Release 4.15 kernel/rcu/srcutree.c

/*
 * Sleepable Read-Copy Update mechanism for mutual exclusion.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, you can access it online at
 * http://www.gnu.org/licenses/gpl-2.0.html.
 *
 * Copyright (C) IBM Corporation, 2006
 * Copyright (C) Fujitsu, 2012
 *
 * Author: Paul McKenney <paulmck@us.ibm.com>
 *         Lai Jiangshan <laijs@cn.fujitsu.com>
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *              Documentation/RCU/ *.txt
 *
 */

#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/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);

static void srcu_invoke_callbacks(struct work_struct *work);
static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
static void process_srcu(struct work_struct *work);

/*
 * Initialize SRCU combining tree.  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_nodes(struct srcu_struct *sp, bool is_static)
{
	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;

	/* Work out the overall tree geometry. */
	sp->level[0] = &sp->node[0];
	for (i = 1; i < rcu_num_lvls; i++)
		sp->level[i] = sp->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. */
	rcu_for_each_node_breadth_first(sp, snp) {
		raw_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] = 0;
			snp->srcu_data_have_cbs[i] = 0;
		}
		snp->srcu_gp_seq_needed_exp = 0;
		snp->grplo = -1;
		snp->grphi = -1;
		if (snp == &sp->node[0]) {
			/* Root node, special case. */
			snp->srcu_parent = NULL;
			continue;
		}

		/* Non-root node. */
		if (snp == sp->level[level + 1])
			level++;
		snp->srcu_parent = sp->level[level - 1] +
				   (snp - sp->level[level]) /
				   levelspread[level - 1];
	}

	/*
         * 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));
	level = rcu_num_lvls - 1;
	snp_first = sp->level[level];
	for_each_possible_cpu(cpu) {
		sdp = per_cpu_ptr(sp->sda, cpu);
		raw_spin_lock_init(&ACCESS_PRIVATE(sdp, lock));
		rcu_segcblist_init(&sdp->srcu_cblist);
		sdp->srcu_cblist_invoking = false;
		sdp->srcu_gp_seq_needed = sp->srcu_gp_seq;
		sdp->srcu_gp_seq_needed_exp = sp->srcu_gp_seq;
		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->cpu = cpu;
		INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks);
		sdp->sp = sp;
		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
		if (is_static)
			continue;

		/* Dynamically allocated, better be no srcu_read_locks()! */
		for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
			sdp->srcu_lock_count[i] = 0;
			sdp->srcu_unlock_count[i] = 0;
		}
	}
}

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/*
 * Initialize non-compile-time initialized fields, including the
 * associated srcu_node and srcu_data structures.  The is_static
 * parameter is passed through to init_srcu_struct_nodes(), and
 * also tells us that ->sda has already been wired up to srcu_data.
 */


static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
{
	mutex_init(&sp->srcu_cb_mutex);
	mutex_init(&sp->srcu_gp_mutex);
	sp->srcu_idx = 0;
	sp->srcu_gp_seq = 0;
	sp->srcu_barrier_seq = 0;
	mutex_init(&sp->srcu_barrier_mutex);
	atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
	INIT_DELAYED_WORK(&sp->work, process_srcu);
	if (!is_static)
		sp->sda = alloc_percpu(struct srcu_data);
	init_srcu_struct_nodes(sp, is_static);
	sp->srcu_gp_seq_needed_exp = 0;
	sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
	smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
	return sp->sda ? 0 : -ENOMEM;
}

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#ifdef CONFIG_DEBUG_LOCK_ALLOC



int __init_srcu_struct(struct srcu_struct *sp, const char *name,
		       struct lock_class_key *key)
{
	/* Don't re-initialize a lock while it is held. */
	debug_check_no_locks_freed((void *)sp, sizeof(*sp));
	lockdep_init_map(&sp->dep_map, name, key, 0);
	raw_spin_lock_init(&ACCESS_PRIVATE(sp, lock));
	return init_srcu_struct_fields(sp, false);
}
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EXPORT_SYMBOL_GPL(__init_srcu_struct);

#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/**
 * init_srcu_struct - initialize a sleep-RCU structure
 * @sp: 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 *sp)
{
	raw_spin_lock_init(&ACCESS_PRIVATE(sp, lock));
	return init_srcu_struct_fields(sp, false);
}
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EXPORT_SYMBOL_GPL(init_srcu_struct);

#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/*
 * 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 sp->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 *sp)
{
	unsigned long flags;

	WARN_ON_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INIT);
	/* The smp_load_acquire() pairs with the smp_store_release(). */
	if (!rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq_needed))) /*^^^*/
		return; /* Already initialized. */
	raw_spin_lock_irqsave_rcu_node(sp, flags);
	if (!rcu_seq_state(sp->srcu_gp_seq_needed)) {
		raw_spin_unlock_irqrestore_rcu_node(sp, flags);
		return;
	}
	init_srcu_struct_fields(sp, true);
	raw_spin_unlock_irqrestore_rcu_node(sp, flags);
}

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/*
 * 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 *sp, int idx)
{
	int cpu;
	unsigned long sum = 0;

	for_each_possible_cpu(cpu) {
		struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);

		sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
	}
	return sum;
}

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/*
 * 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 *sp, int idx)
{
	int cpu;
	unsigned long sum = 0;

	for_each_possible_cpu(cpu) {
		struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);

		sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
	}
	return sum;
}

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/*
 * Return true if the number of pre-existing readers is determined to
 * be zero.
 */


static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
	unsigned long unlocks;

	unlocks = srcu_readers_unlock_idx(sp, 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 time in between. This does
         * not mean that there are no more readers, as one could have read
         * the current index but not have incremented the lock counter yet.
         *
         * So suppose that the updater is preempted here for so long
         * that more than ULONG_MAX non-nested readers come and go in
         * the meantime.  It turns out that this cannot result in overflow
         * because if a reader modifies its unlock count after we read it
         * above, then that reader's next load of ->srcu_idx is guaranteed
         * to get the new value, which will cause it to operate on the
         * other bank of counters, where it cannot contribute to the
         * overflow of these counters.  This means that there is a maximum
         * of 2*NR_CPUS increments, which cannot overflow given current
         * systems, especially not on 64-bit systems.
         *
         * OK, how about nesting?  This does impose a limit on nesting
         * of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient,
         * especially on 64-bit systems.
         */
	return srcu_readers_lock_idx(sp, idx) == unlocks;
}

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/**
 * srcu_readers_active - returns true if there are readers. and false
 *                       otherwise
 * @sp: 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 *sp)
{
	int cpu;
	unsigned long sum = 0;

	for_each_possible_cpu(cpu) {
		struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);

		sum += READ_ONCE(cpuc->srcu_lock_count[0]);
		sum += READ_ONCE(cpuc->srcu_lock_count[1]);
		sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
		sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
	}
	return sum;
}

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#define SRCU_INTERVAL		1

/*
 * Return grace-period delay, zero if there are expedited grace
 * periods pending, SRCU_INTERVAL otherwise.
 */


static unsigned long srcu_get_delay(struct srcu_struct *sp)
{
	if (ULONG_CMP_LT(READ_ONCE(sp->srcu_gp_seq),
			 READ_ONCE(sp->srcu_gp_seq_needed_exp)))
		return 0;
	return SRCU_INTERVAL;
}

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/**
 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 * @sp: 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 *sp)
{
	int cpu;

	if (WARN_ON(!srcu_get_delay(sp)))
		return; /* Leakage unless caller handles error. */
	if (WARN_ON(srcu_readers_active(sp)))
		return; /* Leakage unless caller handles error. */
	flush_delayed_work(&sp->work);
	for_each_possible_cpu(cpu)
		flush_delayed_work(&per_cpu_ptr(sp->sda, cpu)->work);
	if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
	    WARN_ON(srcu_readers_active(sp))) {
		pr_info("cleanup_srcu_struct: Active srcu_struct %p state: %d\n", sp, rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
		return; /* Caller forgot to stop doing call_srcu()? */
	}
	free_percpu(sp->sda);
	sp->sda = NULL;
}
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

/*
 * 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 *sp)
{
	int idx;

	idx = READ_ONCE(sp->srcu_idx) & 0x1;
	this_cpu_inc(sp->sda->srcu_lock_count[idx]);
	smp_mb(); /* B */  /* Avoid leaking the critical section. */
	return idx;
}
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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 *sp, int idx)
{
	smp_mb(); /* C */  /* Avoid leaking the critical section. */
	this_cpu_inc(sp->sda->srcu_unlock_count[idx]);
}
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EXPORT_SYMBOL_GPL(__srcu_read_unlock);

/*
 * We use an adaptive strategy for synchronize_srcu() and especially for
 * synchronize_srcu_expedited().  We spin for a fixed time period
 * (defined below) to allow SRCU readers to exit their read-side critical
 * sections.  If there are still some readers after a few microseconds,
 * we repeatedly block for 1-millisecond time periods.
 */

#define SRCU_RETRY_CHECK_DELAY		5

/*
 * Start an SRCU grace period.
 */


static void srcu_gp_start(struct srcu_struct *sp)
{
	struct srcu_data *sdp = this_cpu_ptr(sp->sda);
	int state;

	lockdep_assert_held(&sp->lock);
	WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
	rcu_segcblist_advance(&sdp->srcu_cblist,
			      rcu_seq_current(&sp->srcu_gp_seq));
	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
				       rcu_seq_snap(&sp->srcu_gp_seq));
	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
	rcu_seq_start(&sp->srcu_gp_seq);
	state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
}

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/*
 * Track online CPUs to guide callback workqueue placement.
 */
DEFINE_PER_CPU(bool, srcu_online);



void srcu_online_cpu(unsigned int cpu)
{
	WRITE_ONCE(per_cpu(srcu_online, cpu), true);
}

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void srcu_offline_cpu(unsigned int cpu)
{
	WRITE_ONCE(per_cpu(srcu_online, cpu), false);
}

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/*
 * Place the workqueue handler on the specified CPU if online, otherwise
 * just run it whereever.  This is useful for placing workqueue handlers
 * that are to invoke the specified CPU's callbacks.
 */


static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
				       struct delayed_work *dwork,
				       unsigned long delay)
{
	bool ret;

	preempt_disable();
	if (READ_ONCE(per_cpu(srcu_online, cpu)))
		ret = queue_delayed_work_on(cpu, wq, dwork, delay);
	else
		ret = queue_delayed_work(wq, dwork, delay);
	preempt_enable();
	return ret;
}

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/*
 * 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->cpu, system_power_efficient_wq,
				   &sdp->work, delay);
}

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/*
 * 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 *sp, struct srcu_node *snp,
				  unsigned long mask, unsigned long delay)
{
	int cpu;

	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
		if (!(mask & (1 << (cpu - snp->grplo))))
			continue;
		srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), delay);
	}
}

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/*
 * 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 *sp)
{
	unsigned long cbdelay;
	bool cbs;
	int cpu;
	unsigned long flags;
	unsigned long gpseq;
	int idx;
	int idxnext;
	unsigned long mask;
	struct srcu_data *sdp;
	struct srcu_node *snp;

	/* Prevent more than one additional grace period. */
	mutex_lock(&sp->srcu_cb_mutex);

	/* End the current grace period. */
	raw_spin_lock_irq_rcu_node(sp);
	idx = rcu_seq_state(sp->srcu_gp_seq);
	WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
	cbdelay = srcu_get_delay(sp);
	sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
	rcu_seq_end(&sp->srcu_gp_seq);
	gpseq = rcu_seq_current(&sp->srcu_gp_seq);
	if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
		sp->srcu_gp_seq_needed_exp = gpseq;
	raw_spin_unlock_irq_rcu_node(sp);
	mutex_unlock(&sp->srcu_gp_mutex);
	/* A new grace period can start at this point.  But only one. */

	/* Initiate callback invocation as needed. */
	idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
	idxnext = (idx + 1) % ARRAY_SIZE(snp->srcu_have_cbs);
	rcu_for_each_node_breadth_first(sp, snp) {
		raw_spin_lock_irq_rcu_node(snp);
		cbs = false;
		if (snp >= sp->level[rcu_num_lvls - 1])
			cbs = snp->srcu_have_cbs[idx] == gpseq;
		snp->srcu_have_cbs[idx] = gpseq;
		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
		if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
			snp->srcu_gp_seq_needed_exp = gpseq;
		mask = snp->srcu_data_have_cbs[idx];
		snp->srcu_data_have_cbs[idx] = 0;
		raw_spin_unlock_irq_rcu_node(snp);
		if (cbs)
			srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);

		/* Occasionally prevent srcu_data counter wrap. */
		if (!(gpseq & counter_wrap_check))
			for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
				sdp = per_cpu_ptr(sp->sda, cpu);
				raw_spin_lock_irqsave_rcu_node(sdp, flags);
				if (ULONG_CMP_GE(gpseq,
						 sdp->srcu_gp_seq_needed + 100))
					sdp->srcu_gp_seq_needed = gpseq;
				raw_spin_unlock_irqrestore_rcu_node(sdp, flags);
			}
	}

	/* Callback initiation done, allow grace periods after next. */
	mutex_unlock(&sp->srcu_cb_mutex);

	/* Start a new grace period if needed. */
	raw_spin_lock_irq_rcu_node(sp);
	gpseq = rcu_seq_current(&sp->srcu_gp_seq);
	if (!rcu_seq_state(gpseq) &&
	    ULONG_CMP_LT(gpseq, sp->srcu_gp_seq_needed)) {
		srcu_gp_start(sp);
		raw_spin_unlock_irq_rcu_node(sp);
		/* Throttle expedited grace periods: Should be rare! */
		srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
				    ? 0 : SRCU_INTERVAL);
	} else {
		raw_spin_unlock_irq_rcu_node(sp);
	}
}

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/*
 * 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 *sp, struct srcu_node *snp,
				  unsigned long s)
{
	unsigned long flags;

	for (; snp != NULL; snp = snp->srcu_parent) {
		if (rcu_seq_done(&sp->srcu_gp_seq, s) ||
		    ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
			return;
		raw_spin_lock_irqsave_rcu_node(snp, flags);
		if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
			raw_spin_unlock_irqrestore_rcu_node(snp, flags);
			return;
		}
		WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
		raw_spin_unlock_irqrestore_rcu_node(snp, flags);
	}
	raw_spin_lock_irqsave_rcu_node(sp, flags);
	if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
		sp->srcu_gp_seq_needed_exp = s;
	raw_spin_unlock_irqrestore_rcu_node(sp, flags);
}

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/*
 * 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.
 */


static void srcu_funnel_gp_start(struct srcu_struct *sp, 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);
	struct srcu_node *snp = sdp->mynode;
	unsigned long snp_seq;

	/* Each pass through the loop does one level of the srcu_node tree. */
	for (; snp != NULL; snp = snp->srcu_parent) {
		if (rcu_seq_done(&sp->srcu_gp_seq, s) && snp != sdp->mynode)
			return; /* GP already done and CBs recorded. */
		raw_spin_lock_irqsave_rcu_node(snp, flags);
		if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
			snp_seq = snp->srcu_have_cbs[idx];
			if (snp == sdp->mynode && snp_seq == s)
				snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
			raw_spin_unlock_irqrestore_rcu_node(snp, flags);
			if (snp == sdp->mynode && snp_seq != s) {
				srcu_schedule_cbs_sdp(sdp, do_norm
							   ? SRCU_INTERVAL
							   : 0);
				return;
			}
			if (!do_norm)
				srcu_funnel_exp_start(sp, snp, s);
			return;
		}
		snp->srcu_have_cbs[idx] = s;
		if (snp == sdp->mynode)
			snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
		if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
			snp->srcu_gp_seq_needed_exp = s;
		raw_spin_unlock_irqrestore_rcu_node(snp, flags);
	}

	/* Top of tree, must ensure the grace period will be started. */
	raw_spin_lock_irqsave_rcu_node(sp, flags);
	if (ULONG_CMP_LT(sp->srcu_gp_seq_needed, s)) {
		/*
                 * Record need for grace period s.  Pair with load
                 * acquire setting up for initialization.
                 */
		smp_store_release(&sp->srcu_gp_seq_needed, s); /*^^^*/
	}
	if (!do_norm && ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
		sp->srcu_gp_seq_needed_exp = s;

	/* If grace period not already done and none in progress, start it. */
	if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
	    rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) {
		WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
		srcu_gp_start(sp);
		queue_delayed_work(system_power_efficient_wq, &sp->work,
				   srcu_get_delay(sp));
	}
	raw_spin_unlock_irqrestore_rcu_node(sp, flags);
}

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/*
 * 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 *sp, int idx, int trycount)
{
	for (;;) {
		if (srcu_readers_active_idx_check(sp, idx))
			return true;
		if (--trycount + !srcu_get_delay(sp) <= 0)
			return false;
		udelay