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

   // SPDX-License-Identifier: GPL-2.0+
   /*
    * Sleepable Read-Copy Update mechanism for mutual exclusion.
    *
    * Copyright (C) IBM Corporation, 2006
    * Copyright (C) Fujitsu, 2012
    *
    * Authors: Paul McKenney <paulmck@linux.ibm.com>
    *         Lai Jiangshan <laijs@cn.fujitsu.com>
   *
   * For detailed explanation of Read-Copy Update mechanism see -
   *              Documentation/RCU/ *.txt
   *
   */
  
  #define pr_fmt(fmt) "rcu: " fmt
  
  #include <linux/export.h>
  #include <linux/mutex.h>
  #include <linux/percpu.h>
  #include <linux/preempt.h>
  #include <linux/rcupdate_wait.h>
  #include <linux/sched.h>
  #include <linux/smp.h>
  #include <linux/delay.h>
  #include <linux/module.h>
  #include <linux/slab.h>
  #include <linux/srcu.h>
  
  #include "rcu.h"
  #include "rcu_segcblist.h"
  
  /* Holdoff in nanoseconds for auto-expediting. */
  #define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000)
  static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
  module_param(exp_holdoff, ulong, 0444);
  
  /* Overflow-check frequency.  N bits roughly says every 2**N grace periods. */
  static ulong counter_wrap_check = (ULONG_MAX >> 2);
  module_param(counter_wrap_check, ulong, 0444);
  
  /*
   * Control conversion to SRCU_SIZE_BIG:
   *    0: Don't convert at all.
   *    1: Convert at init_srcu_struct() time.
   *    2: Convert when rcutorture invokes srcu_torture_stats_print().
   *    3: Decide at boot time based on system shape (default).
   * 0x1x: Convert when excessive contention encountered.
   */
  #define SRCU_SIZING_NONE        0
  #define SRCU_SIZING_INIT        1
  #define SRCU_SIZING_TORTURE     2
  #define SRCU_SIZING_AUTO        3
  #define SRCU_SIZING_CONTEND     0x10
  #define SRCU_SIZING_IS(x) ((convert_to_big & ~SRCU_SIZING_CONTEND) == x)
  #define SRCU_SIZING_IS_NONE() (SRCU_SIZING_IS(SRCU_SIZING_NONE))
  #define SRCU_SIZING_IS_INIT() (SRCU_SIZING_IS(SRCU_SIZING_INIT))
  #define SRCU_SIZING_IS_TORTURE() (SRCU_SIZING_IS(SRCU_SIZING_TORTURE))
  #define SRCU_SIZING_IS_CONTEND() (convert_to_big & SRCU_SIZING_CONTEND)
  static int convert_to_big = SRCU_SIZING_AUTO;
  module_param(convert_to_big, int, 0444);
  
  /* Number of CPUs to trigger init_srcu_struct()-time transition to big. */
  static int big_cpu_lim __read_mostly = 128;
  module_param(big_cpu_lim, int, 0444);
  
  /* Contention events per jiffy to initiate transition to big. */
  static int small_contention_lim __read_mostly = 100;
  module_param(small_contention_lim, int, 0444);
  
  /* Early-boot callback-management, so early that no lock is required! */
  static LIST_HEAD(srcu_boot_list);
  static bool __read_mostly srcu_init_done;
  
  static void srcu_invoke_callbacks(struct work_struct *work);
  static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
  static void process_srcu(struct work_struct *work);
  static void srcu_delay_timer(struct timer_list *t);
  
  /* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */
  #define spin_lock_rcu_node(p)                                                   \
  do {                                                                            \
          spin_lock(&ACCESS_PRIVATE(p, lock));                                    \
          smp_mb__after_unlock_lock();                                            \
  } while (0)
  
  #define spin_unlock_rcu_node(p) spin_unlock(&ACCESS_PRIVATE(p, lock))
  
  #define spin_lock_irq_rcu_node(p)                                               \
  do {                                                                            \
          spin_lock_irq(&ACCESS_PRIVATE(p, lock));                                \
          smp_mb__after_unlock_lock();                                            \
  } while (0)
  
  #define spin_unlock_irq_rcu_node(p)                                             \
          spin_unlock_irq(&ACCESS_PRIVATE(p, lock))
  
  #define spin_lock_irqsave_rcu_node(p, flags)                                    \
  do {                                                                            \
         spin_lock_irqsave(&ACCESS_PRIVATE(p, lock), flags);                     \
         smp_mb__after_unlock_lock();                                            \
 } while (0)
 
 #define spin_trylock_irqsave_rcu_node(p, flags)                                 \
 ({                                                                              \
         bool ___locked = spin_trylock_irqsave(&ACCESS_PRIVATE(p, lock), flags); \
                                                                                 \
         if (___locked)                                                          \
                 smp_mb__after_unlock_lock();                                    \
         ___locked;                                                              \
 })
 
 #define spin_unlock_irqrestore_rcu_node(p, flags)                               \
         spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags)                 \
 
 /*
  * Initialize SRCU per-CPU data.  Note that statically allocated
  * srcu_struct structures might already have srcu_read_lock() and
  * srcu_read_unlock() running against them.  So if the is_static parameter
  * is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
  */
 static void init_srcu_struct_data(struct srcu_struct *ssp)
 {
         int cpu;
         struct srcu_data *sdp;
 
         /*
          * Initialize the per-CPU srcu_data array, which feeds into the
          * leaves of the srcu_node tree.
          */
         WARN_ON_ONCE(ARRAY_SIZE(sdp->srcu_lock_count) !=
                      ARRAY_SIZE(sdp->srcu_unlock_count));
         for_each_possible_cpu(cpu) {
                 sdp = per_cpu_ptr(ssp->sda, cpu);
                 spin_lock_init(&ACCESS_PRIVATE(sdp, lock));
                 rcu_segcblist_init(&sdp->srcu_cblist);
                 sdp->srcu_cblist_invoking = false;
                 sdp->srcu_gp_seq_needed = ssp->srcu_sup->srcu_gp_seq;
                 sdp->srcu_gp_seq_needed_exp = ssp->srcu_sup->srcu_gp_seq;
                 sdp->mynode = NULL;
                 sdp->cpu = cpu;
                 INIT_WORK(&sdp->work, srcu_invoke_callbacks);
                 timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
                 sdp->ssp = ssp;
         }
 }
 
 /* Invalid seq state, used during snp node initialization */
 #define SRCU_SNP_INIT_SEQ               0x2
 
 /*
  * Check whether sequence number corresponding to snp node,
  * is invalid.
  */
 static inline bool srcu_invl_snp_seq(unsigned long s)
 {
         return s == SRCU_SNP_INIT_SEQ;
 }
 
 /*
  * Allocated and initialize SRCU combining tree.  Returns @true if
  * allocation succeeded and @false otherwise.
  */
 static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
 {
         int cpu;
         int i;
         int level = 0;
         int levelspread[RCU_NUM_LVLS];
         struct srcu_data *sdp;
         struct srcu_node *snp;
         struct srcu_node *snp_first;
 
         /* Initialize geometry if it has not already been initialized. */
         rcu_init_geometry();
         ssp->srcu_sup->node = kcalloc(rcu_num_nodes, sizeof(*ssp->srcu_sup->node), gfp_flags);
         if (!ssp->srcu_sup->node)
                 return false;
 
         /* Work out the overall tree geometry. */
         ssp->srcu_sup->level[0] = &ssp->srcu_sup->node[0];
         for (i = 1; i < rcu_num_lvls; i++)
                 ssp->srcu_sup->level[i] = ssp->srcu_sup->level[i - 1] + num_rcu_lvl[i - 1];
         rcu_init_levelspread(levelspread, num_rcu_lvl);
 
         /* Each pass through this loop initializes one srcu_node structure. */
         srcu_for_each_node_breadth_first(ssp, snp) {
                 spin_lock_init(&ACCESS_PRIVATE(snp, lock));
                 WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
                              ARRAY_SIZE(snp->srcu_data_have_cbs));
                 for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
                         snp->srcu_have_cbs[i] = SRCU_SNP_INIT_SEQ;
                         snp->srcu_data_have_cbs[i] = 0;
                 }
                 snp->srcu_gp_seq_needed_exp = SRCU_SNP_INIT_SEQ;
                 snp->grplo = -1;
                 snp->grphi = -1;
                 if (snp == &ssp->srcu_sup->node[0]) {
                         /* Root node, special case. */
                         snp->srcu_parent = NULL;
                         continue;
                 }
 
                 /* Non-root node. */
                 if (snp == ssp->srcu_sup->level[level + 1])
                         level++;
                 snp->srcu_parent = ssp->srcu_sup->level[level - 1] +
                                    (snp - ssp->srcu_sup->level[level]) /
                                    levelspread[level - 1];
         }
 
         /*
          * Initialize the per-CPU srcu_data array, which feeds into the
          * leaves of the srcu_node tree.
          */
         level = rcu_num_lvls - 1;
         snp_first = ssp->srcu_sup->level[level];
         for_each_possible_cpu(cpu) {
                 sdp = per_cpu_ptr(ssp->sda, cpu);
                 sdp->mynode = &snp_first[cpu / levelspread[level]];
                 for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
                         if (snp->grplo < 0)
                                 snp->grplo = cpu;
                         snp->grphi = cpu;
                 }
                 sdp->grpmask = 1UL << (cpu - sdp->mynode->grplo);
         }
         smp_store_release(&ssp->srcu_sup->srcu_size_state, SRCU_SIZE_WAIT_BARRIER);
         return true;
 }
 
 /*
  * Initialize non-compile-time initialized fields, including the
  * associated srcu_node and srcu_data structures.  The is_static parameter
  * tells us that ->sda has already been wired up to srcu_data.
  */
 static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 {
         if (!is_static)
                 ssp->srcu_sup = kzalloc(sizeof(*ssp->srcu_sup), GFP_KERNEL);
         if (!ssp->srcu_sup)
                 return -ENOMEM;
         if (!is_static)
                 spin_lock_init(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
         ssp->srcu_sup->srcu_size_state = SRCU_SIZE_SMALL;
         ssp->srcu_sup->node = NULL;
         mutex_init(&ssp->srcu_sup->srcu_cb_mutex);
         mutex_init(&ssp->srcu_sup->srcu_gp_mutex);
         ssp->srcu_idx = 0;
         ssp->srcu_sup->srcu_gp_seq = 0;
         ssp->srcu_sup->srcu_barrier_seq = 0;
         mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
         atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
         INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);
         ssp->srcu_sup->sda_is_static = is_static;
         if (!is_static)
                 ssp->sda = alloc_percpu(struct srcu_data);
         if (!ssp->sda)
                 goto err_free_sup;
         init_srcu_struct_data(ssp);
         ssp->srcu_sup->srcu_gp_seq_needed_exp = 0;
         ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
         if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
                 if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC))
                         goto err_free_sda;
                 WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_BIG);
         }
         ssp->srcu_sup->srcu_ssp = ssp;
         smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done. */
         return 0;
 
 err_free_sda:
         if (!is_static) {
                 free_percpu(ssp->sda);
                 ssp->sda = NULL;
         }
 err_free_sup:
         if (!is_static) {
                 kfree(ssp->srcu_sup);
                 ssp->srcu_sup = NULL;
         }
         return -ENOMEM;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
                        struct lock_class_key *key)
 {
         /* Don't re-initialize a lock while it is held. */
         debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
         lockdep_init_map(&ssp->dep_map, name, key, 0);
         return init_srcu_struct_fields(ssp, false);
 }
 EXPORT_SYMBOL_GPL(__init_srcu_struct);
 
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /**
  * init_srcu_struct - initialize a sleep-RCU structure
  * @ssp: structure to initialize.
  *
  * Must invoke this on a given srcu_struct before passing that srcu_struct
  * to any other function.  Each srcu_struct represents a separate domain
  * of SRCU protection.
  */
 int init_srcu_struct(struct srcu_struct *ssp)
 {
         return init_srcu_struct_fields(ssp, false);
 }
 EXPORT_SYMBOL_GPL(init_srcu_struct);
 
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
  * Initiate a transition to SRCU_SIZE_BIG with lock held.
  */
 static void __srcu_transition_to_big(struct srcu_struct *ssp)
 {
         lockdep_assert_held(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
         smp_store_release(&ssp->srcu_sup->srcu_size_state, SRCU_SIZE_ALLOC);
 }
 
 /*
  * Initiate an idempotent transition to SRCU_SIZE_BIG.
  */
 static void srcu_transition_to_big(struct srcu_struct *ssp)
 {
         unsigned long flags;
 
         /* Double-checked locking on ->srcu_size-state. */
         if (smp_load_acquire(&ssp->srcu_sup->srcu_size_state) != SRCU_SIZE_SMALL)
                 return;
         spin_lock_irqsave_rcu_node(ssp->srcu_sup, flags);
         if (smp_load_acquire(&ssp->srcu_sup->srcu_size_state) != SRCU_SIZE_SMALL) {
                 spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);
                 return;
         }
         __srcu_transition_to_big(ssp);
         spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);
 }
 
 /*
  * Check to see if the just-encountered contention event justifies
  * a transition to SRCU_SIZE_BIG.
  */
 static void spin_lock_irqsave_check_contention(struct srcu_struct *ssp)
 {
         unsigned long j;
 
         if (!SRCU_SIZING_IS_CONTEND() || ssp->srcu_sup->srcu_size_state)
                 return;
         j = jiffies;
         if (ssp->srcu_sup->srcu_size_jiffies != j) {
                 ssp->srcu_sup->srcu_size_jiffies = j;
                 ssp->srcu_sup->srcu_n_lock_retries = 0;
         }
         if (++ssp->srcu_sup->srcu_n_lock_retries <= small_contention_lim)
                 return;
         __srcu_transition_to_big(ssp);
 }
 
 /*
  * Acquire the specified srcu_data structure's ->lock, but check for
  * excessive contention, which results in initiation of a transition
  * to SRCU_SIZE_BIG.  But only if the srcutree.convert_to_big module
  * parameter permits this.
  */
 static void spin_lock_irqsave_sdp_contention(struct srcu_data *sdp, unsigned long *flags)
 {
         struct srcu_struct *ssp = sdp->ssp;
 
         if (spin_trylock_irqsave_rcu_node(sdp, *flags))
                 return;
         spin_lock_irqsave_rcu_node(ssp->srcu_sup, *flags);
         spin_lock_irqsave_check_contention(ssp);
         spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, *flags);
         spin_lock_irqsave_rcu_node(sdp, *flags);
 }
 
 /*
  * Acquire the specified srcu_struct structure's ->lock, but check for
  * excessive contention, which results in initiation of a transition
  * to SRCU_SIZE_BIG.  But only if the srcutree.convert_to_big module
  * parameter permits this.
  */
 static void spin_lock_irqsave_ssp_contention(struct srcu_struct *ssp, unsigned long *flags)
 {
         if (spin_trylock_irqsave_rcu_node(ssp->srcu_sup, *flags))
                 return;
         spin_lock_irqsave_rcu_node(ssp->srcu_sup, *flags);
         spin_lock_irqsave_check_contention(ssp);
 }
 
 /*
  * First-use initialization of statically allocated srcu_struct
  * structure.  Wiring up the combining tree is more than can be
  * done with compile-time initialization, so this check is added
  * to each update-side SRCU primitive.  Use ssp->lock, which -is-
  * compile-time initialized, to resolve races involving multiple
  * CPUs trying to garner first-use privileges.
  */
 static void check_init_srcu_struct(struct srcu_struct *ssp)
 {
         unsigned long flags;
 
         /* The smp_load_acquire() pairs with the smp_store_release(). */
         if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed))) /*^^^*/
                 return; /* Already initialized. */
         spin_lock_irqsave_rcu_node(ssp->srcu_sup, flags);
         if (!rcu_seq_state(ssp->srcu_sup->srcu_gp_seq_needed)) {
                 spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);
                 return;
         }
         init_srcu_struct_fields(ssp, true);
         spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);
 }
 
 /*
  * Returns approximate total of the readers' ->srcu_lock_count[] values
  * for the rank of per-CPU counters specified by idx.
  */
 static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
 {
         int cpu;
         unsigned long sum = 0;
 
         for_each_possible_cpu(cpu) {
                 struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
                 sum += atomic_long_read(&cpuc->srcu_lock_count[idx]);
         }
         return sum;
 }
 
 /*
  * Returns approximate total of the readers' ->srcu_unlock_count[] values
  * for the rank of per-CPU counters specified by idx.
  */
 static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
 {
         int cpu;
         unsigned long mask = 0;
         unsigned long sum = 0;
 
         for_each_possible_cpu(cpu) {
                 struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
                 sum += atomic_long_read(&cpuc->srcu_unlock_count[idx]);
                 if (IS_ENABLED(CONFIG_PROVE_RCU))
                         mask = mask | READ_ONCE(cpuc->srcu_nmi_safety);
         }
         WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
                   "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
         return sum;
 }
 
 /*
  * Return true if the number of pre-existing readers is determined to
  * be zero.
  */
 static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 {
         unsigned long unlocks;
 
         unlocks = srcu_readers_unlock_idx(ssp, idx);
 
         /*
          * Make sure that a lock is always counted if the corresponding
          * unlock is counted. Needs to be a smp_mb() as the read side may
          * contain a read from a variable that is written to before the
          * synchronize_srcu() in the write side. In this case smp_mb()s
          * A and B act like the store buffering pattern.
          *
          * This smp_mb() also pairs with smp_mb() C to prevent accesses
          * after the synchronize_srcu() from being executed before the
          * grace period ends.
          */
         smp_mb(); /* A */
 
         /*
          * If the locks are the same as the unlocks, then there must have
          * been no readers on this index at some point in this function.
          * But there might be more readers, as a task might have read
          * the current ->srcu_idx but not yet have incremented its CPU's
          * ->srcu_lock_count[idx] counter.  In fact, it is possible
          * that most of the tasks have been preempted between fetching
          * ->srcu_idx and incrementing ->srcu_lock_count[idx].  And there
          * could be almost (ULONG_MAX / sizeof(struct task_struct)) tasks
          * in a system whose address space was fully populated with memory.
          * Call this quantity Nt.
          *
          * So suppose that the updater is preempted at this point in the
          * code for a long time.  That now-preempted updater has already
          * flipped ->srcu_idx (possibly during the preceding grace period),
          * done an smp_mb() (again, possibly during the preceding grace
          * period), and summed up the ->srcu_unlock_count[idx] counters.
          * How many times can a given one of the aforementioned Nt tasks
          * increment the old ->srcu_idx value's ->srcu_lock_count[idx]
          * counter, in the absence of nesting?
          *
          * It can clearly do so once, given that it has already fetched
          * the old value of ->srcu_idx and is just about to use that value
          * to index its increment of ->srcu_lock_count[idx].  But as soon as
          * it leaves that SRCU read-side critical section, it will increment
          * ->srcu_unlock_count[idx], which must follow the updater's above
          * read from that same value.  Thus, as soon the reading task does
          * an smp_mb() and a later fetch from ->srcu_idx, that task will be
          * guaranteed to get the new index.  Except that the increment of
          * ->srcu_unlock_count[idx] in __srcu_read_unlock() is after the
          * smp_mb(), and the fetch from ->srcu_idx in __srcu_read_lock()
          * is before the smp_mb().  Thus, that task might not see the new
          * value of ->srcu_idx until the -second- __srcu_read_lock(),
          * which in turn means that this task might well increment
          * ->srcu_lock_count[idx] for the old value of ->srcu_idx twice,
          * not just once.
          *
          * However, it is important to note that a given smp_mb() takes
          * effect not just for the task executing it, but also for any
          * later task running on that same CPU.
          *
          * That is, there can be almost Nt + Nc further increments of
          * ->srcu_lock_count[idx] for the old index, where Nc is the number
          * of CPUs.  But this is OK because the size of the task_struct
          * structure limits the value of Nt and current systems limit Nc
          * to a few thousand.
          *
          * OK, but what about nesting?  This does impose a limit on
          * nesting of half of the size of the task_struct structure
          * (measured in bytes), which should be sufficient.  A late 2022
          * TREE01 rcutorture run reported this size to be no less than
          * 9408 bytes, allowing up to 4704 levels of nesting, which is
          * comfortably beyond excessive.  Especially on 64-bit systems,
          * which are unlikely to be configured with an address space fully
          * populated with memory, at least not anytime soon.
          */
         return srcu_readers_lock_idx(ssp, idx) == unlocks;
 }
 
 /**
  * srcu_readers_active - returns true if there are readers. and false
  *                       otherwise
  * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
  *
  * Note that this is not an atomic primitive, and can therefore suffer
  * severe errors when invoked on an active srcu_struct.  That said, it
  * can be useful as an error check at cleanup time.
  */
 static bool srcu_readers_active(struct srcu_struct *ssp)
 {
         int cpu;
         unsigned long sum = 0;
 
         for_each_possible_cpu(cpu) {
                 struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
                 sum += atomic_long_read(&cpuc->srcu_lock_count[0]);
                 sum += atomic_long_read(&cpuc->srcu_lock_count[1]);
                 sum -= atomic_long_read(&cpuc->srcu_unlock_count[0]);
                 sum -= atomic_long_read(&cpuc->srcu_unlock_count[1]);
         }
         return sum;
 }
 
 /*
  * We use an adaptive strategy for synchronize_srcu() and especially for
  * synchronize_srcu_expedited().  We spin for a fixed time period
  * (defined below, boot time configurable) to allow SRCU readers to exit
  * their read-side critical sections.  If there are still some readers
  * after one jiffy, we repeatedly block for one jiffy time periods.
  * The blocking time is increased as the grace-period age increases,
  * with max blocking time capped at 10 jiffies.
  */
 #define SRCU_DEFAULT_RETRY_CHECK_DELAY          5
 
 static ulong srcu_retry_check_delay = SRCU_DEFAULT_RETRY_CHECK_DELAY;
 module_param(srcu_retry_check_delay, ulong, 0444);
 
 #define SRCU_INTERVAL           1               // Base delay if no expedited GPs pending.
 #define SRCU_MAX_INTERVAL       10              // Maximum incremental delay from slow readers.
 
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_LO       3UL     // Lowmark on default per-GP-phase
                                                         // no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_HI       1000UL  // Highmark on default per-GP-phase
                                                         // no-delay instances.
 
 #define SRCU_UL_CLAMP_LO(val, low)      ((val) > (low) ? (val) : (low))
 #define SRCU_UL_CLAMP_HI(val, high)     ((val) < (high) ? (val) : (high))
 #define SRCU_UL_CLAMP(val, low, high)   SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO((val), (low)), (high))
 // per-GP-phase no-delay instances adjusted to allow non-sleeping poll upto
 // one jiffies time duration. Mult by 2 is done to factor in the srcu_get_delay()
 // called from process_srcu().
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED \
         (2UL * USEC_PER_SEC / HZ / SRCU_DEFAULT_RETRY_CHECK_DELAY)
 
 // Maximum per-GP-phase consecutive no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE  \
         SRCU_UL_CLAMP(SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED,  \
                       SRCU_DEFAULT_MAX_NODELAY_PHASE_LO,        \
                       SRCU_DEFAULT_MAX_NODELAY_PHASE_HI)
 
 static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE;
 module_param(srcu_max_nodelay_phase, ulong, 0444);
 
 // Maximum consecutive no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY        (SRCU_DEFAULT_MAX_NODELAY_PHASE > 100 ? \
                                          SRCU_DEFAULT_MAX_NODELAY_PHASE : 100)
 
 static ulong srcu_max_nodelay = SRCU_DEFAULT_MAX_NODELAY;
 module_param(srcu_max_nodelay, ulong, 0444);
 
 /*
  * Return grace-period delay, zero if there are expedited grace
  * periods pending, SRCU_INTERVAL otherwise.
  */
 static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 {
         unsigned long gpstart;
         unsigned long j;
         unsigned long jbase = SRCU_INTERVAL;
         struct srcu_usage *sup = ssp->srcu_sup;
 
         if (ULONG_CMP_LT(READ_ONCE(sup->srcu_gp_seq), READ_ONCE(sup->srcu_gp_seq_needed_exp)))
                 jbase = 0;
         if (rcu_seq_state(READ_ONCE(sup->srcu_gp_seq))) {
                 j = jiffies - 1;
                 gpstart = READ_ONCE(sup->srcu_gp_start);
                 if (time_after(j, gpstart))
                         jbase += j - gpstart;
                 if (!jbase) {
                         WRITE_ONCE(sup->srcu_n_exp_nodelay, READ_ONCE(sup->srcu_n_exp_nodelay) + 1);
                         if (READ_ONCE(sup->srcu_n_exp_nodelay) > srcu_max_nodelay_phase)
                                 jbase = 1;
                 }
         }
         return jbase > SRCU_MAX_INTERVAL ? SRCU_MAX_INTERVAL : jbase;
 }
 
 /**
  * cleanup_srcu_struct - deconstruct a sleep-RCU structure
  * @ssp: structure to clean up.
  *
  * Must invoke this after you are finished using a given srcu_struct that
  * was initialized via init_srcu_struct(), else you leak memory.
  */
 void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
         int cpu;
         struct srcu_usage *sup = ssp->srcu_sup;
 
         if (WARN_ON(!srcu_get_delay(ssp)))
                 return; /* Just leak it! */
         if (WARN_ON(srcu_readers_active(ssp)))
                 return; /* Just leak it! */
         flush_delayed_work(&sup->work);
         for_each_possible_cpu(cpu) {
                 struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
 
                 del_timer_sync(&sdp->delay_work);
                 flush_work(&sdp->work);
                 if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
                         return; /* Forgot srcu_barrier(), so just leak it! */
         }
         if (WARN_ON(rcu_seq_state(READ_ONCE(sup->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
             WARN_ON(rcu_seq_current(&sup->srcu_gp_seq) != sup->srcu_gp_seq_needed) ||
             WARN_ON(srcu_readers_active(ssp))) {
                 pr_info("%s: Active srcu_struct %p read state: %d gp state: %lu/%lu\n",
                         __func__, ssp, rcu_seq_state(READ_ONCE(sup->srcu_gp_seq)),
                         rcu_seq_current(&sup->srcu_gp_seq), sup->srcu_gp_seq_needed);
                 return; // Caller forgot to stop doing call_srcu()?
                         // Or caller invoked start_poll_synchronize_srcu()
                         // and then cleanup_srcu_struct() before that grace
                         // period ended?
         }
         kfree(sup->node);
         sup->node = NULL;
         sup->srcu_size_state = SRCU_SIZE_SMALL;
         if (!sup->sda_is_static) {
                 free_percpu(ssp->sda);
                 ssp->sda = NULL;
                 kfree(sup);
                 ssp->srcu_sup = NULL;
         }
 }
 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 
 #ifdef CONFIG_PROVE_RCU
 /*
  * Check for consistent NMI safety.
  */
 void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe)
 {
         int nmi_safe_mask = 1 << nmi_safe;
         int old_nmi_safe_mask;
         struct srcu_data *sdp;
 
         /* NMI-unsafe use in NMI is a bad sign */
         WARN_ON_ONCE(!nmi_safe && in_nmi());
         sdp = raw_cpu_ptr(ssp->sda);
         old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety);
         if (!old_nmi_safe_mask) {
                 WRITE_ONCE(sdp->srcu_nmi_safety, nmi_safe_mask);
                 return;
         }
         WARN_ONCE(old_nmi_safe_mask != nmi_safe_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_nmi_safe_mask, nmi_safe_mask);
 }
 EXPORT_SYMBOL_GPL(srcu_check_nmi_safety);
 #endif /* CONFIG_PROVE_RCU */
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.
  * Returns an index that must be passed to the matching srcu_read_unlock().
  */
 int __srcu_read_lock(struct srcu_struct *ssp)
 {
         int idx;
 
         idx = READ_ONCE(ssp->srcu_idx) & 0x1;
         this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
         smp_mb(); /* B */  /* Avoid leaking the critical section. */
         return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);
 
 /*
  * Removes the count for the old reader from the appropriate per-CPU
  * element of the srcu_struct.  Note that this may well be a different
  * CPU than that which was incremented by the corresponding srcu_read_lock().
  */
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
 {
         smp_mb(); /* C */  /* Avoid leaking the critical section. */
         this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 
 #ifdef CONFIG_NEED_SRCU_NMI_SAFE
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct, but in an NMI-safe manner using RMW atomics.
  * Returns an index that must be passed to the matching srcu_read_unlock().
  */
 int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
 {
         int idx;
         struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
 
         idx = READ_ONCE(ssp->srcu_idx) & 0x1;
         atomic_long_inc(&sdp->srcu_lock_count[idx]);
         smp_mb__after_atomic(); /* B */  /* Avoid leaking the critical section. */
         return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock_nmisafe);
 
 /*
  * Removes the count for the old reader from the appropriate per-CPU
  * element of the srcu_struct.  Note that this may well be a different
  * CPU than that which was incremented by the corresponding srcu_read_lock().
  */
 void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 {
         struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
 
         smp_mb__before_atomic(); /* C */  /* Avoid leaking the critical section. */
         atomic_long_inc(&sdp->srcu_unlock_count[idx]);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);
 
 #endif // CONFIG_NEED_SRCU_NMI_SAFE
 
 /*
  * Start an SRCU grace period.
  */
 static void srcu_gp_start(struct srcu_struct *ssp)
 {
         int state;
 
         lockdep_assert_held(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
         WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_sup->srcu_gp_seq, ssp->srcu_sup->srcu_gp_seq_needed));
         WRITE_ONCE(ssp->srcu_sup->srcu_gp_start, jiffies);
         WRITE_ONCE(ssp->srcu_sup->srcu_n_exp_nodelay, 0);
         smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
         rcu_seq_start(&ssp->srcu_sup->srcu_gp_seq);
         state = rcu_seq_state(ssp->srcu_sup->srcu_gp_seq);
         WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
 }
 
 
 static void srcu_delay_timer(struct timer_list *t)
 {
         struct srcu_data *sdp = container_of(t, struct srcu_data, delay_work);
 
         queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
 }
 
 static void srcu_queue_delayed_work_on(struct srcu_data *sdp,
                                        unsigned long delay)
 {
         if (!delay) {
                 queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
                 return;
         }
 
         timer_reduce(&sdp->delay_work, jiffies + delay);
 }
 
 /*
  * Schedule callback invocation for the specified srcu_data structure,
  * if possible, on the corresponding CPU.
  */
 static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
 {
         srcu_queue_delayed_work_on(sdp, delay);
 }
 
 /*
  * Schedule callback invocation for all srcu_data structures associated
  * with the specified srcu_node structure that have callbacks for the
  * just-completed grace period, the one corresponding to idx.  If possible,
  * schedule this invocation on the corresponding CPUs.
  */
 static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp,
                                   unsigned long mask, unsigned long delay)
 {
         int cpu;
 
         for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
                 if (!(mask & (1UL << (cpu - snp->grplo))))
                         continue;
                 srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
         }
 }
 
 /*
  * Note the end of an SRCU grace period.  Initiates callback invocation
  * and starts a new grace period if needed.
  *
  * The ->srcu_cb_mutex acquisition does not protect any data, but
  * instead prevents more than one grace period from starting while we
  * are initiating callback invocation.  This allows the ->srcu_have_cbs[]
  * array to have a finite number of elements.
  */
 static void srcu_gp_end(struct srcu_struct *ssp)
 {
         unsigned long cbdelay = 1;
         bool cbs;
         bool last_lvl;
         int cpu;
         unsigned long gpseq;
         int idx;
         unsigned long mask;
         struct srcu_data *sdp;
         unsigned long sgsne;
         struct srcu_node *snp;
         int ss_state;
         struct srcu_usage *sup = ssp->srcu_sup;
 
         /* Prevent more than one additional grace period. */
         mutex_lock(&sup->srcu_cb_mutex);
 
         /* End the current grace period. */
         spin_lock_irq_rcu_node(sup);
         idx = rcu_seq_state(sup->srcu_gp_seq);
         WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
         if (ULONG_CMP_LT(READ_ONCE(sup->srcu_gp_seq), READ_ONCE(sup->srcu_gp_seq_needed_exp)))
                 cbdelay = 0;
 
         WRITE_ONCE(sup->srcu_last_gp_end, ktime_get_mono_fast_ns());
         rcu_seq_end(&sup->srcu_gp_seq);
         gpseq = rcu_seq_current(&sup->srcu_gp_seq);
         if (ULONG_CMP_LT(sup->srcu_gp_seq_needed_exp, gpseq))
                 WRITE_ONCE(sup->srcu_gp_seq_needed_exp, gpseq);
         spin_unlock_irq_rcu_node(sup);
         mutex_unlock(&sup->srcu_gp_mutex);
         /* A new grace period can start at this point.  But only one. */
 
         /* Initiate callback invocation as needed. */
         ss_state = smp_load_acquire(&sup->srcu_size_state);
         if (ss_state < SRCU_SIZE_WAIT_BARRIER) {
                 srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, get_boot_cpu_id()),
                                         cbdelay);
         } else {
                 idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
                 srcu_for_each_node_breadth_first(ssp, snp) {
                         spin_lock_irq_rcu_node(snp);
                         cbs = false;
                         last_lvl = snp >= sup->level[rcu_num_lvls - 1];
                         if (last_lvl)
                                 cbs = ss_state < SRCU_SIZE_BIG || snp->srcu_have_cbs[idx] == gpseq;
                         snp->srcu_have_cbs[idx] = gpseq;
                         rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
                         sgsne = snp->srcu_gp_seq_needed_exp;
                         if (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, gpseq))
                                 WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
                         if (ss_state < SRCU_SIZE_BIG)
                                 mask = ~0;
                         else
                                 mask = snp->srcu_data_have_cbs[idx];
                         snp->srcu_data_have_cbs[idx] = 0;
                         spin_unlock_irq_rcu_node(snp);
                         if (cbs)
                                 srcu_schedule_cbs_snp(ssp, snp, mask, cbdelay);
                 }
         }
 
         /* Occasionally prevent srcu_data counter wrap. */
         if (!(gpseq & counter_wrap_check))
                 for_each_possible_cpu(cpu) {
                         sdp = per_cpu_ptr(ssp->sda, cpu);
                         spin_lock_irq_rcu_node(sdp);
                         if (ULONG_CMP_GE(gpseq, sdp->srcu_gp_seq_needed + 100))
                                 sdp->srcu_gp_seq_needed = gpseq;
                         if (ULONG_CMP_GE(gpseq, sdp->srcu_gp_seq_needed_exp + 100))
                                 sdp->srcu_gp_seq_needed_exp = gpseq;
                         spin_unlock_irq_rcu_node(sdp);
                 }
 
         /* Callback initiation done, allow grace periods after next. */
         mutex_unlock(&sup->srcu_cb_mutex);
 
         /* Start a new grace period if needed. */
         spin_lock_irq_rcu_node(sup);
         gpseq = rcu_seq_current(&sup->srcu_gp_seq);
         if (!rcu_seq_state(gpseq) &&
             ULONG_CMP_LT(gpseq, sup->srcu_gp_seq_needed)) {
                 srcu_gp_start(ssp);
                 spin_unlock_irq_rcu_node(sup);
                 srcu_reschedule(ssp, 0);
         } else {
                 spin_unlock_irq_rcu_node(sup);
         }
 
         /* Transition to big if needed. */
         if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
                 if (ss_state == SRCU_SIZE_ALLOC)
                         init_srcu_struct_nodes(ssp, GFP_KERNEL);
                 else
                         smp_store_release(&sup->srcu_size_state, ss_state + 1);
         }
 }
 
 /*
  * Funnel-locking scheme to scalably mediate many concurrent expedited
  * grace-period requests.  This function is invoked for the first known
  * expedited request for a grace period that has already been requested,
  * but without expediting.  To start a completely new grace period,
  * whether expedited or not, use srcu_funnel_gp_start() instead.
  */
 static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp,
                                   unsigned long s)
 {
         unsigned long flags;
         unsigned long sgsne;
 
         if (snp)
                 for (; snp != NULL; snp = snp->srcu_parent) {
                         sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp);
                         if (WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, s)) ||
                             (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
                                 return;
                         spin_lock_irqsave_rcu_node(snp, flags);
                         sgsne = snp->srcu_gp_seq_needed_exp;
                         if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)) {
                                 spin_unlock_irqrestore_rcu_node(snp, flags);
                                 return;
                         }
                         WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
                         spin_unlock_irqrestore_rcu_node(snp, flags);
                 }
         spin_lock_irqsave_ssp_contention(ssp, &flags);
         if (ULONG_CMP_LT(ssp->srcu_sup->srcu_gp_seq_needed_exp, s))
                 WRITE_ONCE(ssp->srcu_sup->srcu_gp_seq_needed_exp, s);
         spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);
 }
 
 /*
  * Funnel-locking scheme to scalably mediate many concurrent grace-period
  * requests.  The winner has to do the work of actually starting grace
  * period s.  Losers must either ensure that their desired grace-period
  * number is recorded on at least their leaf srcu_node structure, or they
  * must take steps to invoke their own callbacks.
  *
  * Note that this function also does the work of srcu_funnel_exp_start(),
  * in some cases by directly invoking it.
  *
  * The srcu read lock should be hold around this function. And s is a seq snap
  * after holding that lock.
  */
 static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
                                  unsigned long s, bool do_norm)
 {
         unsigned long flags;
         int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
         unsigned long sgsne;
         struct srcu_node *snp;
         struct srcu_node *snp_leaf;
         unsigned long snp_seq;
         struct srcu_usage *sup = ssp->srcu_sup;
 
         /* Ensure that snp node tree is fully initialized before traversing it */
         if (smp_load_acquire(&sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
                 snp_leaf = NULL;
         else
                 snp_leaf = sdp->mynode;
 
         if (snp_leaf)
                 /* Each pass through the loop does one level of the srcu_node tree. */
                 for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
                         if (WARN_ON_ONCE(rcu_seq_done(&sup->srcu_gp_seq, s)) && snp != snp_leaf)
                                 return; /* GP already done and CBs recorded. */
                         spin_lock_irqsave_rcu_node(snp, flags);
                         snp_seq = snp->srcu_have_cbs[idx];
                         if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
                                 if (snp == snp_leaf && snp_seq == s)
                                         snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
                                 spin_unlock_irqrestore_rcu_node(snp, flags);
                                 if (snp == snp_leaf && snp_seq != s) {
                                         srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
                                         return;
                                 }
                                 if (!do_norm)
                                         srcu_funnel_exp_start(ssp, snp, s);
                                 return;
                         }
                         snp->srcu_have_cbs[idx] = s;
                         if (snp == snp_leaf)
                                 snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
                         sgsne = snp->srcu_gp_seq_needed_exp;
                         if (!do_norm && (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, s)))
                                 WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
                         spin_unlock_irqrestore_rcu_node(snp, flags);
                 }
 
         /* Top of tree, must ensure the grace period will be started. */
         spin_lock_irqsave_ssp_contention(ssp, &flags);
         if (ULONG_CMP_LT(sup->srcu_gp_seq_needed, s)) {
                 /*
                  * Record need for grace period s.  Pair with load
                  * acquire setting up for initialization.
                  */
                 smp_store_release(&sup->srcu_gp_seq_needed, s); /*^^^*/
         }
         if (!do_norm && ULONG_CMP_LT(sup->srcu_gp_seq_needed_exp, s))
                 WRITE_ONCE(sup->srcu_gp_seq_needed_exp, s);
 
         /* If grace period not already in progress, start it. */
         if (!WARN_ON_ONCE(rcu_seq_done(&sup->srcu_gp_seq, s)) &&
             rcu_seq_state(sup->srcu_gp_seq) == SRCU_STATE_IDLE) {
                 WARN_ON_ONCE(ULONG_CMP_GE(sup->srcu_gp_seq, sup->srcu_gp_seq_needed));
                 srcu_gp_start(ssp);
 
                 // And how can that list_add() in the "else" clause
                 // possibly be safe for concurrent execution?  Well,
                 // it isn't.  And it does not have to be.  After all, it
                 // can only be executed during early boot when there is only
                 // the one boot CPU running with interrupts still disabled.
                 if (likely(srcu_init_done))
                         queue_delayed_work(rcu_gp_wq, &sup->work,
                                            !!srcu_get_delay(ssp));
                 else if (list_empty(&sup->work.work.entry))
                         list_add(&sup->work.work.entry, &srcu_boot_list);
         }
         spin_unlock_irqrestore_rcu_node(sup, flags);
 }
 
 /*
  * Wait until all readers counted by array index idx complete, but
  * loop an additional time if there is an expedited grace period pending.
  * The caller must ensure that ->srcu_idx is not changed while checking.
  */
 static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
 {
         unsigned long curdelay;
 
         curdelay = !srcu_get_delay(ssp);
 
         for (;;) {
                 if (srcu_readers_active_idx_check(ssp, idx))
                         return true;
                 if ((--trycount + curdelay) <= 0)
                         return false;
                 udelay(srcu_retry_check_delay);
         }
 }
 
 /*
  * Increment the ->srcu_idx counter so that future SRCU readers will
  * use the other rank of the ->srcu_(un)lock_count[] arrays.  This allows
  * us to wait for pre-existing readers in a starvation-free manner.
  */
 static void srcu_flip(struct srcu_struct *ssp)
 {
         /*
          * Because the flip of ->srcu_idx is executed only if the
          * preceding call to srcu_readers_active_idx_check() found that
          * the ->srcu_unlock_count[] and ->srcu_lock_count[] sums matched
          * and because that summing uses atomic_long_read(), there is
          * ordering due to a control dependency between that summing and
          * the WRITE_ONCE() in this call to srcu_flip().  This ordering
          * ensures that if this updater saw a given reader's increment from
          * __srcu_read_lock(), that reader was using a value of ->srcu_idx
          * from before the previous call to srcu_flip(), which should be
          * quite rare.  This ordering thus helps forward progress because
          * the grace period could otherwise be delayed by additional
          * calls to __srcu_read_lock() using that old (soon to be new)
          * value of ->srcu_idx.
          *
          * This sum-equality check and ordering also ensures that if
          * a given call to __srcu_read_lock() uses the new value of
          * ->srcu_idx, this updater's earlier scans cannot have seen
          * that reader's increments, which is all to the good, because
          * this grace period need not wait on that reader.  After all,
          * if those earlier scans had seen that reader, there would have
          * been a sum mismatch and this code would not be reached.
          *
          * This means that the following smp_mb() is redundant, but
          * it stays until either (1) Compilers learn about this sort of
          * control dependency or (2) Some production workload running on
          * a production system is unduly delayed by this slowpath smp_mb().
          */
         smp_mb(); /* E */  /* Pairs with B and C. */
 
         WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); // Flip the counter.
 
         /*
          * Ensure that if the updater misses an __srcu_read_unlock()
          * increment, that task's __srcu_read_lock() following its next
          * __srcu_read_lock() or __srcu_read_unlock() will see the above
          * counter update.  Note that both this memory barrier and the
          * one in srcu_readers_active_idx_check() provide the guarantee
          * for __srcu_read_lock().
          */
         smp_mb(); /* D */  /* Pairs with C. */
 }
 
 /*
  * If SRCU is likely idle, return true, otherwise return false.
  *
  * Note that it is OK for several current from-idle requests for a new
  * grace period from idle to specify expediting because they will all end
  * up requesting the same grace period anyhow.  So no loss.
  *
  * Note also that if any CPU (including the current one) is still invoking
  * callbacks, this function will nevertheless say "idle".  This is not
  * ideal, but the overhead of checking all CPUs' callback lists is even
  * less ideal, especially on large systems.  Furthermore, the wakeup
  * can happen before the callback is fully removed, so we have no choice
  * but to accept this type of error.
  *
  * This function is also subject to counter-wrap errors, but let's face
  * it, if this function was preempted for enough time for the counters
  * to wrap, it really doesn't matter whether or not we expedite the grace
  * period.  The extra overhead of a needlessly expedited grace period is
  * negligible when amortized over that time period, and the extra latency
  * of a needlessly non-expedited grace period is similarly negligible.
  */
 static bool srcu_might_be_idle(struct srcu_struct *ssp)
 {
         unsigned long curseq;
         unsigned long flags;
         struct srcu_data *sdp;
         unsigned long t;
         unsigned long tlast;
 
         check_init_srcu_struct(ssp);
         /* If the local srcu_data structure has callbacks, not idle.  */
         sdp = raw_cpu_ptr(ssp->sda);
         spin_lock_irqsave_rcu_node(sdp, flags);
         if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
                 spin_unlock_irqrestore_rcu_node(sdp, flags);
                 return false; /* Callbacks already present, so not idle. */
         }
         spin_unlock_irqrestore_rcu_node(sdp, flags);
 
         /*
          * No local callbacks, so probabilistically probe global state.
          * Exact information would require acquiring locks, which would
          * kill scalability, hence the probabilistic nature of the probe.
          */
 
         /* First, see if enough time has passed since the last GP. */
         t = ktime_get_mono_fast_ns();
         tlast = READ_ONCE(ssp->srcu_sup->srcu_last_gp_end);
         if (exp_holdoff == 0 ||
             time_in_range_open(t, tlast, tlast + exp_holdoff))
                 return false; /* Too soon after last GP. */
 
         /* Next, check for probable idleness. */
         curseq = rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq);
         smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
         if (ULONG_CMP_LT(curseq, READ_ONCE(ssp->srcu_sup->srcu_gp_seq_needed)))
                 return false; /* Grace period in progress, so not idle. */
         smp_mb(); /* Order ->srcu_gp_seq with prior access. */
         if (curseq != rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq))
                 return false; /* GP # changed, so not idle. */
         return true; /* With reasonable probability, idle! */
 }
 
 /*
  * SRCU callback function to leak a callback.
  */
 static void srcu_leak_callback(struct rcu_head *rhp)
 {
 }
 
 /*
  * Start an SRCU grace period, and also queue the callback if non-NULL.
  */
 static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
                                              struct rcu_head *rhp, bool do_norm)
 {
         unsigned long flags;
         int idx;
         bool needexp = false;
         bool needgp = false;
         unsigned long s;
         struct srcu_data *sdp;
         struct srcu_node *sdp_mynode;
         int ss_state;
 
         check_init_srcu_struct(ssp);
         /*
          * While starting a new grace period, make sure we are in an
          * SRCU read-side critical section so that the grace-period
          * sequence number cannot wrap around in the meantime.
          */
         idx = __srcu_read_lock_nmisafe(ssp);
         ss_state = smp_load_acquire(&ssp->srcu_sup->srcu_size_state);
         if (ss_state < SRCU_SIZE_WAIT_CALL)
                 sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id());
         else
                 sdp = raw_cpu_ptr(ssp->sda);
         spin_lock_irqsave_sdp_contention(sdp, &flags);
         if (rhp)
                 rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
         /*
          * It's crucial to capture the snapshot 's' for acceleration before
          * reading the current gp_seq that is used for advancing. This is
          * essential because if the acceleration snapshot is taken after a
          * failed advancement attempt, there's a risk that a grace period may
          * conclude and a new one may start in the interim. If the snapshot is
          * captured after this sequence of events, the acceleration snapshot 's'
          * could be excessively advanced, leading to acceleration failure.
          * In such a scenario, an 'acceleration leak' can occur, where new
          * callbacks become indefinitely stuck in the RCU_NEXT_TAIL segment.
          * Also note that encountering advancing failures is a normal
          * occurrence when the grace period for RCU_WAIT_TAIL is in progress.
          *
          * To see this, consider the following events which occur if
          * rcu_seq_snap() were to be called after advance:
          *
          *  1) The RCU_WAIT_TAIL segment has callbacks (gp_num = X + 4) and the
          *     RCU_NEXT_READY_TAIL also has callbacks (gp_num = X + 8).
          *
          *  2) The grace period for RCU_WAIT_TAIL is seen as started but not
          *     completed so rcu_seq_current() returns X + SRCU_STATE_SCAN1.
          *
          *  3) This value is passed to rcu_segcblist_advance() which can't move
          *     any segment forward and fails.
          *
          *  4) srcu_gp_start_if_needed() still proceeds with callback acceleration.
          *     But then the call to rcu_seq_snap() observes the grace period for the
          *     RCU_WAIT_TAIL segment as completed and the subsequent one for the
          *     RCU_NEXT_READY_TAIL segment as started (ie: X + 4 + SRCU_STATE_SCAN1)
          *     so it returns a snapshot of the next grace period, which is X + 12.
          *
          *  5) The value of X + 12 is passed to rcu_segcblist_accelerate() but the
          *     freshly enqueued callback in RCU_NEXT_TAIL can't move to
          *     RCU_NEXT_READY_TAIL which already has callbacks for a previous grace
          *     period (gp_num = X + 8). So acceleration fails.
          */
         s = rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq);
         if (rhp) {
                 rcu_segcblist_advance(&sdp->srcu_cblist,
                                       rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
                 /*
                  * Acceleration can never fail because the base current gp_seq
                  * used for acceleration is <= the value of gp_seq used for
                  * advancing. This means that RCU_NEXT_TAIL segment will
                  * always be able to be emptied by the acceleration into the
                  * RCU_NEXT_READY_TAIL or RCU_WAIT_TAIL segments.
                  */
                 WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s));
         }
         if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
                 sdp->srcu_gp_seq_needed = s;
                 needgp = true;
         }
         if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
                 sdp->srcu_gp_seq_needed_exp = s;
                 needexp = true;
         }
         spin_unlock_irqrestore_rcu_node(sdp, flags);
 
         /* Ensure that snp node tree is fully initialized before traversing it */
         if (ss_state < SRCU_SIZE_WAIT_BARRIER)
                 sdp_mynode = NULL;
         else
                 sdp_mynode = sdp->mynode;
 
         if (needgp)
                 srcu_funnel_gp_start(ssp, sdp, s, do_norm);
         else if (needexp)
                 srcu_funnel_exp_start(ssp, sdp_mynode, s);
         __srcu_read_unlock_nmisafe(ssp, idx);
         return s;
 }
 
 /*
  * Enqueue an SRCU callback on the srcu_data structure associated with
  * the current CPU and the specified srcu_struct structure, initiating
  * grace-period processing if it is not already running.
  *
  * Note that all CPUs must agree that the grace period extended beyond
  * all pre-existing SRCU 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 corresponding SRCU read-side critical section whose beginning
  * preceded the call to call_srcu().  It also means that each CPU executing
  * an SRCU read-side critical section that continues beyond the start of
  * "func()" must have executed a memory barrier after the call_srcu()
  * but before the beginning of that SRCU 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_srcu() and CPU B invoked the
  * resulting SRCU 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_srcu() and the invocation of "func()".
  * This guarantee applies even if CPU A and CPU B are the same CPU (but
  * again only if the system has more than one CPU).
  *
  * Of course, these guarantees apply only for invocations of call_srcu(),
  * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
  * srcu_struct structure.
  */
 static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
                         rcu_callback_t func, bool do_norm)
 {
         if (debug_rcu_head_queue(rhp)) {
                 /* Probable double call_srcu(), so leak the callback. */
                 WRITE_ONCE(rhp->func, srcu_leak_callback);
                 WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
                 return;
         }
         rhp->func = func;
         (void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
 }
 
 /**
  * call_srcu() - Queue a callback for invocation after an SRCU grace period
  * @ssp: srcu_struct in queue the callback
  * @rhp: structure to be used for queueing the SRCU callback.
  * @func: function to be invoked after the SRCU grace period
  *
  * The callback function will be invoked some time after a full SRCU
  * grace period elapses, in other words after all pre-existing SRCU
  * read-side critical sections have completed.  However, the callback
  * function might well execute concurrently with other SRCU read-side
  * critical sections that started after call_srcu() was invoked.  SRCU
  * read-side critical sections are delimited by srcu_read_lock() and
  * srcu_read_unlock(), and may be nested.
  *
  * The callback will be invoked from process context, but must nevertheless
  * be fast and must not block.
  */
 void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
                rcu_callback_t func)
 {
         __call_srcu(ssp, rhp, func, true);
 }
 EXPORT_SYMBOL_GPL(call_srcu);
 
 /*
  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
  */
 static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
 {
         struct rcu_synchronize rcu;
 
         srcu_lock_sync(&ssp->dep_map);
 
         RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
                          lock_is_held(&rcu_bh_lock_map) ||
                          lock_is_held(&rcu_lock_map) ||
                          lock_is_held(&rcu_sched_lock_map),
                          "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
 
         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
                 return;
         might_sleep();
         check_init_srcu_struct(ssp);
         init_completion(&rcu.completion);
         init_rcu_head_on_stack(&rcu.head);
         __call_srcu(ssp, &rcu.head, wakeme_after_rcu, do_norm);
         wait_for_completion(&rcu.completion);
         destroy_rcu_head_on_stack(&rcu.head);
 
         /*
          * Make sure that later code is ordered after the SRCU grace
          * period.  This pairs with the spin_lock_irq_rcu_node()
          * in srcu_invoke_callbacks().  Unlike Tree RCU, this is needed
          * because the current CPU might have been totally uninvolved with
          * (and thus unordered against) that grace period.
          */
         smp_mb();
 }
 
 /**
  * synchronize_srcu_expedited - Brute-force SRCU grace period
  * @ssp: srcu_struct with which to synchronize.
  *
  * Wait for an SRCU grace period to elapse, but be more aggressive about
  * spinning rather than blocking when waiting.
  *
  * Note that synchronize_srcu_expedited() has the same deadlock and
  * memory-ordering properties as does synchronize_srcu().
  */
 void synchronize_srcu_expedited(struct srcu_struct *ssp)
 {
         __synchronize_srcu(ssp, rcu_gp_is_normal());
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 
 /**
  * synchronize_srcu - wait for prior SRCU read-side critical-section completion
  * @ssp: srcu_struct with which to synchronize.
  *
  * Wait for the count to drain to zero of both indexes. To avoid the
  * possible starvation of synchronize_srcu(), it waits for the count of
  * the index=((->srcu_idx & 1) ^ 1) to drain to zero at first,
  * and then flip the srcu_idx and wait for the count of the other index.
  *
  * Can block; must be called from process context.
  *
  * Note that it is illegal to call synchronize_srcu() from the corresponding
  * SRCU read-side critical section; doing so will result in deadlock.
  * However, it is perfectly legal to call synchronize_srcu() on one
  * srcu_struct from some other srcu_struct's read-side critical section,
  * as long as the resulting graph of srcu_structs is acyclic.
  *
  * There are memory-ordering constraints implied by synchronize_srcu().
  * On systems with more than one CPU, when synchronize_srcu() returns,
  * each CPU is guaranteed to have executed a full memory barrier since
  * the end of its last corresponding SRCU read-side critical section
  * whose beginning preceded the call to synchronize_srcu().  In addition,
  * each CPU having an SRCU read-side critical section that extends beyond
  * the return from synchronize_srcu() is guaranteed to have executed a
  * full memory barrier after the beginning of synchronize_srcu() and before
  * the beginning of that SRCU 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_srcu(), 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_srcu().  This guarantee applies even if CPU A and CPU B
  * are the same CPU, but again only if the system has more than one CPU.
  *
  * Of course, these memory-ordering guarantees apply only when
  * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
  * passed the same srcu_struct structure.
  *
  * Implementation of these memory-ordering guarantees is similar to
  * that of synchronize_rcu().
  *
  * If SRCU is likely idle, expedite the first request.  This semantic
  * was provided by Classic SRCU, and is relied upon by its users, so TREE
  * SRCU must also provide it.  Note that detecting idleness is heuristic
  * and subject to both false positives and negatives.
  */
 void synchronize_srcu(struct srcu_struct *ssp)
 {
         if (srcu_might_be_idle(ssp) || rcu_gp_is_expedited())
                 synchronize_srcu_expedited(ssp);
         else
                 __synchronize_srcu(ssp, true);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
 /**
  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
  * @ssp: srcu_struct to provide cookie for.
  *
  * This function returns a cookie that can be passed to
  * poll_state_synchronize_srcu(), which will return true if a full grace
  * period has elapsed in the meantime.  It is the caller's responsibility
  * to make sure that grace period happens, for example, by invoking
  * call_srcu() after return from get_state_synchronize_srcu().
  */
 unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
 {
         // Any prior manipulation of SRCU-protected data must happen
         // before the load from ->srcu_gp_seq.
         smp_mb();
         return rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
 
 /**
  * start_poll_synchronize_srcu - Provide cookie and start grace period
  * @ssp: srcu_struct to provide cookie for.
  *
  * This function returns a cookie that can be passed to
  * poll_state_synchronize_srcu(), which will return true if a full grace
  * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
  * this function also ensures that any needed SRCU grace period will be
  * started.  This convenience does come at a cost in terms of CPU overhead.
  */
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
 {
         return srcu_gp_start_if_needed(ssp, NULL, true);
 }
 EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
 
 /**
  * poll_state_synchronize_srcu - Has cookie's grace period ended?
  * @ssp: srcu_struct to provide cookie for.
  * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
  *
  * This function takes the cookie that was returned from either
  * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
  * returns @true if an SRCU grace period elapsed since the time that the
  * cookie was created.
  *
  * Because cookies are finite in size, wrapping/overflow is possible.
  * This is more pronounced on 32-bit systems where cookies are 32 bits,
  * where in theory wrapping could happen in about 14 hours assuming
  * 25-microsecond expedited SRCU grace periods.  However, a more likely
  * overflow lower bound is on the order of 24 days in the case of
  * one-millisecond SRCU grace periods.  Of course, wrapping in a 64-bit
  * system requires geologic timespans, as in more than seven million years
  * even for expedited SRCU grace periods.
  *
  * Wrapping/overflow is much more of an issue for CONFIG_SMP=n systems
  * that also have CONFIG_PREEMPTION=n, which selects Tiny SRCU.  This uses
  * a 16-bit cookie, which rcutorture routinely wraps in a matter of a
  * few minutes.  If this proves to be a problem, this counter will be
  * expanded to the same size as for Tree SRCU.
  */
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
 {
         if (cookie != SRCU_GET_STATE_COMPLETED &&
             !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie))
                 return false;
         // Ensure that the end of the SRCU grace period happens before
         // any subsequent code that the caller might execute.
         smp_mb(); // ^^^
         return true;
 }
 EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
 
 /*
  * Callback function for srcu_barrier() use.
  */
 static void srcu_barrier_cb(struct rcu_head *rhp)
 {
         struct srcu_data *sdp;
         struct srcu_struct *ssp;
 
         sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
         ssp = sdp->ssp;
         if (atomic_dec_and_test(&ssp->srcu_sup->srcu_barrier_cpu_cnt))
                 complete(&ssp->srcu_sup->srcu_barrier_completion);
 }
 
 /*
  * Enqueue an srcu_barrier() callback on the specified srcu_data
  * structure's ->cblist.  but only if that ->cblist already has at least one
  * callback enqueued.  Note that if a CPU already has callbacks enqueue,
  * it must have already registered the need for a future grace period,
  * so all we need do is enqueue a callback that will use the same grace
  * period as the last callback already in the queue.
  */
 static void srcu_barrier_one_cpu(struct srcu_struct *ssp, struct srcu_data *sdp)
 {
         spin_lock_irq_rcu_node(sdp);
         atomic_inc(&ssp->srcu_sup->srcu_barrier_cpu_cnt);
         sdp->srcu_barrier_head.func = srcu_barrier_cb;
         debug_rcu_head_queue(&sdp->srcu_barrier_head);
         if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
                                    &sdp->srcu_barrier_head)) {
                 debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
                 atomic_dec(&ssp->srcu_sup->srcu_barrier_cpu_cnt);
         }
         spin_unlock_irq_rcu_node(sdp);
 }
 
 /**
  * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
  * @ssp: srcu_struct on which to wait for in-flight callbacks.
  */
 void srcu_barrier(struct srcu_struct *ssp)
 {
         int cpu;
         int idx;
         unsigned long s = rcu_seq_snap(&ssp->srcu_sup->srcu_barrier_seq);
 
         check_init_srcu_struct(ssp);
         mutex_lock(&ssp->srcu_sup->srcu_barrier_mutex);
         if (rcu_seq_done(&ssp->srcu_sup->srcu_barrier_seq, s)) {
                 smp_mb(); /* Force ordering following return. */
                 mutex_unlock(&ssp->srcu_sup->srcu_barrier_mutex);
                 return; /* Someone else did our work for us. */
         }
         rcu_seq_start(&ssp->srcu_sup->srcu_barrier_seq);
         init_completion(&ssp->srcu_sup->srcu_barrier_completion);
 
         /* Initial count prevents reaching zero until all CBs are posted. */
         atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 1);
 
         idx = __srcu_read_lock_nmisafe(ssp);
         if (smp_load_acquire(&ssp->srcu_sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
                 srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, get_boot_cpu_id()));
         else
                 for_each_possible_cpu(cpu)
                         srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu));
         __srcu_read_unlock_nmisafe(ssp, idx);
 
         /* Remove the initial count, at which point reaching zero can happen. */
         if (atomic_dec_and_test(&ssp->srcu_sup->srcu_barrier_cpu_cnt))
                 complete(&ssp->srcu_sup->srcu_barrier_completion);
         wait_for_completion(&ssp->srcu_sup->srcu_barrier_completion);
 
         rcu_seq_end(&ssp->srcu_sup->srcu_barrier_seq);
         mutex_unlock(&ssp->srcu_sup->srcu_barrier_mutex);
 }
 EXPORT_SYMBOL_GPL(srcu_barrier);
 
 /**
  * srcu_batches_completed - return batches completed.
  * @ssp: srcu_struct on which to report batch completion.
  *
  * Report the number of batches, correlated with, but not necessarily
  * precisely the same as, the number of grace periods that have elapsed.
  */
 unsigned long srcu_batches_completed(struct srcu_struct *ssp)
 {
         return READ_ONCE(ssp->srcu_idx);
 }
 EXPORT_SYMBOL_GPL(srcu_batches_completed);
 
 /*
  * Core SRCU state machine.  Push state bits of ->srcu_gp_seq
  * to SRCU_STATE_SCAN2, and invoke srcu_gp_end() when scan has
  * completed in that state.
  */
 static void srcu_advance_state(struct srcu_struct *ssp)
 {
         int idx;
 
         mutex_lock(&ssp->srcu_sup->srcu_gp_mutex);
 
         /*
          * Because readers might be delayed for an extended period after
          * fetching ->srcu_idx for their index, at any point in time there
          * might well be readers using both idx=0 and idx=1.  We therefore
          * need to wait for readers to clear from both index values before
          * invoking a callback.
          *
          * The load-acquire ensures that we see the accesses performed
          * by the prior grace period.
          */
         idx = rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq)); /* ^^^ */
         if (idx == SRCU_STATE_IDLE) {
                 spin_lock_irq_rcu_node(ssp->srcu_sup);
                 if (ULONG_CMP_GE(ssp->srcu_sup->srcu_gp_seq, ssp->srcu_sup->srcu_gp_seq_needed)) {
                         WARN_ON_ONCE(rcu_seq_state(ssp->srcu_sup->srcu_gp_seq));
                         spin_unlock_irq_rcu_node(ssp->srcu_sup);
                         mutex_unlock(&ssp->srcu_sup->srcu_gp_mutex);
                         return;
                 }
                 idx = rcu_seq_state(READ_ONCE(ssp->srcu_sup->srcu_gp_seq));
                 if (idx == SRCU_STATE_IDLE)
                         srcu_gp_start(ssp);
                 spin_unlock_irq_rcu_node(ssp->srcu_sup);
                 if (idx != SRCU_STATE_IDLE) {
                         mutex_unlock(&ssp->srcu_sup->srcu_gp_mutex);
                         return; /* Someone else started the grace period. */
                 }
         }
 
         if (rcu_seq_state(READ_ONCE(ssp->srcu_sup->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
                 idx = 1 ^ (ssp->srcu_idx & 1);
                 if (!try_check_zero(ssp, idx, 1)) {
                         mutex_unlock(&ssp->srcu_sup->srcu_gp_mutex);
                         return; /* readers present, retry later. */
                 }
                 srcu_flip(ssp);
                 spin_lock_irq_rcu_node(ssp->srcu_sup);
                 rcu_seq_set_state(&ssp->srcu_sup->srcu_gp_seq, SRCU_STATE_SCAN2);
                 ssp->srcu_sup->srcu_n_exp_nodelay = 0;
                 spin_unlock_irq_rcu_node(ssp->srcu_sup);
         }
 
         if (rcu_seq_state(READ_ONCE(ssp->srcu_sup->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
 
                 /*
                  * SRCU read-side critical sections are normally short,
                  * so check at least twice in quick succession after a flip.
                  */
                 idx = 1 ^ (ssp->srcu_idx & 1);
                 if (!try_check_zero(ssp, idx, 2)) {
                         mutex_unlock(&ssp->srcu_sup->srcu_gp_mutex);
                         return; /* readers present, retry later. */
                 }
                 ssp->srcu_sup->srcu_n_exp_nodelay = 0;
                 srcu_gp_end(ssp);  /* Releases ->srcu_gp_mutex. */
         }
 }
 
 /*
  * Invoke a limited number of SRCU callbacks that have passed through
  * their grace period.  If there are more to do, SRCU will reschedule
  * the workqueue.  Note that needed memory barriers have been executed
  * in this task's context by srcu_readers_active_idx_check().
  */
 static void srcu_invoke_callbacks(struct work_struct *work)
 {
         long len;
         bool more;
         struct rcu_cblist ready_cbs;
         struct rcu_head *rhp;
         struct srcu_data *sdp;
         struct srcu_struct *ssp;
 
         sdp = container_of(work, struct srcu_data, work);
 
         ssp = sdp->ssp;
         rcu_cblist_init(&ready_cbs);
         spin_lock_irq_rcu_node(sdp);
         WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
         rcu_segcblist_advance(&sdp->srcu_cblist,
                               rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
         /*
          * Although this function is theoretically re-entrant, concurrent
          * callbacks invocation is disallowed to avoid executing an SRCU barrier
          * too early.
          */
         if (sdp->srcu_cblist_invoking ||
             !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
                 spin_unlock_irq_rcu_node(sdp);
                 return;  /* Someone else on the job or nothing to do. */
         }
 
         /* We are on the job!  Extract and invoke ready callbacks. */
         sdp->srcu_cblist_invoking = true;
         rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
         len = ready_cbs.len;
         spin_unlock_irq_rcu_node(sdp);
         rhp = rcu_cblist_dequeue(&ready_cbs);
         for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
                 debug_rcu_head_unqueue(rhp);
                 debug_rcu_head_callback(rhp);
                 local_bh_disable();
                 rhp->func(rhp);
                 local_bh_enable();
         }
         WARN_ON_ONCE(ready_cbs.len);
 
         /*
          * Update counts, accelerate new callbacks, and if needed,
          * schedule another round of callback invocation.
          */
         spin_lock_irq_rcu_node(sdp);
         rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
         sdp->srcu_cblist_invoking = false;
         more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
         spin_unlock_irq_rcu_node(sdp);
         /* An SRCU barrier or callbacks from previous nesting work pending */
         if (more)
                 srcu_schedule_cbs_sdp(sdp, 0);
 }
 
 /*
  * Finished one round of SRCU grace period.  Start another if there are
  * more SRCU callbacks queued, otherwise put SRCU into not-running state.
  */
 static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay)
 {
         bool pushgp = true;
 
         spin_lock_irq_rcu_node(ssp->srcu_sup);
         if (ULONG_CMP_GE(ssp->srcu_sup->srcu_gp_seq, ssp->srcu_sup->srcu_gp_seq_needed)) {
                 if (!WARN_ON_ONCE(rcu_seq_state(ssp->srcu_sup->srcu_gp_seq))) {
                         /* All requests fulfilled, time to go idle. */
                         pushgp = false;
                 }
         } else if (!rcu_seq_state(ssp->srcu_sup->srcu_gp_seq)) {
                 /* Outstanding request and no GP.  Start one. */
                 srcu_gp_start(ssp);
         }
         spin_unlock_irq_rcu_node(ssp->srcu_sup);
 
         if (pushgp)
                 queue_delayed_work(rcu_gp_wq, &ssp->srcu_sup->work, delay);
 }
 
 /*
  * This is the work-queue function that handles SRCU grace periods.
  */
 static void process_srcu(struct work_struct *work)
 {
         unsigned long curdelay;
         unsigned long j;
         struct srcu_struct *ssp;
         struct srcu_usage *sup;
 
         sup = container_of(work, struct srcu_usage, work.work);
         ssp = sup->srcu_ssp;
 
         srcu_advance_state(ssp);
         curdelay = srcu_get_delay(ssp);
         if (curdelay) {
                 WRITE_ONCE(sup->reschedule_count, 0);
         } else {
                 j = jiffies;
                 if (READ_ONCE(sup->reschedule_jiffies) == j) {
                         WRITE_ONCE(sup->reschedule_count, READ_ONCE(sup->reschedule_count) + 1);
                         if (READ_ONCE(sup->reschedule_count) > srcu_max_nodelay)
                                 curdelay = 1;
                 } else {
                         WRITE_ONCE(sup->reschedule_count, 1);
                         WRITE_ONCE(sup->reschedule_jiffies, j);
                 }
         }
         srcu_reschedule(ssp, curdelay);
 }
 
 void srcutorture_get_gp_data(struct srcu_struct *ssp, int *flags,
                              unsigned long *gp_seq)
 {
         *flags = 0;
         *gp_seq = rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
 
 static const char * const srcu_size_state_name[] = {
         "SRCU_SIZE_SMALL",
         "SRCU_SIZE_ALLOC",
         "SRCU_SIZE_WAIT_BARRIER",
         "SRCU_SIZE_WAIT_CALL",
         "SRCU_SIZE_WAIT_CBS1",
         "SRCU_SIZE_WAIT_CBS2",
         "SRCU_SIZE_WAIT_CBS3",
         "SRCU_SIZE_WAIT_CBS4",
         "SRCU_SIZE_BIG",
         "SRCU_SIZE_???",
 };
 
 void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 {
         int cpu;
         int idx;
         unsigned long s0 = 0, s1 = 0;
         int ss_state = READ_ONCE(ssp->srcu_sup->srcu_size_state);
         int ss_state_idx = ss_state;
 
         idx = ssp->srcu_idx & 0x1;
         if (ss_state < 0 || ss_state >= ARRAY_SIZE(srcu_size_state_name))
                 ss_state_idx = ARRAY_SIZE(srcu_size_state_name) - 1;
         pr_alert("%s%s Tree SRCU g%ld state %d (%s)",
                  tt, tf, rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq), ss_state,
                  srcu_size_state_name[ss_state_idx]);
         if (!ssp->sda) {
                 // Called after cleanup_srcu_struct(), perhaps.
                 pr_cont(" No per-CPU srcu_data structures (->sda == NULL).\n");
         } else {
                 pr_cont(" per-CPU(idx=%d):", idx);
                 for_each_possible_cpu(cpu) {
                         unsigned long l0, l1;
                         unsigned long u0, u1;
                         long c0, c1;
                         struct srcu_data *sdp;
 
                         sdp = per_cpu_ptr(ssp->sda, cpu);
                         u0 = data_race(atomic_long_read(&sdp->srcu_unlock_count[!idx]));
                         u1 = data_race(atomic_long_read(&sdp->srcu_unlock_count[idx]));
 
                         /*
                          * Make sure that a lock is always counted if the corresponding
                          * unlock is counted.
                          */
                         smp_rmb();
 
                         l0 = data_race(atomic_long_read(&sdp->srcu_lock_count[!idx]));
                         l1 = data_race(atomic_long_read(&sdp->srcu_lock_count[idx]));
 
                         c0 = l0 - u0;
                         c1 = l1 - u1;
                         pr_cont(" %d(%ld,%ld %c)",
                                 cpu, c0, c1,
                                 "C."[rcu_segcblist_empty(&sdp->srcu_cblist)]);
                         s0 += c0;
                         s1 += c1;
                 }
                 pr_cont(" T(%ld,%ld)\n", s0, s1);
         }
         if (SRCU_SIZING_IS_TORTURE())
                 srcu_transition_to_big(ssp);
 }
 EXPORT_SYMBOL_GPL(srcu_torture_stats_print);
 
 static int __init srcu_bootup_announce(void)
 {
         pr_info("Hierarchical SRCU implementation.\n");
         if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
                 pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
         if (srcu_retry_check_delay != SRCU_DEFAULT_RETRY_CHECK_DELAY)
                 pr_info("\tNon-default retry check delay of %lu us.\n", srcu_retry_check_delay);
         if (srcu_max_nodelay != SRCU_DEFAULT_MAX_NODELAY)
                 pr_info("\tNon-default max no-delay of %lu.\n", srcu_max_nodelay);
         pr_info("\tMax phase no-delay instances is %lu.\n", srcu_max_nodelay_phase);
         return 0;
 }
 early_initcall(srcu_bootup_announce);
 
 void __init srcu_init(void)
 {
         struct srcu_usage *sup;
 
         /* Decide on srcu_struct-size strategy. */
         if (SRCU_SIZING_IS(SRCU_SIZING_AUTO)) {
                 if (nr_cpu_ids >= big_cpu_lim) {
                         convert_to_big = SRCU_SIZING_INIT; // Don't bother waiting for contention.
                         pr_info("%s: Setting srcu_struct sizes to big.\n", __func__);
                 } else {
                         convert_to_big = SRCU_SIZING_NONE | SRCU_SIZING_CONTEND;
                         pr_info("%s: Setting srcu_struct sizes based on contention.\n", __func__);
                 }
         }
 
         /*
          * Once that is set, call_srcu() can follow the normal path and
          * queue delayed work. This must follow RCU workqueues creation
          * and timers initialization.
          */
         srcu_init_done = true;
         while (!list_empty(&srcu_boot_list)) {
                 sup = list_first_entry(&srcu_boot_list, struct srcu_usage,
                                       work.work.entry);
                 list_del_init(&sup->work.work.entry);
                 if (SRCU_SIZING_IS(SRCU_SIZING_INIT) &&
                     sup->srcu_size_state == SRCU_SIZE_SMALL)
                         sup->srcu_size_state = SRCU_SIZE_ALLOC;
                 queue_work(rcu_gp_wq, &sup->work.work);
         }
 }
 
 #ifdef CONFIG_MODULES
 
 /* Initialize any global-scope srcu_struct structures used by this module. */
 static int srcu_module_coming(struct module *mod)
 {
         int i;
         struct srcu_struct *ssp;
         struct srcu_struct **sspp = mod->srcu_struct_ptrs;
 
         for (i = 0; i < mod->num_srcu_structs; i++) {
                 ssp = *(sspp++);
                 ssp->sda = alloc_percpu(struct srcu_data);
                 if (WARN_ON_ONCE(!ssp->sda))
                         return -ENOMEM;
         }
         return 0;
 }
 
 /* Clean up any global-scope srcu_struct structures used by this module. */
 static void srcu_module_going(struct module *mod)
 {
         int i;
         struct srcu_struct *ssp;
         struct srcu_struct **sspp = mod->srcu_struct_ptrs;
 
         for (i = 0; i < mod->num_srcu_structs; i++) {
                 ssp = *(sspp++);
                 if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed)) &&
                     !WARN_ON_ONCE(!ssp->srcu_sup->sda_is_static))
                         cleanup_srcu_struct(ssp);
                 if (!WARN_ON(srcu_readers_active(ssp)))
                         free_percpu(ssp->sda);
         }
 }
 
 /* Handle one module, either coming or going. */
 static int srcu_module_notify(struct notifier_block *self,
                               unsigned long val, void *data)
 {
         struct module *mod = data;
         int ret = 0;
 
         switch (val) {
         case MODULE_STATE_COMING:
                 ret = srcu_module_coming(mod);
                 break;
         case MODULE_STATE_GOING:
                 srcu_module_going(mod);
                 break;
         default:
                 break;
         }
         return ret;
 }
 
 static struct notifier_block srcu_module_nb = {
         .notifier_call = srcu_module_notify,
         .priority = 0,
 };
 
 static __init int init_srcu_module_notifier(void)
 {
         int ret;
 
         ret = register_module_notifier(&srcu_module_nb);
         if (ret)
                 pr_warn("Failed to register srcu module notifier\n");
         return ret;
 }
 late_initcall(init_srcu_module_notifier);
 
 #endif /* #ifdef CONFIG_MODULES */
 

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