* Report quiescent state -before- changing ->qsmaskinitnext! */
                 rcu_disable_urgency_upon_qs(rdp);
                 rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
         }
         WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         arch_spin_unlock(&rcu_state.ofl_lock);
         rdp->cpu_started = false;
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*
  * The outgoing CPU has just passed through the dying-idle state, and we
  * are being invoked from the CPU that was IPIed to continue the offline
  * operation.  Migrate the outgoing CPU's callbacks to the current CPU.
  */
 void rcutree_migrate_callbacks(int cpu)
 {
         unsigned long flags;
         struct rcu_data *my_rdp;
         struct rcu_node *my_rnp;
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
         bool needwake;
 
         if (rcu_rdp_is_offloaded(rdp))
                 return;
 
         raw_spin_lock_irqsave(&rcu_state.barrier_lock, flags);
         if (rcu_segcblist_empty(&rdp->cblist)) {
                 raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
                 return;  /* No callbacks to migrate. */
         }
 
         WARN_ON_ONCE(rcu_rdp_cpu_online(rdp));
         rcu_barrier_entrain(rdp);
         my_rdp = this_cpu_ptr(&rcu_data);
         my_rnp = my_rdp->mynode;
         rcu_nocb_lock(my_rdp); /* irqs already disabled. */
         WARN_ON_ONCE(!rcu_nocb_flush_bypass(my_rdp, NULL, jiffies, false));
         raw_spin_lock_rcu_node(my_rnp); /* irqs already disabled. */
         /* Leverage recent GPs and set GP for new callbacks. */
         needwake = rcu_advance_cbs(my_rnp, rdp) ||
                    rcu_advance_cbs(my_rnp, my_rdp);
         rcu_segcblist_merge(&my_rdp->cblist, &rdp->cblist);
         raw_spin_unlock(&rcu_state.barrier_lock); /* irqs remain disabled. */
         needwake = needwake || rcu_advance_cbs(my_rnp, my_rdp);
         rcu_segcblist_disable(&rdp->cblist);
         WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) != !rcu_segcblist_n_cbs(&my_rdp->cblist));
         check_cb_ovld_locked(my_rdp, my_rnp);
         if (rcu_rdp_is_offloaded(my_rdp)) {
                 raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */
                 __call_rcu_nocb_wake(my_rdp, true, flags);
         } else {
                 rcu_nocb_unlock(my_rdp); /* irqs remain disabled. */
                 raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */
         }
         local_irq_restore(flags);
         if (needwake)
                 rcu_gp_kthread_wake();
         lockdep_assert_irqs_enabled();
         WARN_ONCE(rcu_segcblist_n_cbs(&rdp->cblist) != 0 ||
                   !rcu_segcblist_empty(&rdp->cblist),
                   "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, 1stCB=%p\n",
                   cpu, rcu_segcblist_n_cbs(&rdp->cblist),
                   rcu_segcblist_first_cb(&rdp->cblist));
 }
 
 /*
  * The CPU has been completely removed, and some other CPU is reporting
  * this fact from process context.  Do the remainder of the cleanup.
  * There can only be one CPU hotplug operation at a time, so no need for
  * explicit locking.
  */
 int rcutree_dead_cpu(unsigned int cpu)
 {
         ASSERT_EXCLUSIVE_WRITER(rcu_state.n_online_cpus);
         WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
         // Stop-machine done, so allow nohz_full to disable tick.
         tick_dep_clear(TICK_DEP_BIT_RCU);
         return 0;
 }
 
 /*
  * Near the end of the offline process.  Trace the fact that this CPU
  * is going offline.
  */
 int rcutree_dying_cpu(unsigned int cpu)
 {
         bool blkd;
         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
         struct rcu_node *rnp = rdp->mynode;
 
         blkd = !!(READ_ONCE(rnp->qsmask) & rdp->grpmask);
         trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
                                blkd ? TPS("cpuofl-bgp") : TPS("cpuofl"));
         return 0;
 }
 
 /*
  * Near the beginning of the process.  The CPU is still very much alive
  * with pretty much all services enabled.
  */
 int rcutree_offline_cpu(unsigned int cpu)
 {
         unsigned long flags;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
 
         rdp = per_cpu_ptr(&rcu_data, cpu);
         rnp = rdp->mynode;
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         rnp->ffmask &= ~rdp->grpmask;
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
         rcutree_affinity_setting(cpu, cpu);
 
         // nohz_full CPUs need the tick for stop-machine to work quickly
         tick_dep_set(TICK_DEP_BIT_RCU);
         return 0;
 }
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
 /*
  * On non-huge systems, use expedited RCU grace periods to make suspend
  * and hibernation run faster.
  */
 static int rcu_pm_notify(struct notifier_block *self,
                          unsigned long action, void *hcpu)
 {
         switch (action) {
         case PM_HIBERNATION_PREPARE:
         case PM_SUSPEND_PREPARE:
                 rcu_async_hurry();
                 rcu_expedite_gp();
                 break;
         case PM_POST_HIBERNATION:
         case PM_POST_SUSPEND:
                 rcu_unexpedite_gp();
                 rcu_async_relax();
                 break;
         default:
                 break;
         }
         return NOTIFY_OK;
 }
 
 /*
  * Spawn the kthreads that handle RCU's grace periods.
  */
 static int __init rcu_spawn_gp_kthread(void)
 {
         unsigned long flags;
         struct rcu_node *rnp;
         struct sched_param sp;
         struct task_struct *t;
         struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
         rcu_scheduler_fully_active = 1;
         t = kthread_create(rcu_gp_kthread, NULL, "%s", rcu_state.name);
         if (WARN_ONCE(IS_ERR(t), "%s: Could not start grace-period kthread, OOM is now expected behavior\n", __func__))
                 return 0;
         if (kthread_prio) {
                 sp.sched_priority = kthread_prio;
                 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
         }
         rnp = rcu_get_root();
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         WRITE_ONCE(rcu_state.gp_activity, jiffies);
         WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
         // Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
         smp_store_release(&rcu_state.gp_kthread, t);  /* ^^^ */
         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         wake_up_process(t);
         /* This is a pre-SMP initcall, we expect a single CPU */
         WARN_ON(num_online_cpus() > 1);
         /*
          * Those kthreads couldn't be created on rcu_init() -> rcutree_prepare_cpu()
          * due to rcu_scheduler_fully_active.
          */
         rcu_spawn_cpu_nocb_kthread(smp_processor_id());
         rcu_spawn_rnp_kthreads(rdp->mynode);
         rcu_spawn_core_kthreads();
         /* Create kthread worker for expedited GPs */
         rcu_start_exp_gp_kworker();
         return 0;
 }
 early_initcall(rcu_spawn_gp_kthread);
 
 /*
  * This function is invoked towards the end of the scheduler's
  * initialization process.  Before this is called, the idle task might
  * contain synchronous grace-period primitives (during which time, this idle
  * task is booting the system, and such primitives are no-ops).  After this
  * function is called, any synchronous grace-period primitives are run as
  * expedited, with the requesting task driving the grace period forward.
  * A later core_initcall() rcu_set_runtime_mode() will switch to full
  * runtime RCU functionality.
  */
 void rcu_scheduler_starting(void)
 {
         unsigned long flags;
         struct rcu_node *rnp;
 
         WARN_ON(num_online_cpus() != 1);
         WARN_ON(nr_context_switches() > 0);
         rcu_test_sync_prims();
 
         // Fix up the ->gp_seq counters.
         local_irq_save(flags);
         rcu_for_each_node_breadth_first(rnp)
                 rnp->gp_seq_needed = rnp->gp_seq = rcu_state.gp_seq;
         local_irq_restore(flags);
 
         // Switch out of early boot mode.
         rcu_scheduler_active = RCU_SCHEDULER_INIT;
         rcu_test_sync_prims();
 }
 
 /*
  * Helper function for rcu_init() that initializes the rcu_state structure.
  */
 static void __init rcu_init_one(void)
 {
         static const char * const buf[] = RCU_NODE_NAME_INIT;
         static const char * const fqs[] = RCU_FQS_NAME_INIT;
         static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
         static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
 
         int levelspread[RCU_NUM_LVLS];          /* kids/node in each level. */
         int cpustride = 1;
         int i;
         int j;
         struct rcu_node *rnp;
 
         BUILD_BUG_ON(RCU_NUM_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
 
         /* Silence gcc 4.8 false positive about array index out of range. */
         if (rcu_num_lvls <= 0 || rcu_num_lvls > RCU_NUM_LVLS)
                 panic("rcu_init_one: rcu_num_lvls out of range");
 
         /* Initialize the level-tracking arrays. */
 
         for (i = 1; i < rcu_num_lvls; i++)
                 rcu_state.level[i] =
                         rcu_state.level[i - 1] + num_rcu_lvl[i - 1];
         rcu_init_levelspread(levelspread, num_rcu_lvl);
 
         /* Initialize the elements themselves, starting from the leaves. */
 
         for (i = rcu_num_lvls - 1; i >= 0; i--) {
                 cpustride *= levelspread[i];
                 rnp = rcu_state.level[i];
                 for (j = 0; j < num_rcu_lvl[i]; j++, rnp++) {
                         raw_spin_lock_init(&ACCESS_PRIVATE(rnp, lock));
                         lockdep_set_class_and_name(&ACCESS_PRIVATE(rnp, lock),
                                                    &rcu_node_class[i], buf[i]);
                         raw_spin_lock_init(&rnp->fqslock);
                         lockdep_set_class_and_name(&rnp->fqslock,
                                                    &rcu_fqs_class[i], fqs[i]);
                         rnp->gp_seq = rcu_state.gp_seq;
                         rnp->gp_seq_needed = rcu_state.gp_seq;
                         rnp->completedqs = rcu_state.gp_seq;
                         rnp->qsmask = 0;
                         rnp->qsmaskinit = 0;
                         rnp->grplo = j * cpustride;
                         rnp->grphi = (j + 1) * cpustride - 1;
                         if (rnp->grphi >= nr_cpu_ids)
                                 rnp->grphi = nr_cpu_ids - 1;
                         if (i == 0) {
                                 rnp->grpnum = 0;
                                 rnp->grpmask = 0;
                                 rnp->parent = NULL;
                         } else {
                                 rnp->grpnum = j % levelspread[i - 1];
                                 rnp->grpmask = BIT(rnp->grpnum);
                                 rnp->parent = rcu_state.level[i - 1] +
                                               j / levelspread[i - 1];
                         }
                         rnp->level = i;
                         INIT_LIST_HEAD(&rnp->blkd_tasks);
                         rcu_init_one_nocb(rnp);
                         init_waitqueue_head(&rnp->exp_wq[0]);
                         init_waitqueue_head(&rnp->exp_wq[1]);
                         init_waitqueue_head(&rnp->exp_wq[2]);
                         init_waitqueue_head(&rnp->exp_wq[3]);
                         spin_lock_init(&rnp->exp_lock);
                         mutex_init(&rnp->kthread_mutex);
                         raw_spin_lock_init(&rnp->exp_poll_lock);
                         rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
                         INIT_WORK(&rnp->exp_poll_wq, sync_rcu_do_polled_gp);
                 }
         }
 
         init_swait_queue_head(&rcu_state.gp_wq);
         init_swait_queue_head(&rcu_state.expedited_wq);
         rnp = rcu_first_leaf_node();
         for_each_possible_cpu(i) {
                 while (i > rnp->grphi)
                         rnp++;
                 per_cpu_ptr(&rcu_data, i)->mynode = rnp;
                 rcu_boot_init_percpu_data(i);
         }
 }
 
 /*
  * Force priority from the kernel command-line into range.
  */
 static void __init sanitize_kthread_prio(void)
 {
         int kthread_prio_in = kthread_prio;
 
         if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 2
             && IS_BUILTIN(CONFIG_RCU_TORTURE_TEST))
                 kthread_prio = 2;
         else if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 1)
                 kthread_prio = 1;
         else if (kthread_prio < 0)
                 kthread_prio = 0;
         else if (kthread_prio > 99)
                 kthread_prio = 99;
 
         if (kthread_prio != kthread_prio_in)
                 pr_alert("%s: Limited prio to %d from %d\n",
                          __func__, kthread_prio, kthread_prio_in);
 }
 
 /*
  * Compute the rcu_node tree geometry from kernel parameters.  This cannot
  * replace the definitions in tree.h because those are needed to size
  * the ->node array in the rcu_state structure.
  */
 void rcu_init_geometry(void)
 {
         ulong d;
         int i;
         static unsigned long old_nr_cpu_ids;
         int rcu_capacity[RCU_NUM_LVLS];
         static bool initialized;
 
         if (initialized) {
                 /*
                  * Warn if setup_nr_cpu_ids() had not yet been invoked,
                  * unless nr_cpus_ids == NR_CPUS, in which case who cares?
                  */
                 WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids);
                 return;
         }
 
         old_nr_cpu_ids = nr_cpu_ids;
         initialized = true;
 
         /*
          * Initialize any unspecified boot parameters.
          * The default values of jiffies_till_first_fqs and
          * jiffies_till_next_fqs are set to the RCU_JIFFIES_TILL_FORCE_QS
          * value, which is a function of HZ, then adding one for each
          * RCU_JIFFIES_FQS_DIV CPUs that might be on the system.
          */
         d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
         if (jiffies_till_first_fqs == ULONG_MAX)
                 jiffies_till_first_fqs = d;
         if (jiffies_till_next_fqs == ULONG_MAX)
                 jiffies_till_next_fqs = d;
         adjust_jiffies_till_sched_qs();
 
         /* If the compile-time values are accurate, just leave. */
         if (rcu_fanout_leaf == RCU_FANOUT_LEAF &&
             nr_cpu_ids == NR_CPUS)
                 return;
         pr_info("Adjusting geometry for rcu_fanout_leaf=%d, nr_cpu_ids=%u\n",
                 rcu_fanout_leaf, nr_cpu_ids);
 
         /*
          * The boot-time rcu_fanout_leaf parameter must be at least two
          * and cannot exceed the number of bits in the rcu_node masks.
          * Complain and fall back to the compile-time values if this
          * limit is exceeded.
          */
         if (rcu_fanout_leaf < 2 ||
             rcu_fanout_leaf > sizeof(unsigned long) * 8) {
                 rcu_fanout_leaf = RCU_FANOUT_LEAF;
                 WARN_ON(1);
                 return;
         }
 
         /*
          * Compute number of nodes that can be handled an rcu_node tree
          * with the given number of levels.
          */
         rcu_capacity[0] = rcu_fanout_leaf;
         for (i = 1; i < RCU_NUM_LVLS; i++)
                 rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
 
         /*
          * The tree must be able to accommodate the configured number of CPUs.
          * If this limit is exceeded, fall back to the compile-time values.
          */
         if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1]) {
                 rcu_fanout_leaf = RCU_FANOUT_LEAF;
                 WARN_ON(1);
                 return;
         }
 
         /* Calculate the number of levels in the tree. */
         for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
         }
         rcu_num_lvls = i + 1;
 
         /* Calculate the number of rcu_nodes at each level of the tree. */
         for (i = 0; i < rcu_num_lvls; i++) {
                 int cap = rcu_capacity[(rcu_num_lvls - 1) - i];
                 num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
         }
 
         /* Calculate the total number of rcu_node structures. */
         rcu_num_nodes = 0;
         for (i = 0; i < rcu_num_lvls; i++)
                 rcu_num_nodes += num_rcu_lvl[i];
 }
 
 /*
  * Dump out the structure of the rcu_node combining tree associated
  * with the rcu_state structure.
  */
 static void __init rcu_dump_rcu_node_tree(void)
 {
         int level = 0;
         struct rcu_node *rnp;
 
         pr_info("rcu_node tree layout dump\n");
         pr_info(" ");
         rcu_for_each_node_breadth_first(rnp) {
                 if (rnp->level != level) {
                         pr_cont("\n");
                         pr_info(" ");
                         level = rnp->level;
                 }
                 pr_cont("%d:%d ^%d  ", rnp->grplo, rnp->grphi, rnp->grpnum);
         }
         pr_cont("\n");
 }
 
 struct workqueue_struct *rcu_gp_wq;
 
 static void __init kfree_rcu_batch_init(void)
 {
         int cpu;
         int i, j;
         struct shrinker *kfree_rcu_shrinker;
 
         /* Clamp it to [0:100] seconds interval. */
         if (rcu_delay_page_cache_fill_msec < 0 ||
                 rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) {
 
                 rcu_delay_page_cache_fill_msec =
                         clamp(rcu_delay_page_cache_fill_msec, 0,
                                 (int) (100 * MSEC_PER_SEC));
 
                 pr_info("Adjusting rcutree.rcu_delay_page_cache_fill_msec to %d ms.\n",
                         rcu_delay_page_cache_fill_msec);
         }
 
         for_each_possible_cpu(cpu) {
                 struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
                 for (i = 0; i < KFREE_N_BATCHES; i++) {
                         INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
                         krcp->krw_arr[i].krcp = krcp;
 
                         for (j = 0; j < FREE_N_CHANNELS; j++)
                                 INIT_LIST_HEAD(&krcp->krw_arr[i].bulk_head_free[j]);
                 }
 
                 for (i = 0; i < FREE_N_CHANNELS; i++)
                         INIT_LIST_HEAD(&krcp->bulk_head[i]);
 
                 INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
                 INIT_DELAYED_WORK(&krcp->page_cache_work, fill_page_cache_func);
                 krcp->initialized = true;
         }
 
         kfree_rcu_shrinker = shrinker_alloc(0, "rcu-kfree");
         if (!kfree_rcu_shrinker) {
                 pr_err("Failed to allocate kfree_rcu() shrinker!\n");
                 return;
         }
 
         kfree_rcu_shrinker->count_objects = kfree_rcu_shrink_count;
         kfree_rcu_shrinker->scan_objects = kfree_rcu_shrink_scan;
 
         shrinker_register(kfree_rcu_shrinker);
 }
 
 void __init rcu_init(void)
 {
         int cpu = smp_processor_id();
 
         rcu_early_boot_tests();
 
         kfree_rcu_batch_init();
         rcu_bootup_announce();
         sanitize_kthread_prio();
         rcu_init_geometry();
         rcu_init_one();
         if (dump_tree)
                 rcu_dump_rcu_node_tree();
         if (use_softirq)
                 open_softirq(RCU_SOFTIRQ, rcu_core_si);
 
         /*
          * We don't need protection against CPU-hotplug here because
          * this is called early in boot, before either interrupts
          * or the scheduler are operational.
          */
         pm_notifier(rcu_pm_notify, 0);
         WARN_ON(num_online_cpus() > 1); // Only one CPU this early in boot.
         rcutree_prepare_cpu(cpu);
         rcutree_report_cpu_starting(cpu);
         rcutree_online_cpu(cpu);
 
         /* Create workqueue for Tree SRCU and for expedited GPs. */
         rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
         WARN_ON(!rcu_gp_wq);
 
         sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
         WARN_ON(!sync_wq);
 
         /* Fill in default value for rcutree.qovld boot parameter. */
         /* -After- the rcu_node ->lock fields are initialized! */
         if (qovld < 0)
                 qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
         else
                 qovld_calc = qovld;
 
         // Kick-start in case any polled grace periods started early.
         (void)start_poll_synchronize_rcu_expedited();
 
         rcu_test_sync_prims();
 
         tasks_cblist_init_generic();
 }
 
 #include "tree_stall.h"
 #include "tree_exp.h"
 #include "tree_nocb.h"
 #include "tree_plugin.h"