[urcu.git] / urcu.c

/*
 * urcu.c
 *
 * Userspace RCU library
 *
 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
 * 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
#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 "urcu-static.h"
/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
#include "urcu.h"

#ifdef RCU_MEMBARRIER
static int init_done;
int 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;

int gp_futex;

/*
 * Global grace period counter.
 * Contains the current RCU_GP_CTR_PHASE.
 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
 * Written to only by writer with mutex taken. Read by both writer and readers.
 */
long rcu_gp_ctr = RCU_GP_COUNT;

/*
 * Written to only by each individual reader. Read by both the reader and the
 * writers.
 */
struct rcu_reader __thread rcu_reader;

#ifdef DEBUG_YIELD
unsigned int yield_active;
unsigned int __thread rand_yield;
#endif

static LIST_HEAD(registry);

static void mutex_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);
                }
                if (rcu_reader.need_mb) {
                        smp_mb();
                        rcu_reader.need_mb = 0;
                        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) {
                perror("Error in pthread mutex unlock");
                exit(-1);
        }
}

#ifdef RCU_MEMBARRIER
static void smp_mb_master(int group)
{
        if (likely(has_sys_membarrier))
                membarrier(MEMBARRIER_EXPEDITED);
        else
                smp_mb();
}
#endif

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

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

        /*
         * Ask for each threads to execute a smp_mb() so we can consider the
         * compiler barriers around rcu read lock as real memory barriers.
         */
        if (list_empty(&registry))
                return;
        /*
         * pthread_kill has a 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 smp_mc() to make sure the
         * cache flush is enforced.
         */
        list_for_each_entry(index, &registry, head) {
                index->need_mb = 1;
                smp_mc();       /* write need_mb before sending the signal */
                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).
         */
        list_for_each_entry(index, &registry, head) {
                while (index->need_mb) {
                        pthread_kill(index->tid, SIGRCU);
                        poll(NULL, 0, 1);
                }
        }
        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(&gp_futex) == -1)
                futex_async(&gp_futex, FUTEX_WAIT, -1,
                      NULL, NULL, 0);
}

void update_counter_and_wait(void)
{
        LIST_HEAD(qsreaders);
        int wait_loops = 0;
        struct rcu_reader *index, *tmp;

        /* Switch parity: 0 -> 1, 1 -> 0 */
        STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);

        /*
         * Must commit qparity update to memory before waiting for other parity
         * quiescent state. Failure to do so could result in the writer waiting
         * forever while new readers are always accessing data (no progress).
         * Ensured by STORE_SHARED and LOAD_SHARED.
         */

        /*
         * Adding a 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.
         */
        smp_mb();

        /*
         * Wait for each thread rcu_reader.ctr count to become 0.
         */
        for (;;) {
                wait_loops++;
                if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                        uatomic_dec(&gp_futex);
                        /* Write futex before read reader_gp */
                        smp_mb_master(RCU_MB_GROUP);
                }

                list_for_each_entry_safe(index, tmp, &registry, head) {
                        if (!rcu_old_gp_ongoing(&index->ctr))
                                list_move(&index->head, &qsreaders);
                }

#ifndef HAS_INCOHERENT_CACHES
                if (list_empty(&registry)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
                                uatomic_set(&gp_futex, 0);
                        }
                        break;
                } else {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
                                wait_gp();
                        else
                                cpu_relax();
                }
#else /* #ifndef HAS_INCOHERENT_CACHES */
                /*
                 * BUSY-LOOP. Force the reader thread to commit its
                 * rcu_reader.ctr update to memory if we wait for too long.
                 */
                if (list_empty(&registry)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
                                uatomic_set(&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:
                                cpu_relax();
                        }
                }
#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
        }
        /* put back the reader list in the registry */
        list_splice(&qsreaders, &registry);
}

void synchronize_rcu(void)
{
        mutex_lock(&rcu_gp_lock);

        if (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 previous parity to be empty of readers.
         */
        update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */

        /*
         * Must finish waiting for quiescent state for parity 0 before
         * committing qparity update to memory. Failure to do so could result in
         * the writer waiting forever while new readers are always accessing
         * data (no progress).
         * Ensured by STORE_SHARED and LOAD_SHARED.
         */

        /*
         * Adding a 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.
         */
        smp_mb();

        /*
         * Wait for previous parity to be empty of readers.
         */
        update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */

        /* 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);
}

/*
 * 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();
}

void rcu_register_thread(void)
{
        rcu_reader.tid = pthread_self();
        assert(rcu_reader.need_mb == 0);
        assert(rcu_reader.ctr == 0);

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

void rcu_unregister_thread(void)
{
        mutex_lock(&rcu_gp_lock);
        list_del(&rcu_reader.head);
        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))
                has_sys_membarrier = 1;
}
#endif

#ifdef RCU_SIGNAL
static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
{
        /*
         * Executing this smp_mb() is the only purpose of this signal handler.
         * It punctually promotes barrier() into smp_mb() on every thread it is
         * executed on.
         */
        smp_mb();
        rcu_reader.need_mb = 0;
        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) {
                perror("Error in sigaction");
                exit(-1);
        }
}

void rcu_exit(void)
{
        struct sigaction act;
        int ret;

        ret = sigaction(SIGRCU, NULL, &act);
        if (ret) {
                perror("Error in sigaction");
                exit(-1);
        }
        assert(act.sa_sigaction == sigrcu_handler);
        assert(list_empty(&registry));
}
#endif /* #ifdef RCU_SIGNAL */