urcu-defer: fix futex wakeup value

[userspace-rcu.git] / urcu-defer.c

/*
 * urcu-defer.c
 *
 * Userspace RCU library - batch memory reclamation
 *
 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <poll.h>
#include <sys/time.h>
#include <syscall.h>
#include <unistd.h>

#include "urcu-defer-static.h"
/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
#include "urcu-defer.h"

#define futex(...)      syscall(__NR_futex, __VA_ARGS__)
#define FUTEX_WAIT              0
#define FUTEX_WAKE              1

void __attribute__((destructor)) urcu_defer_exit(void);

extern void synchronize_rcu(void);

/*
 * urcu_defer_mutex nests inside defer_thread_mutex.
 */
static pthread_mutex_t urcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;

static int defer_thread_futex;

/*
 * Written to only by each individual deferer. Read by both the deferer and
 * the reclamation tread.
 */
static struct defer_queue __thread defer_queue;

/* Thread IDs of registered deferers */
#define INIT_NUM_THREADS 4

struct deferer_registry {
        pthread_t tid;
        struct defer_queue *defer_queue;
        unsigned long last_head;
};

static struct deferer_registry *registry;
static int num_deferers, alloc_deferers;

static pthread_t tid_defer;

static void internal_urcu_lock(pthread_mutex_t *mutex)
{
        int ret;

#ifndef DISTRUST_SIGNALS_EXTREME
        ret = pthread_mutex_lock(mutex);
        if (ret) {
                perror("Error in pthread mutex lock");
                exit(-1);
        }
#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
        while ((ret = pthread_mutex_trylock(mutex)) != 0) {
                if (ret != EBUSY && ret != EINTR) {
                        printf("ret = %d, errno = %d\n", ret, errno);
                        perror("Error in pthread mutex lock");
                        exit(-1);
                }
                pthread_testcancel();
                poll(NULL,0,10);
        }
#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
}

static void internal_urcu_unlock(pthread_mutex_t *mutex)
{
        int ret;

        ret = pthread_mutex_unlock(mutex);
        if (ret) {
                perror("Error in pthread mutex unlock");
                exit(-1);
        }
}

/*
 * Wake-up any waiting defer thread. Called from many concurrent threads.
 */
static void wake_up_defer(void)
{
        if (unlikely(atomic_read(&defer_thread_futex) == -1)) {
                atomic_set(&defer_thread_futex, 0);
                futex(&defer_thread_futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);
        }
}

static unsigned long rcu_defer_num_callbacks(void)
{
        unsigned long num_items = 0, head;
        struct deferer_registry *index;

        internal_urcu_lock(&urcu_defer_mutex);
        for (index = registry; index < registry + num_deferers; index++) {
                head = LOAD_SHARED(index->defer_queue->head);
                num_items += head - index->defer_queue->tail;
        }
        internal_urcu_unlock(&urcu_defer_mutex);
        return num_items;
}

/*
 * Defer thread waiting. Single thread.
 */
static void wait_defer(void)
{
        atomic_dec(&defer_thread_futex);
        smp_mb();       /* Write futex before read queue */
        if (rcu_defer_num_callbacks()) {
                smp_mb();       /* Read queue before write futex */
                /* Callbacks are queued, don't wait. */
                atomic_set(&defer_thread_futex, 0);
        } else {
                smp_rmb();      /* Read queue before read futex */
                if (atomic_read(&defer_thread_futex) == -1)
                        futex(&defer_thread_futex, FUTEX_WAIT, -1,
                              NULL, NULL, 0);
        }
}

/*
 * Must be called after Q.S. is reached.
 */
static void rcu_defer_barrier_queue(struct defer_queue *queue,
                                    unsigned long head)
{
        unsigned long i;
        void (*fct)(void *p);
        void *p;

        /*
         * Tail is only modified when lock is held.
         * Head is only modified by owner thread.
         */

        for (i = queue->tail; i != head;) {
                smp_rmb();       /* read head before q[]. */
                p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                if (unlikely(DQ_IS_FCT_BIT(p))) {
                        DQ_CLEAR_FCT_BIT(p);
                        queue->last_fct_out = p;
                        p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                } else if (unlikely(p == DQ_FCT_MARK)) {
                        p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                        queue->last_fct_out = p;
                        p = LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                }
                fct = queue->last_fct_out;
                fct(p);
        }
        smp_mb();       /* push tail after having used q[] */
        STORE_SHARED(queue->tail, i);
}

static void _rcu_defer_barrier_thread(void)
{
        unsigned long head, num_items;

        head = defer_queue.head;
        num_items = head - defer_queue.tail;
        if (unlikely(!num_items))
                return;
        synchronize_rcu();
        rcu_defer_barrier_queue(&defer_queue, head);
}


void rcu_defer_barrier_thread(void)
{
        internal_urcu_lock(&urcu_defer_mutex);
        _rcu_defer_barrier_thread();
        internal_urcu_unlock(&urcu_defer_mutex);
}

/*
 * rcu_defer_barrier - Execute all queued rcu callbacks.
 *
 * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
 * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
 * are guaranteed to be executed.
 * Callbacks queued by other threads concurrently with rcu_defer_barrier()
 * execution are not guaranteed to be executed in the current batch (could
 * be left for the next batch). These callbacks queued by other threads are only
 * guaranteed to be executed if there is explicit synchronization between
 * the thread adding to the queue and the thread issuing the defer_barrier call.
 */

void rcu_defer_barrier(void)
{
        struct deferer_registry *index;
        unsigned long num_items = 0;

        if (!registry)
                return;

        internal_urcu_lock(&urcu_defer_mutex);
        for (index = registry; index < registry + num_deferers; index++) {
                index->last_head = LOAD_SHARED(index->defer_queue->head);
                num_items += index->last_head - index->defer_queue->tail;
        }
        if (likely(!num_items)) {
                /*
                 * We skip the grace period because there are no queued
                 * callbacks to execute.
                 */
                goto end;
        }
        synchronize_rcu();
        for (index = registry; index < registry + num_deferers; index++)
                rcu_defer_barrier_queue(index->defer_queue,
                                          index->last_head);
end:
        internal_urcu_unlock(&urcu_defer_mutex);
}

/*
 * _rcu_defer_queue - Queue a RCU callback.
 */
void _rcu_defer_queue(void (*fct)(void *p), void *p)
{
        unsigned long head, tail;

        /*
         * Head is only modified by ourself. Tail can be modified by reclamation
         * thread.
         */
        head = defer_queue.head;
        tail = LOAD_SHARED(defer_queue.tail);

        /*
         * If queue is full, empty it ourself.
         * Worse-case: must allow 2 supplementary entries for fct pointer.
         */
        if (unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
                assert(head - tail <= DEFER_QUEUE_SIZE);
                rcu_defer_barrier_thread();
                assert(head - LOAD_SHARED(defer_queue.tail) == 0);
        }

        if (unlikely(defer_queue.last_fct_in != fct)) {
                defer_queue.last_fct_in = fct;
                if (unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
                        /*
                         * If the function to encode is not aligned or the
                         * marker, write DQ_FCT_MARK followed by the function
                         * pointer.
                         */
                        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
                                      DQ_FCT_MARK);
                        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
                                      fct);
                } else {
                        DQ_SET_FCT_BIT(fct);
                        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
                                      fct);
                }
        } else {
                if (unlikely(DQ_IS_FCT_BIT(p) || p == DQ_FCT_MARK)) {
                        /*
                         * If the data to encode is not aligned or the marker,
                         * write DQ_FCT_MARK followed by the function pointer.
                         */
                        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
                                      DQ_FCT_MARK);
                        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK],
                                      fct);
                }
        }
        _STORE_SHARED(defer_queue.q[head++ & DEFER_QUEUE_MASK], p);
        smp_wmb();      /* Publish new pointer before head */
                        /* Write q[] before head. */
        STORE_SHARED(defer_queue.head, head);
        smp_mb();       /* Write queue head before read futex */
        /*
         * Wake-up any waiting defer thread.
         */
        wake_up_defer();
}

void *thr_defer(void *args)
{
        for (;;) {
                pthread_testcancel();
                /*
                 * "Be green". Don't wake up the CPU if there is no RCU work
                 * to perform whatsoever. Aims at saving laptop battery life by
                 * leaving the processor in sleep state when idle.
                 */
                wait_defer();
                /* Sleeping after wait_defer to let many callbacks enqueue */
                poll(NULL,0,100);       /* wait for 100ms */
                rcu_defer_barrier();
        }

        return NULL;
}

/*
 * library wrappers to be used by non-LGPL compatible source code.
 */

void rcu_defer_queue(void (*fct)(void *p), void *p)
{
        _rcu_defer_queue(fct, p);
}

static void rcu_add_deferer(pthread_t id)
{
        struct deferer_registry *oldarray;

        if (!registry) {
                alloc_deferers = INIT_NUM_THREADS;
                num_deferers = 0;
                registry =
                        malloc(sizeof(struct deferer_registry) * alloc_deferers);
        }
        if (alloc_deferers < num_deferers + 1) {
                oldarray = registry;
                registry = malloc(sizeof(struct deferer_registry)
                                * (alloc_deferers << 1));
                memcpy(registry, oldarray,
                        sizeof(struct deferer_registry) * alloc_deferers);
                alloc_deferers <<= 1;
                free(oldarray);
        }
        registry[num_deferers].tid = id;
        /* reference to the TLS of _this_ deferer thread. */
        registry[num_deferers].defer_queue = &defer_queue;
        registry[num_deferers].last_head = 0;
        num_deferers++;
}

/*
 * Never shrink (implementation limitation).
 * This is O(nb threads). Eventually use a hash table.
 */
static void rcu_remove_deferer(pthread_t id)
{
        struct deferer_registry *index;

        assert(registry != NULL);
        for (index = registry; index < registry + num_deferers; index++) {
                if (pthread_equal(index->tid, id)) {
                        memcpy(index, &registry[num_deferers - 1],
                                sizeof(struct deferer_registry));
                        registry[num_deferers - 1].tid = 0;
                        registry[num_deferers - 1].defer_queue = NULL;
                        registry[num_deferers - 1].last_head = 0;
                        num_deferers--;
                        return;
                }
        }
        /* Hrm not found, forgot to register ? */
        assert(0);
}

static void start_defer_thread(void)
{
        int ret;

        ret = pthread_create(&tid_defer, NULL, thr_defer,
                NULL);
        assert(!ret);
}

static void stop_defer_thread(void)
{
        int ret;
        void *tret;

        pthread_cancel(tid_defer);
        wake_up_defer();
        ret = pthread_join(tid_defer, &tret);
        assert(!ret);
}

void rcu_defer_register_thread(void)
{
        int deferers;

        internal_urcu_lock(&defer_thread_mutex);
        internal_urcu_lock(&urcu_defer_mutex);
        defer_queue.q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
        rcu_add_deferer(pthread_self());
        deferers = num_deferers;
        internal_urcu_unlock(&urcu_defer_mutex);

        if (deferers == 1)
                start_defer_thread();
        internal_urcu_unlock(&defer_thread_mutex);
}

void rcu_defer_unregister_thread(void)
{
        int deferers;

        internal_urcu_lock(&defer_thread_mutex);
        internal_urcu_lock(&urcu_defer_mutex);
        rcu_remove_deferer(pthread_self());
        _rcu_defer_barrier_thread();
        free(defer_queue.q);
        defer_queue.q = NULL;
        deferers = num_deferers;
        internal_urcu_unlock(&urcu_defer_mutex);

        if (deferers == 0)
                stop_defer_thread();
        internal_urcu_unlock(&defer_thread_mutex);
}

void urcu_defer_exit(void)
{
        free(registry);
}