doc/examples: add membarrier flavor

[urcu.git] / urcu.c

/*
 * urcu.c
 *
 * Userspace RCU 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.
 */

#define _BSD_SOURCE
#define _GNU_SOURCE
#define _LGPL_SOURCE
#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <assert.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <poll.h>

#include "urcu/wfcqueue.h"
#include "urcu/map/urcu.h"
#include "urcu/static/urcu.h"
#include "urcu-pointer.h"
#include "urcu/tls-compat.h"

#include "urcu-die.h"
#include "urcu-wait.h"

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

/*
 * If a reader is really non-cooperative and refuses to commit its
 * rcu_active_readers count to memory (there is no barrier in the reader
 * per-se), kick it after a few loops waiting for it.
 */
#define KICK_READER_LOOPS 10000

/*
 * Active attempts to check for reader Q.S. before calling futex().
 */
#define RCU_QS_ACTIVE_ATTEMPTS 100

/*
 * RCU_MEMBARRIER is only possibly available on Linux.
 */
#if defined(RCU_MEMBARRIER) && defined(__linux__)
#include <syscall.h>
#endif

/* If the headers do not support SYS_membarrier, fall back on RCU_MB */
#ifdef SYS_membarrier
# define membarrier(...)                syscall(SYS_membarrier, __VA_ARGS__)
#else
# define membarrier(...)                -ENOSYS
#endif

#define MEMBARRIER_EXPEDITED            (1 << 0)
#define MEMBARRIER_DELAYED              (1 << 1)
#define MEMBARRIER_QUERY                (1 << 16)

#ifdef RCU_MEMBARRIER
static int init_done;
int rcu_has_sys_membarrier;

void __attribute__((constructor)) rcu_init(void);
#endif

#ifdef RCU_MB
void rcu_init(void)
{
}
#endif

#ifdef RCU_SIGNAL
static int init_done;

void __attribute__((constructor)) rcu_init(void);
void __attribute__((destructor)) rcu_exit(void);
#endif

static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };

/*
 * Written to only by each individual reader. Read by both the reader and the
 * writers.
 */
DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);

#ifdef DEBUG_YIELD
unsigned int rcu_yield_active;
DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
#endif

static CDS_LIST_HEAD(registry);

/*
 * Queue keeping threads awaiting to wait for a grace period. Contains
 * struct gp_waiters_thread objects.
 */
static DEFINE_URCU_WAIT_QUEUE(gp_waiters);

static void mutex_lock(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);
                if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
                        cmm_smp_mb();
                        _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
                        cmm_smp_mb();
                }
                poll(NULL,0,10);
        }
#endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
}

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

        ret = pthread_mutex_unlock(mutex);
        if (ret)
                urcu_die(ret);
}

#ifdef RCU_MEMBARRIER
static void smp_mb_master(int group)
{
        if (caa_likely(rcu_has_sys_membarrier))
                (void) membarrier(MEMBARRIER_EXPEDITED);
        else
                cmm_smp_mb();
}
#endif

#ifdef RCU_MB
static void smp_mb_master(int group)
{
        cmm_smp_mb();
}
#endif

#ifdef RCU_SIGNAL
static void force_mb_all_readers(void)
{
        struct rcu_reader *index;

        /*
         * Ask for each threads to execute a cmm_smp_mb() so we can consider the
         * compiler barriers around rcu read lock as real memory barriers.
         */
        if (cds_list_empty(&registry))
                return;
        /*
         * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
         * a cache flush on architectures with non-coherent cache. Let's play
         * safe and don't assume anything : we use cmm_smp_mc() to make sure the
         * cache flush is enforced.
         */
        cds_list_for_each_entry(index, &registry, node) {
                CMM_STORE_SHARED(index->need_mb, 1);
                pthread_kill(index->tid, SIGRCU);
        }
        /*
         * Wait for sighandler (and thus mb()) to execute on every thread.
         *
         * Note that the pthread_kill() will never be executed on systems
         * that correctly deliver signals in a timely manner.  However, it
         * is not uncommon for kernels to have bugs that can result in
         * lost or unduly delayed signals.
         *
         * If you are seeing the below pthread_kill() executing much at
         * all, we suggest testing the underlying kernel and filing the
         * relevant bug report.  For Linux kernels, we recommend getting
         * the Linux Test Project (LTP).
         */
        cds_list_for_each_entry(index, &registry, node) {
                while (CMM_LOAD_SHARED(index->need_mb)) {
                        pthread_kill(index->tid, SIGRCU);
                        poll(NULL, 0, 1);
                }
        }
        cmm_smp_mb();   /* read ->need_mb before ending the barrier */
}

static void smp_mb_master(int group)
{
        force_mb_all_readers();
}
#endif /* #ifdef RCU_SIGNAL */

/*
 * synchronize_rcu() waiting. Single thread.
 */
static void wait_gp(void)
{
        /* Read reader_gp before read futex */
        smp_mb_master(RCU_MB_GROUP);
        if (uatomic_read(&rcu_gp.futex) == -1)
                futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
                      NULL, NULL, 0);
}

static void wait_for_readers(struct cds_list_head *input_readers,
                        struct cds_list_head *cur_snap_readers,
                        struct cds_list_head *qsreaders)
{
        int wait_loops = 0;
        struct rcu_reader *index, *tmp;

        /*
         * Wait for each thread URCU_TLS(rcu_reader).ctr to either
         * indicate quiescence (not nested), or observe the current
         * rcu_gp.ctr value.
         */
        for (;;) {
                wait_loops++;
                if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                        uatomic_dec(&rcu_gp.futex);
                        /* Write futex before read reader_gp */
                        smp_mb_master(RCU_MB_GROUP);
                }

                cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
                        switch (rcu_reader_state(&index->ctr)) {
                        case RCU_READER_ACTIVE_CURRENT:
                                if (cur_snap_readers) {
                                        cds_list_move(&index->node,
                                                cur_snap_readers);
                                        break;
                                }
                                /* Fall-through */
                        case RCU_READER_INACTIVE:
                                cds_list_move(&index->node, qsreaders);
                                break;
                        case RCU_READER_ACTIVE_OLD:
                                /*
                                 * Old snapshot. Leaving node in
                                 * input_readers will make us busy-loop
                                 * until the snapshot becomes current or
                                 * the reader becomes inactive.
                                 */
                                break;
                        }
                }

#ifndef HAS_INCOHERENT_CACHES
                if (cds_list_empty(input_readers)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
                                uatomic_set(&rcu_gp.futex, 0);
                        }
                        break;
                } else {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
                                wait_gp();
                        else
                                caa_cpu_relax();
                }
#else /* #ifndef HAS_INCOHERENT_CACHES */
                /*
                 * BUSY-LOOP. Force the reader thread to commit its
                 * URCU_TLS(rcu_reader).ctr update to memory if we wait
                 * for too long.
                 */
                if (cds_list_empty(input_readers)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
                                uatomic_set(&rcu_gp.futex, 0);
                        }
                        break;
                } else {
                        switch (wait_loops) {
                        case RCU_QS_ACTIVE_ATTEMPTS:
                                wait_gp();
                                break; /* only escape switch */
                        case KICK_READER_LOOPS:
                                smp_mb_master(RCU_MB_GROUP);
                                wait_loops = 0;
                                break; /* only escape switch */
                        default:
                                caa_cpu_relax();
                        }
                }
#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
        }
}

void synchronize_rcu(void)
{
        CDS_LIST_HEAD(cur_snap_readers);
        CDS_LIST_HEAD(qsreaders);
        DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
        struct urcu_waiters waiters;

        /*
         * Add ourself to gp_waiters queue of threads awaiting to wait
         * for a grace period. Proceed to perform the grace period only
         * if we are the first thread added into the queue.
         * The implicit memory barrier before urcu_wait_add()
         * orders prior memory accesses of threads put into the wait
         * queue before their insertion into the wait queue.
         */
        if (urcu_wait_add(&gp_waiters, &wait) != 0) {
                /* Not first in queue: will be awakened by another thread. */
                urcu_adaptative_busy_wait(&wait);
                /* Order following memory accesses after grace period. */
                cmm_smp_mb();
                return;
        }
        /* We won't need to wake ourself up */
        urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);

        mutex_lock(&rcu_gp_lock);

        /*
         * Move all waiters into our local queue.
         */
        urcu_move_waiters(&waiters, &gp_waiters);

        if (cds_list_empty(&registry))
                goto out;

        /* All threads should read qparity before accessing data structure
         * where new ptr points to. Must be done within rcu_gp_lock because it
         * iterates on reader threads.*/
        /* Write new ptr before changing the qparity */
        smp_mb_master(RCU_MB_GROUP);

        /*
         * Wait for readers to observe original parity or be quiescent.
         */
        wait_for_readers(&registry, &cur_snap_readers, &qsreaders);

        /*
         * Must finish waiting for quiescent state for original parity before
         * committing next rcu_gp.ctr update to memory. Failure to do so could
         * result in the writer waiting forever while new readers are always
         * accessing data (no progress).  Enforce compiler-order of load
         * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
         */
        cmm_barrier();

        /*
         * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
         * model easier to understand. It does not have a big performance impact
         * anyway, given this is the write-side.
         */
        cmm_smp_mb();

        /* Switch parity: 0 -> 1, 1 -> 0 */
        CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);

        /*
         * Must commit rcu_gp.ctr update to memory before waiting for quiescent
         * state. Failure to do so could result in the writer waiting forever
         * while new readers are always accessing data (no progress). Enforce
         * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
         */
        cmm_barrier();

        /*
         *
         * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
         * model easier to understand. It does not have a big performance impact
         * anyway, given this is the write-side.
         */
        cmm_smp_mb();

        /*
         * Wait for readers to observe new parity or be quiescent.
         */
        wait_for_readers(&cur_snap_readers, NULL, &qsreaders);

        /*
         * Put quiescent reader list back into registry.
         */
        cds_list_splice(&qsreaders, &registry);

        /* Finish waiting for reader threads before letting the old ptr being
         * freed. Must be done within rcu_gp_lock because it iterates on reader
         * threads. */
        smp_mb_master(RCU_MB_GROUP);
out:
        mutex_unlock(&rcu_gp_lock);

        /*
         * Wakeup waiters only after we have completed the grace period
         * and have ensured the memory barriers at the end of the grace
         * period have been issued.
         */
        urcu_wake_all_waiters(&waiters);
}

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

void rcu_read_lock(void)
{
        _rcu_read_lock();
}

void rcu_read_unlock(void)
{
        _rcu_read_unlock();
}

int rcu_read_ongoing(void)
{
        return _rcu_read_ongoing();
}

void rcu_register_thread(void)
{
        URCU_TLS(rcu_reader).tid = pthread_self();
        assert(URCU_TLS(rcu_reader).need_mb == 0);
        assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));

        mutex_lock(&rcu_gp_lock);
        rcu_init();     /* In case gcc does not support constructor attribute */
        cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
        mutex_unlock(&rcu_gp_lock);
}

void rcu_unregister_thread(void)
{
        mutex_lock(&rcu_gp_lock);
        cds_list_del(&URCU_TLS(rcu_reader).node);
        mutex_unlock(&rcu_gp_lock);
}

#ifdef RCU_MEMBARRIER
void rcu_init(void)
{
        if (init_done)
                return;
        init_done = 1;
        if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
                rcu_has_sys_membarrier = 1;
}
#endif

#ifdef RCU_SIGNAL
static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
{
        /*
         * Executing this cmm_smp_mb() is the only purpose of this signal handler.
         * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
         * executed on.
         */
        cmm_smp_mb();
        _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
        cmm_smp_mb();
}

/*
 * rcu_init constructor. Called when the library is linked, but also when
 * reader threads are calling rcu_register_thread().
 * Should only be called by a single thread at a given time. This is ensured by
 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
 * load time, which should not be executed by multiple threads nor concurrently
 * with rcu_register_thread() anyway.
 */
void rcu_init(void)
{
        struct sigaction act;
        int ret;

        if (init_done)
                return;
        init_done = 1;

        act.sa_sigaction = sigrcu_handler;
        act.sa_flags = SA_SIGINFO | SA_RESTART;
        sigemptyset(&act.sa_mask);
        ret = sigaction(SIGRCU, &act, NULL);
        if (ret)
                urcu_die(errno);
}

void rcu_exit(void)
{
        /*
         * Don't unregister the SIGRCU signal handler anymore, because
         * call_rcu threads could still be using it shortly before the
         * application exits.
         * Assertion disabled because call_rcu threads are now rcu
         * readers, and left running at exit.
         * assert(cds_list_empty(&registry));
         */
}

#endif /* #ifdef RCU_SIGNAL */

DEFINE_RCU_FLAVOR(rcu_flavor);

#include "urcu-call-rcu-impl.h"
#include "urcu-defer-impl.h"