srcu: Implement more-efficient reader counts

[linux.git] / kernel / rcu / srcu.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.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/delay.h>
#include <linux/srcu.h>

#include "rcu.h"

/*
 * Initialize an rcu_batch structure to empty.
 */
static inline void rcu_batch_init(struct rcu_batch *b)
{
        b->head = NULL;
        b->tail = &b->head;
}

/*
 * Enqueue a callback onto the tail of the specified rcu_batch structure.
 */
static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
{
        *b->tail = head;
        b->tail = &head->next;
}

/*
 * Is the specified rcu_batch structure empty?
 */
static inline bool rcu_batch_empty(struct rcu_batch *b)
{
        return b->tail == &b->head;
}

/*
 * Remove the callback at the head of the specified rcu_batch structure
 * and return a pointer to it, or return NULL if the structure is empty.
 */
static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
{
        struct rcu_head *head;

        if (rcu_batch_empty(b))
                return NULL;

        head = b->head;
        b->head = head->next;
        if (b->tail == &head->next)
                rcu_batch_init(b);

        return head;
}

/*
 * Move all callbacks from the rcu_batch structure specified by "from" to
 * the structure specified by "to".
 */
static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
{
        if (!rcu_batch_empty(from)) {
                *to->tail = from->head;
                to->tail = from->tail;
                rcu_batch_init(from);
        }
}

static int init_srcu_struct_fields(struct srcu_struct *sp)
{
        sp->completed = 0;
        spin_lock_init(&sp->queue_lock);
        sp->running = false;
        rcu_batch_init(&sp->batch_queue);
        rcu_batch_init(&sp->batch_check0);
        rcu_batch_init(&sp->batch_check1);
        rcu_batch_init(&sp->batch_done);
        INIT_DELAYED_WORK(&sp->work, process_srcu);
        sp->per_cpu_ref = alloc_percpu(struct srcu_array);
        return sp->per_cpu_ref ? 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);
        return init_srcu_struct_fields(sp);
}
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)
{
        return init_srcu_struct_fields(sp);
}
EXPORT_SYMBOL_GPL(init_srcu_struct);

#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/*
 * Returns approximate total of the readers' ->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_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);

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

/*
 * Returns approximate total of the readers' ->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_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);

                sum += READ_ONCE(cpuc->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 ->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 ->completed 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 ->completed.
         */
        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_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);

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

/**
 * 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)
{
        if (WARN_ON(srcu_readers_active(sp)))
                return; /* Leakage unless caller handles error. */
        free_percpu(sp->per_cpu_ref);
        sp->per_cpu_ref = 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->completed) & 0x1;
        __this_cpu_inc(sp->per_cpu_ref->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->per_cpu_ref->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 10 microseconds,
 * we repeatedly block for 1-millisecond time periods.  This approach
 * has done well in testing, so there is no need for a config parameter.
 */
#define SRCU_RETRY_CHECK_DELAY          5
#define SYNCHRONIZE_SRCU_TRYCOUNT       2
#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT   12

/*
 * @@@ Wait until all pre-existing readers complete.  Such readers
 * will have used the index specified by "idx".
 * the caller should ensures the ->completed is not changed while checking
 * and idx = (->completed & 1) ^ 1
 */
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 <= 0)
                        return false;
                udelay(SRCU_RETRY_CHECK_DELAY);
        }
}

/*
 * Increment the ->completed counter so that future SRCU readers will
 * use the other rank of the ->(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)
{
        sp->completed++;
}

/*
 * Enqueue an SRCU callback on 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 *head,
               rcu_callback_t func)
{
        unsigned long flags;

        head->next = NULL;
        head->func = func;
        spin_lock_irqsave(&sp->queue_lock, flags);
        rcu_batch_queue(&sp->batch_queue, head);
        if (!sp->running) {
                sp->running = true;
                queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
        }
        spin_unlock_irqrestore(&sp->queue_lock, flags);
}
EXPORT_SYMBOL_GPL(call_srcu);

static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
static void srcu_reschedule(struct srcu_struct *sp);

/*
 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 */
static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
{
        struct rcu_synchronize rcu;
        struct rcu_head *head = &rcu.head;
        bool done = false;

        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");

        might_sleep();
        init_completion(&rcu.completion);

        head->next = NULL;
        head->func = wakeme_after_rcu;
        spin_lock_irq(&sp->queue_lock);
        if (!sp->running) {
                /* steal the processing owner */
                sp->running = true;
                rcu_batch_queue(&sp->batch_check0, head);
                spin_unlock_irq(&sp->queue_lock);

                srcu_advance_batches(sp, trycount);
                if (!rcu_batch_empty(&sp->batch_done)) {
                        BUG_ON(sp->batch_done.head != head);
                        rcu_batch_dequeue(&sp->batch_done);
                        done = true;
                }
                /* give the processing owner to work_struct */
                srcu_reschedule(sp);
        } else {
                rcu_batch_queue(&sp->batch_queue, head);
                spin_unlock_irq(&sp->queue_lock);
        }

        if (!done)
                wait_for_completion(&rcu.completion);
}

/**
 * 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=((->completed & 1) ^ 1) to drain to zero at first,
 * and then flip the completed 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)
{
        __synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
                           ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
                           : SYNCHRONIZE_SRCU_TRYCOUNT);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

/**
 * 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)
{
        __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);

/**
 * 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)
{
        synchronize_srcu(sp);
}
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->completed;
}
EXPORT_SYMBOL_GPL(srcu_batches_completed);

#define SRCU_CALLBACK_BATCH     10
#define SRCU_INTERVAL           1

/*
 * Move any new SRCU callbacks to the first stage of the SRCU grace
 * period pipeline.
 */
static void srcu_collect_new(struct srcu_struct *sp)
{
        if (!rcu_batch_empty(&sp->batch_queue)) {
                spin_lock_irq(&sp->queue_lock);
                rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
                spin_unlock_irq(&sp->queue_lock);
        }
}

/*
 * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
 * ->batch_check1 and then to ->batch_done as readers drain.
 */
static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
{
        int idx = 1 ^ (sp->completed & 1);

        /*
         * Because readers might be delayed for an extended period after
         * fetching ->completed 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.
         */

        if (rcu_batch_empty(&sp->batch_check0) &&
            rcu_batch_empty(&sp->batch_check1))
                return; /* no callbacks need to be advanced */

        if (!try_check_zero(sp, idx, trycount))
                return; /* failed to advance, will try after SRCU_INTERVAL */

        /*
         * The callbacks in ->batch_check1 have already done with their
         * first zero check and flip back when they were enqueued on
         * ->batch_check0 in a previous invocation of srcu_advance_batches().
         * (Presumably try_check_zero() returned false during that
         * invocation, leaving the callbacks stranded on ->batch_check1.)
         * They are therefore ready to invoke, so move them to ->batch_done.
         */
        rcu_batch_move(&sp->batch_done, &sp->batch_check1);

        if (rcu_batch_empty(&sp->batch_check0))
                return; /* no callbacks need to be advanced */
        srcu_flip(sp);

        /*
         * The callbacks in ->batch_check0 just finished their
         * first check zero and flip, so move them to ->batch_check1
         * for future checking on the other idx.
         */
        rcu_batch_move(&sp->batch_check1, &sp->batch_check0);

        /*
         * SRCU read-side critical sections are normally short, so check
         * at least twice in quick succession after a flip.
         */
        trycount = trycount < 2 ? 2 : trycount;
        if (!try_check_zero(sp, idx^1, trycount))
                return; /* failed to advance, will try after SRCU_INTERVAL */

        /*
         * The callbacks in ->batch_check1 have now waited for all
         * pre-existing readers using both idx values.  They are therefore
         * ready to invoke, so move them to ->batch_done.
         */
        rcu_batch_move(&sp->batch_done, &sp->batch_check1);
}

/*
 * 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.
 */
static void srcu_invoke_callbacks(struct srcu_struct *sp)
{
        int i;
        struct rcu_head *head;

        for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
                head = rcu_batch_dequeue(&sp->batch_done);
                if (!head)
                        break;
                local_bh_disable();
                head->func(head);
                local_bh_enable();
        }
}

/*
 * 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)
{
        bool pending = true;

        if (rcu_batch_empty(&sp->batch_done) &&
            rcu_batch_empty(&sp->batch_check1) &&
            rcu_batch_empty(&sp->batch_check0) &&
            rcu_batch_empty(&sp->batch_queue)) {
                spin_lock_irq(&sp->queue_lock);
                if (rcu_batch_empty(&sp->batch_done) &&
                    rcu_batch_empty(&sp->batch_check1) &&
                    rcu_batch_empty(&sp->batch_check0) &&
                    rcu_batch_empty(&sp->batch_queue)) {
                        sp->running = false;
                        pending = false;
                }
                spin_unlock_irq(&sp->queue_lock);
        }

        if (pending)
                queue_delayed_work(system_power_efficient_wq,
                                   &sp->work, SRCU_INTERVAL);
}

/*
 * 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_collect_new(sp);
        srcu_advance_batches(sp, 1);
        srcu_invoke_callbacks(sp);
        srcu_reschedule(sp);
}
EXPORT_SYMBOL_GPL(process_srcu);