srcu: Expedite srcu_schedule_cbs_snp() callback invocation

[mirror_ubuntu-bionic-kernel.git] / 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/srcu.h>

#include "rcu.h"

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

/*
 * 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) {
                spin_lock_init(&snp->lock);
                for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++)
                        snp->srcu_have_cbs[i] = 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);
                spin_lock_init(&sdp->lock);
                rcu_segcblist_init(&sdp->srcu_cblist);
                sdp->srcu_cblist_invoking = false;
                sdp->srcu_gp_seq_needed = 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;
                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;
                }
        }
}

/*
 * 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;
        atomic_set(&sp->srcu_exp_cnt, 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);
        smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
        return sp->sda ? 0 : -ENOMEM;
}

#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);
        spin_lock_init(&sp->gp_lock);
        return init_srcu_struct_fields(sp, false);
}
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)
{
        spin_lock_init(&sp->gp_lock);
        return init_srcu_struct_fields(sp, false);
}
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 ->gp_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. */
        spin_lock_irqsave(&sp->gp_lock, flags);
        if (!rcu_seq_state(sp->srcu_gp_seq_needed)) {
                spin_unlock_irqrestore(&sp->gp_lock, flags);
                return;
        }
        init_srcu_struct_fields(sp, true);
        spin_unlock_irqrestore(&sp->gp_lock, 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 *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;
}

/*
 * 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;
}

/*
 * 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.
         *
         * Possible bug: There is no guarantee that there haven't been
         * ULONG_MAX increments of ->srcu_lock_count[] since the unlocks were
         * counted, meaning that this could return true even if there are
         * still active readers.  Since there are no memory barriers around
         * srcu_flip(), the CPU is not required to increment ->srcu_idx
         * before running srcu_readers_unlock_idx(), which means that there
         * could be an arbitrarily large number of critical sections that
         * execute after srcu_readers_unlock_idx() but use the old value
         * of ->srcu_idx.
         */
        return srcu_readers_lock_idx(sp, idx) == unlocks;
}

/**
 * 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;
}

#define SRCU_INTERVAL           1

/**
 * 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;

        WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
        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;
}
EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

/*
 * Counts the new reader in the appropriate per-CPU element of the
 * srcu_struct.  Must be called from process context.
 * 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;
}
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().
 * Must be called from process context.
 */
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]);
}
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;

        RCU_LOCKDEP_WARN(!lockdep_is_held(&sp->gp_lock),
                         "Invoked srcu_gp_start() without ->gp_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));
        rcu_seq_start(&sp->srcu_gp_seq);
        state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
        WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
}

/*
 * 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);
}

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

/*
 * 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;
}

/*
 * 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);
}

/*
 * Schedule callback invocation for all srcu_data structures associated
 * with the specified srcu_node structure, if possible, on the corresponding
 * CPUs.
 */
static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp)
{
        int cpu;

        for (cpu = snp->grplo; cpu <= snp->grphi; cpu++)
                srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu),
                                      atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
}

/*
 * 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)
{
        bool cbs;
        unsigned long gpseq;
        int idx;
        int idxnext;
        struct srcu_node *snp;

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

        /* End the current grace period. */
        spin_lock_irq(&sp->gp_lock);
        idx = rcu_seq_state(sp->srcu_gp_seq);
        WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
        rcu_seq_end(&sp->srcu_gp_seq);
        gpseq = rcu_seq_current(&sp->srcu_gp_seq);
        spin_unlock_irq(&sp->gp_lock);
        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) {
                spin_lock_irq(&snp->lock);
                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);
                spin_unlock_irq(&snp->lock);
                if (cbs) {
                        smp_mb(); /* GP end before CB invocation. */
                        srcu_schedule_cbs_snp(sp, snp);
                }
        }

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

        /* Start a new grace period if needed. */
        spin_lock_irq(&sp->gp_lock);
        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);
                spin_unlock_irq(&sp->gp_lock);
                /* Throttle expedited grace periods: Should be rare! */
                srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) &&
                                    rcu_seq_ctr(gpseq) & 0xf
                                    ? 0
                                    : SRCU_INTERVAL);
        } else {
                spin_unlock_irq(&sp->gp_lock);
        }
}

/*
 * 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)
{
        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. */
                spin_lock_irqsave(&snp->lock, flags);
                if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
                        snp_seq = snp->srcu_have_cbs[idx];
                        spin_unlock_irqrestore(&snp->lock, flags);
                        if (snp == sdp->mynode && snp_seq != s) {
                                smp_mb(); /* CBs after GP! */
                                srcu_schedule_cbs_sdp(sdp, 0);
                        }
                        return;
                }
                snp->srcu_have_cbs[idx] = s;
                spin_unlock_irqrestore(&snp->lock, flags);
        }

        /* Top of tree, must ensure the grace period will be started. */
        spin_lock_irqsave(&sp->gp_lock, 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 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,
                                   atomic_read(&sp->srcu_exp_cnt)
                                   ? 0
                                   : SRCU_INTERVAL);
        }
        spin_unlock_irqrestore(&sp->gp_lock, 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 *sp, int idx, int trycount)
{
        for (;;) {
                if (srcu_readers_active_idx_check(sp, idx))
                        return true;
                if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 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 *sp)
{
        WRITE_ONCE(sp->srcu_idx, sp->srcu_idx + 1);

        /*
         * Ensure that if the updater misses an __srcu_read_unlock()
         * increment, that task's next __srcu_read_lock() 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. */
}

/*
 * 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_rcu().  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_rcu()
 * 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_rcu() 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_rcu() 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.
 */
void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
               rcu_callback_t func)
{
        unsigned long flags;
        bool needgp = false;
        unsigned long s;
        struct srcu_data *sdp;

        check_init_srcu_struct(sp);
        rhp->func = func;
        local_irq_save(flags);
        sdp = this_cpu_ptr(sp->sda);
        spin_lock(&sdp->lock);
        rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, false);
        rcu_segcblist_advance(&sdp->srcu_cblist,
                              rcu_seq_current(&sp->srcu_gp_seq));
        s = rcu_seq_snap(&sp->srcu_gp_seq);
        (void)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;
        }
        spin_unlock_irqrestore(&sdp->lock, flags);
        if (needgp)
                srcu_funnel_gp_start(sp, sdp, s);
}
EXPORT_SYMBOL_GPL(call_srcu);

/*
 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 */
static void __synchronize_srcu(struct srcu_struct *sp)
{
        struct rcu_synchronize rcu;

        RCU_LOCKDEP_WARN(lock_is_held(&sp->dep_map) ||
                         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(sp);
        init_completion(&rcu.completion);
        init_rcu_head_on_stack(&rcu.head);
        call_srcu(sp, &rcu.head, wakeme_after_rcu);
        wait_for_completion(&rcu.completion);
        destroy_rcu_head_on_stack(&rcu.head);
}

/**
 * synchronize_srcu_expedited - Brute-force SRCU grace period
 * @sp: 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 *sp)
{
        bool do_norm = rcu_gp_is_normal();

        check_init_srcu_struct(sp);
        if (!do_norm) {
                atomic_inc(&sp->srcu_exp_cnt);
                smp_mb__after_atomic(); /* increment before GP. */
        }
        __synchronize_srcu(sp);
        if (!do_norm) {
                smp_mb__before_atomic(); /* GP before decrement. */
                WARN_ON_ONCE(atomic_dec_return(&sp->srcu_exp_cnt) < 0);
        }
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);

/**
 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 * @sp: 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-sched 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.
 */
void synchronize_srcu(struct srcu_struct *sp)
{
        if (rcu_gp_is_expedited())
                synchronize_srcu_expedited(sp);
        else
                __synchronize_srcu(sp);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

/*
 * Callback function for srcu_barrier() use.
 */
static void srcu_barrier_cb(struct rcu_head *rhp)
{
        struct srcu_data *sdp;
        struct srcu_struct *sp;

        sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
        sp = sdp->sp;
        if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
                complete(&sp->srcu_barrier_completion);
}

/**
 * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
 * @sp: srcu_struct on which to wait for in-flight callbacks.
 */
void srcu_barrier(struct srcu_struct *sp)
{
        int cpu;
        struct srcu_data *sdp;
        unsigned long s = rcu_seq_snap(&sp->srcu_barrier_seq);

        check_init_srcu_struct(sp);
        mutex_lock(&sp->srcu_barrier_mutex);
        if (rcu_seq_done(&sp->srcu_barrier_seq, s)) {
                smp_mb(); /* Force ordering following return. */
                mutex_unlock(&sp->srcu_barrier_mutex);
                return; /* Someone else did our work for us. */
        }
        rcu_seq_start(&sp->srcu_barrier_seq);
        init_completion(&sp->srcu_barrier_completion);

        /* Initial count prevents reaching zero until all CBs are posted. */
        atomic_set(&sp->srcu_barrier_cpu_cnt, 1);

        /*
         * Each pass through this loop enqueues a callback, but only
         * on CPUs already having callbacks 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.
         */
        for_each_possible_cpu(cpu) {
                sdp = per_cpu_ptr(sp->sda, cpu);
                spin_lock_irq(&sdp->lock);
                atomic_inc(&sp->srcu_barrier_cpu_cnt);
                sdp->srcu_barrier_head.func = srcu_barrier_cb;
                if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
                                           &sdp->srcu_barrier_head, 0))
                        atomic_dec(&sp->srcu_barrier_cpu_cnt);
                spin_unlock_irq(&sdp->lock);
        }

        /* Remove the initial count, at which point reaching zero can happen. */
        if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
                complete(&sp->srcu_barrier_completion);
        wait_for_completion(&sp->srcu_barrier_completion);

        rcu_seq_end(&sp->srcu_barrier_seq);
        mutex_unlock(&sp->srcu_barrier_mutex);
}
EXPORT_SYMBOL_GPL(srcu_barrier);

/**
 * srcu_batches_completed - return batches completed.
 * @sp: 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 *sp)
{
        return sp->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 *sp)
{
        int idx;

        mutex_lock(&sp->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(&sp->srcu_gp_seq)); /* ^^^ */
        if (idx == SRCU_STATE_IDLE) {
                spin_lock_irq(&sp->gp_lock);
                if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
                        WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq));
                        spin_unlock_irq(&sp->gp_lock);
                        mutex_unlock(&sp->srcu_gp_mutex);
                        return;
                }
                idx = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
                if (idx == SRCU_STATE_IDLE)
                        srcu_gp_start(sp);
                spin_unlock_irq(&sp->gp_lock);
                if (idx != SRCU_STATE_IDLE) {
                        mutex_unlock(&sp->srcu_gp_mutex);
                        return; /* Someone else started the grace period. */
                }
        }

        if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
                idx = 1 ^ (sp->srcu_idx & 1);
                if (!try_check_zero(sp, idx, 1)) {
                        mutex_unlock(&sp->srcu_gp_mutex);
                        return; /* readers present, retry later. */
                }
                srcu_flip(sp);
                rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
        }

        if (rcu_seq_state(READ_ONCE(sp->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 ^ (sp->srcu_idx & 1);
                if (!try_check_zero(sp, idx, 2)) {
                        mutex_unlock(&sp->srcu_gp_mutex);
                        return; /* readers present, retry later. */
                }
                srcu_gp_end(sp);  /* 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)
{
        bool more;
        struct rcu_cblist ready_cbs;
        struct rcu_head *rhp;
        struct srcu_data *sdp;
        struct srcu_struct *sp;

        sdp = container_of(work, struct srcu_data, work.work);
        sp = sdp->sp;
        rcu_cblist_init(&ready_cbs);
        spin_lock_irq(&sdp->lock);
        smp_mb(); /* Old grace periods before callback invocation! */
        rcu_segcblist_advance(&sdp->srcu_cblist,
                              rcu_seq_current(&sp->srcu_gp_seq));
        if (sdp->srcu_cblist_invoking ||
            !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
                spin_unlock_irq(&sdp->lock);
                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);
        spin_unlock_irq(&sdp->lock);
        rhp = rcu_cblist_dequeue(&ready_cbs);
        for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
                local_bh_disable();
                rhp->func(rhp);
                local_bh_enable();
        }

        /*
         * Update counts, accelerate new callbacks, and if needed,
         * schedule another round of callback invocation.
         */
        spin_lock_irq(&sdp->lock);
        rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
        (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
                                       rcu_seq_snap(&sp->srcu_gp_seq));
        sdp->srcu_cblist_invoking = false;
        more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
        spin_unlock_irq(&sdp->lock);
        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 *sp, unsigned long delay)
{
        bool pushgp = true;

        spin_lock_irq(&sp->gp_lock);
        if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
                if (!WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq))) {
                        /* All requests fulfilled, time to go idle. */
                        pushgp = false;
                }
        } else if (!rcu_seq_state(sp->srcu_gp_seq)) {
                /* Outstanding request and no GP.  Start one. */
                srcu_gp_start(sp);
        }
        spin_unlock_irq(&sp->gp_lock);

        if (pushgp)
                queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
}

/*
 * This is the work-queue function that handles SRCU grace periods.
 */
void process_srcu(struct work_struct *work)
{
        struct srcu_struct *sp;

        sp = container_of(work, struct srcu_struct, work.work);

        srcu_advance_state(sp);
        srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
}
EXPORT_SYMBOL_GPL(process_srcu);