wfcqueue: remove misleading comment

[userspace-rcu.git] / urcu-defer-impl.h

#ifndef _URCU_DEFER_IMPL_H
#define _URCU_DEFER_IMPL_H

/*
 * urcu-defer-impl.h
 *
 * Userspace RCU header - memory reclamation.
 *
 * TO BE INCLUDED ONLY FROM URCU LIBRARY CODE. See urcu-defer.h for linking
 * dynamically with the userspace rcu reclamation library.
 *
 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
 *
 * 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
 *
 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
 */

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

#include "urcu/futex.h"

#include <urcu/compiler.h>
#include <urcu/arch.h>
#include <urcu/uatomic.h>
#include <urcu/list.h>
#include <urcu/system.h>
#include <urcu/tls-compat.h>
#include "urcu-die.h"

/*
 * Number of entries in the per-thread defer queue. Must be power of 2.
 */
#define DEFER_QUEUE_SIZE        (1 << 12)
#define DEFER_QUEUE_MASK        (DEFER_QUEUE_SIZE - 1)

/*
 * Typically, data is aligned at least on the architecture size.
 * Use lowest bit to indicate that the current callback is changing.
 * Assumes that (void *)-2L is not used often. Used to encode non-aligned
 * functions and non-aligned data using extra space.
 * We encode the (void *)-2L fct as: -2L, fct, data.
 * We encode the (void *)-2L data as either:
 *   fct | DQ_FCT_BIT, data (if fct is aligned), or
 *   -2L, fct, data (if fct is not aligned).
 * Here, DQ_FCT_MARK == ~DQ_FCT_BIT. Required for the test order.
 */
#define DQ_FCT_BIT              (1 << 0)
#define DQ_IS_FCT_BIT(x)        ((unsigned long)(x) & DQ_FCT_BIT)
#define DQ_SET_FCT_BIT(x)       \
        (x = (void *)((unsigned long)(x) | DQ_FCT_BIT))
#define DQ_CLEAR_FCT_BIT(x)     \
        (x = (void *)((unsigned long)(x) & ~DQ_FCT_BIT))
#define DQ_FCT_MARK             ((void *)(~DQ_FCT_BIT))

/*
 * This code section can only be included in LGPL 2.1 compatible source code.
 * See below for the function call wrappers which can be used in code meant to
 * be only linked with the Userspace RCU library. This comes with a small
 * performance degradation on the read-side due to the added function calls.
 * This is required to permit relinking with newer versions of the library.
 */

#ifdef DEBUG_RCU
#define rcu_assert(args...)     assert(args)
#else
#define rcu_assert(args...)
#endif

/*
 * defer queue.
 * Contains pointers. Encoded to save space when same callback is often used.
 * When looking up the next item:
 * - if DQ_FCT_BIT is set, set the current callback to DQ_CLEAR_FCT_BIT(ptr)
 *   - next element contains pointer to data.
 * - else if item == DQ_FCT_MARK
 *   - set the current callback to next element ptr
 *   - following next element contains pointer to data.
 * - else current element contains data
 */
struct defer_queue {
        unsigned long head;     /* add element at head */
        void *last_fct_in;      /* last fct pointer encoded */
        unsigned long tail;     /* next element to remove at tail */
        void *last_fct_out;     /* last fct pointer encoded */
        void **q;
        /* registry information */
        unsigned long last_head;
        struct cds_list_head list;      /* list of thread queues */
};

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

void __attribute__((destructor)) rcu_defer_exit(void);

extern void synchronize_rcu(void);

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

static int32_t defer_thread_futex;
static int32_t defer_thread_stop;

/*
 * Written to only by each individual deferer. Read by both the deferer and
 * the reclamation tread.
 */
static DEFINE_URCU_TLS(struct defer_queue, defer_queue);
static CDS_LIST_HEAD(registry_defer);
static pthread_t tid_defer;

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

#ifndef DISTRUST_SIGNALS_EXTREME
        ret = pthread_mutex_lock(mutex);
        if (ret)
                urcu_die(ret);
#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
        while ((ret = pthread_mutex_trylock(mutex)) != 0) {
                if (ret != EBUSY && ret != EINTR)
                        urcu_die(ret);
                poll(NULL,0,10);
        }
#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
}

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

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

        mutex_lock_defer(&rcu_defer_mutex);
        cds_list_for_each_entry(index, &registry_defer, list) {
                head = CMM_LOAD_SHARED(index->head);
                num_items += head - index->tail;
        }
        mutex_unlock(&rcu_defer_mutex);
        return num_items;
}

/*
 * Defer thread waiting. Single thread.
 */
static void wait_defer(void)
{
        uatomic_dec(&defer_thread_futex);
        /* Write futex before read queue */
        /* Write futex before read defer_thread_stop */
        cmm_smp_mb();
        if (_CMM_LOAD_SHARED(defer_thread_stop)) {
                uatomic_set(&defer_thread_futex, 0);
                pthread_exit(0);
        }
        if (rcu_defer_num_callbacks()) {
                cmm_smp_mb();   /* Read queue before write futex */
                /* Callbacks are queued, don't wait. */
                uatomic_set(&defer_thread_futex, 0);
        } else {
                cmm_smp_rmb();  /* Read queue before read futex */
                if (uatomic_read(&defer_thread_futex) == -1)
                        futex_noasync(&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;) {
                cmm_smp_rmb();       /* read head before q[]. */
                p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                if (caa_unlikely(DQ_IS_FCT_BIT(p))) {
                        DQ_CLEAR_FCT_BIT(p);
                        queue->last_fct_out = p;
                        p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                } else if (caa_unlikely(p == DQ_FCT_MARK)) {
                        p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                        queue->last_fct_out = p;
                        p = CMM_LOAD_SHARED(queue->q[i++ & DEFER_QUEUE_MASK]);
                }
                fct = queue->last_fct_out;
                fct(p);
        }
        cmm_smp_mb();   /* push tail after having used q[] */
        CMM_STORE_SHARED(queue->tail, i);
}

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

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

void rcu_defer_barrier_thread(void)
{
        mutex_lock_defer(&rcu_defer_mutex);
        _rcu_defer_barrier_thread();
        mutex_unlock(&rcu_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 defer_queue *index;
        unsigned long num_items = 0;

        if (cds_list_empty(&registry_defer))
                return;

        mutex_lock_defer(&rcu_defer_mutex);
        cds_list_for_each_entry(index, &registry_defer, list) {
                index->last_head = CMM_LOAD_SHARED(index->head);
                num_items += index->last_head - index->tail;
        }
        if (caa_likely(!num_items)) {
                /*
                 * We skip the grace period because there are no queued
                 * callbacks to execute.
                 */
                goto end;
        }
        synchronize_rcu();
        cds_list_for_each_entry(index, &registry_defer, list)
                rcu_defer_barrier_queue(index, index->last_head);
end:
        mutex_unlock(&rcu_defer_mutex);
}

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

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

        /*
         * If queue is full, or reached threshold. Empty queue ourself.
         * Worse-case: must allow 2 supplementary entries for fct pointer.
         */
        if (caa_unlikely(head - tail >= DEFER_QUEUE_SIZE - 2)) {
                assert(head - tail <= DEFER_QUEUE_SIZE);
                rcu_defer_barrier_thread();
                assert(head - CMM_LOAD_SHARED(URCU_TLS(defer_queue).tail) == 0);
        }

        /*
         * Encode:
         * if the function is not changed and the data is aligned and it is
         * not the marker:
         *      store the data
         * otherwise if the function is aligned and its not the marker:
         *      store the function with DQ_FCT_BIT
         *      store the data
         * otherwise:
         *      store the marker (DQ_FCT_MARK)
         *      store the function
         *      store the data
         *
         * Decode: see the comments before 'struct defer_queue'
         *         or the code in rcu_defer_barrier_queue().
         */
        if (caa_unlikely(URCU_TLS(defer_queue).last_fct_in != fct
                        || DQ_IS_FCT_BIT(p)
                        || p == DQ_FCT_MARK)) {
                URCU_TLS(defer_queue).last_fct_in = fct;
                if (caa_unlikely(DQ_IS_FCT_BIT(fct) || fct == DQ_FCT_MARK)) {
                        _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
                                      DQ_FCT_MARK);
                        _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
                                      fct);
                } else {
                        DQ_SET_FCT_BIT(fct);
                        _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK],
                                      fct);
                }
        }
        _CMM_STORE_SHARED(URCU_TLS(defer_queue).q[head++ & DEFER_QUEUE_MASK], p);
        cmm_smp_wmb();  /* Publish new pointer before head */
                        /* Write q[] before head. */
        CMM_STORE_SHARED(URCU_TLS(defer_queue).head, head);
        cmm_smp_mb();   /* Write queue head before read futex */
        /*
         * Wake-up any waiting defer thread.
         */
        wake_up_defer();
}

static void *thr_defer(void *args)
{
        for (;;) {
                /*
                 * "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 defer_rcu(void (*fct)(void *p), void *p)
{
        _defer_rcu(fct, p);
}

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;

        _CMM_STORE_SHARED(defer_thread_stop, 1);
        /* Store defer_thread_stop before testing futex */
        cmm_smp_mb();
        wake_up_defer();

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

        CMM_STORE_SHARED(defer_thread_stop, 0);
        /* defer thread should always exit when futex value is 0 */
        assert(uatomic_read(&defer_thread_futex) == 0);
}

int rcu_defer_register_thread(void)
{
        int was_empty;

        assert(URCU_TLS(defer_queue).last_head == 0);
        assert(URCU_TLS(defer_queue).q == NULL);
        URCU_TLS(defer_queue).q = malloc(sizeof(void *) * DEFER_QUEUE_SIZE);
        if (!URCU_TLS(defer_queue).q)
                return -ENOMEM;

        mutex_lock_defer(&defer_thread_mutex);
        mutex_lock_defer(&rcu_defer_mutex);
        was_empty = cds_list_empty(&registry_defer);
        cds_list_add(&URCU_TLS(defer_queue).list, &registry_defer);
        mutex_unlock(&rcu_defer_mutex);

        if (was_empty)
                start_defer_thread();
        mutex_unlock(&defer_thread_mutex);
        return 0;
}

void rcu_defer_unregister_thread(void)
{
        int is_empty;

        mutex_lock_defer(&defer_thread_mutex);
        mutex_lock_defer(&rcu_defer_mutex);
        cds_list_del(&URCU_TLS(defer_queue).list);
        _rcu_defer_barrier_thread();
        free(URCU_TLS(defer_queue).q);
        URCU_TLS(defer_queue).q = NULL;
        is_empty = cds_list_empty(&registry_defer);
        mutex_unlock(&rcu_defer_mutex);

        if (is_empty)
                stop_defer_thread();
        mutex_unlock(&defer_thread_mutex);
}

void rcu_defer_exit(void)
{
        assert(cds_list_empty(&registry_defer));
}

#endif /* _URCU_DEFER_IMPL_H */