Fix: urcu-bp: Bulletproof RCU arena resize bug

[urcu.git] / urcu-call-rcu-impl.h

/*
 * urcu-call-rcu.c
 *
 * Userspace RCU library - batch memory reclamation with kernel API
 *
 * Copyright (c) 2010 Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 *
 * 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
 */

#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 <sys/time.h>
#include <unistd.h>
#include <sched.h>

#include "config.h"
#include "urcu/wfcqueue.h"
#include "urcu-call-rcu.h"
#include "urcu-pointer.h"
#include "urcu/list.h"
#include "urcu/futex.h"
#include "urcu/tls-compat.h"
#include "urcu-die.h"

/* Data structure that identifies a call_rcu thread. */

struct call_rcu_data {
        /*
         * We do not align head on a different cache-line than tail
         * mainly because call_rcu callback-invocation threads use
         * batching ("splice") to get an entire list of callbacks, which
         * effectively empties the queue, and requires to touch the tail
         * anyway.
         */
        struct cds_wfcq_tail cbs_tail;
        struct cds_wfcq_head cbs_head;
        unsigned long flags;
        int32_t futex;
        unsigned long qlen; /* maintained for debugging. */
        pthread_t tid;
        int cpu_affinity;
        struct cds_list_head list;
} __attribute__((aligned(CAA_CACHE_LINE_SIZE)));

struct call_rcu_completion {
        int barrier_count;
        int32_t futex;
};

struct call_rcu_completion_work {
        struct rcu_head head;
        struct call_rcu_completion *completion;
};

/*
 * List of all call_rcu_data structures to keep valgrind happy.
 * Protected by call_rcu_mutex.
 */

static CDS_LIST_HEAD(call_rcu_data_list);

/* Link a thread using call_rcu() to its call_rcu thread. */

static DEFINE_URCU_TLS(struct call_rcu_data *, thread_call_rcu_data);

/*
 * Guard call_rcu thread creation and atfork handlers.
 */
static pthread_mutex_t call_rcu_mutex = PTHREAD_MUTEX_INITIALIZER;

/* If a given thread does not have its own call_rcu thread, this is default. */

static struct call_rcu_data *default_call_rcu_data;

/*
 * If the sched_getcpu() and sysconf(_SC_NPROCESSORS_CONF) calls are
 * available, then we can have call_rcu threads assigned to individual
 * CPUs rather than only to specific threads.
 */

#ifdef HAVE_SCHED_GETCPU

static int urcu_sched_getcpu(void)
{
        return sched_getcpu();
}

#else /* #ifdef HAVE_SCHED_GETCPU */

static int urcu_sched_getcpu(void)
{
        return -1;
}

#endif /* #else #ifdef HAVE_SCHED_GETCPU */

#if defined(HAVE_SYSCONF) && defined(HAVE_SCHED_GETCPU)

/*
 * Pointer to array of pointers to per-CPU call_rcu_data structures
 * and # CPUs. per_cpu_call_rcu_data is a RCU-protected pointer to an
 * array of RCU-protected pointers to call_rcu_data. call_rcu acts as a
 * RCU read-side and reads per_cpu_call_rcu_data and the per-cpu pointer
 * without mutex. The call_rcu_mutex protects updates.
 */

static struct call_rcu_data **per_cpu_call_rcu_data;
static long maxcpus;

static void maxcpus_reset(void)
{
        maxcpus = 0;
}

/* Allocate the array if it has not already been allocated. */

static void alloc_cpu_call_rcu_data(void)
{
        struct call_rcu_data **p;
        static int warned = 0;

        if (maxcpus != 0)
                return;
        maxcpus = sysconf(_SC_NPROCESSORS_CONF);
        if (maxcpus <= 0) {
                return;
        }
        p = malloc(maxcpus * sizeof(*per_cpu_call_rcu_data));
        if (p != NULL) {
                memset(p, '\0', maxcpus * sizeof(*per_cpu_call_rcu_data));
                rcu_set_pointer(&per_cpu_call_rcu_data, p);
        } else {
                if (!warned) {
                        fprintf(stderr, "[error] liburcu: unable to allocate per-CPU pointer array\n");
                }
                warned = 1;
        }
}

#else /* #if defined(HAVE_SYSCONF) && defined(HAVE_SCHED_GETCPU) */

/*
 * per_cpu_call_rcu_data should be constant, but some functions below, used both
 * for cases where cpu number is available and not available, assume it it not
 * constant.
 */
static struct call_rcu_data **per_cpu_call_rcu_data = NULL;
static const long maxcpus = -1;

static void maxcpus_reset(void)
{
}

static void alloc_cpu_call_rcu_data(void)
{
}

#endif /* #else #if defined(HAVE_SYSCONF) && defined(HAVE_SCHED_GETCPU) */

/* Acquire the specified pthread mutex. */

static void call_rcu_lock(pthread_mutex_t *pmp)
{
        int ret;

        ret = pthread_mutex_lock(pmp);
        if (ret)
                urcu_die(ret);
}

/* Release the specified pthread mutex. */

static void call_rcu_unlock(pthread_mutex_t *pmp)
{
        int ret;

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

#if HAVE_SCHED_SETAFFINITY
static
int set_thread_cpu_affinity(struct call_rcu_data *crdp)
{
        cpu_set_t mask;

        if (crdp->cpu_affinity < 0)
                return 0;

        CPU_ZERO(&mask);
        CPU_SET(crdp->cpu_affinity, &mask);
#if SCHED_SETAFFINITY_ARGS == 2
        return sched_setaffinity(0, &mask);
#else
        return sched_setaffinity(0, sizeof(mask), &mask);
#endif
}
#else
static
int set_thread_cpu_affinity(struct call_rcu_data *crdp)
{
        return 0;
}
#endif

static void call_rcu_wait(struct call_rcu_data *crdp)
{
        /* Read call_rcu list before read futex */
        cmm_smp_mb();
        if (uatomic_read(&crdp->futex) == -1)
                futex_async(&crdp->futex, FUTEX_WAIT, -1,
                      NULL, NULL, 0);
}

static void call_rcu_wake_up(struct call_rcu_data *crdp)
{
        /* Write to call_rcu list before reading/writing futex */
        cmm_smp_mb();
        if (caa_unlikely(uatomic_read(&crdp->futex) == -1)) {
                uatomic_set(&crdp->futex, 0);
                futex_async(&crdp->futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);
        }
}

static void call_rcu_completion_wait(struct call_rcu_completion *completion)
{
        /* Read completion barrier count before read futex */
        cmm_smp_mb();
        if (uatomic_read(&completion->futex) == -1)
                futex_async(&completion->futex, FUTEX_WAIT, -1,
                      NULL, NULL, 0);
}

static void call_rcu_completion_wake_up(struct call_rcu_completion *completion)
{
        /* Write to completion barrier count before reading/writing futex */
        cmm_smp_mb();
        if (caa_unlikely(uatomic_read(&completion->futex) == -1)) {
                uatomic_set(&completion->futex, 0);
                futex_async(&completion->futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);
        }
}

/* This is the code run by each call_rcu thread. */

static void *call_rcu_thread(void *arg)
{
        unsigned long cbcount;
        struct call_rcu_data *crdp = (struct call_rcu_data *) arg;
        int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
        int ret;

        ret = set_thread_cpu_affinity(crdp);
        if (ret)
                urcu_die(errno);

        /*
         * If callbacks take a read-side lock, we need to be registered.
         */
        rcu_register_thread();

        URCU_TLS(thread_call_rcu_data) = crdp;
        if (!rt) {
                uatomic_dec(&crdp->futex);
                /* Decrement futex before reading call_rcu list */
                cmm_smp_mb();
        }
        for (;;) {
                struct cds_wfcq_head cbs_tmp_head;
                struct cds_wfcq_tail cbs_tmp_tail;
                struct cds_wfcq_node *cbs, *cbs_tmp_n;
                enum cds_wfcq_ret splice_ret;

                if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) {
                        /*
                         * Pause requested. Become quiescent: remove
                         * ourself from all global lists, and don't
                         * process any callback. The callback lists may
                         * still be non-empty though.
                         */
                        rcu_unregister_thread();
                        cmm_smp_mb__before_uatomic_or();
                        uatomic_or(&crdp->flags, URCU_CALL_RCU_PAUSED);
                        while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) != 0)
                                poll(NULL, 0, 1);
                        rcu_register_thread();
                }

                cds_wfcq_init(&cbs_tmp_head, &cbs_tmp_tail);
                splice_ret = __cds_wfcq_splice_blocking(&cbs_tmp_head,
                        &cbs_tmp_tail, &crdp->cbs_head, &crdp->cbs_tail);
                assert(splice_ret != CDS_WFCQ_RET_WOULDBLOCK);
                assert(splice_ret != CDS_WFCQ_RET_DEST_NON_EMPTY);
                if (splice_ret != CDS_WFCQ_RET_SRC_EMPTY) {
                        synchronize_rcu();
                        cbcount = 0;
                        __cds_wfcq_for_each_blocking_safe(&cbs_tmp_head,
                                        &cbs_tmp_tail, cbs, cbs_tmp_n) {
                                struct rcu_head *rhp;

                                rhp = caa_container_of(cbs,
                                        struct rcu_head, next);
                                rhp->func(rhp);
                                cbcount++;
                        }
                        uatomic_sub(&crdp->qlen, cbcount);
                }
                if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOP)
                        break;
                rcu_thread_offline();
                if (!rt) {
                        if (cds_wfcq_empty(&crdp->cbs_head,
                                        &crdp->cbs_tail)) {
                                call_rcu_wait(crdp);
                                poll(NULL, 0, 10);
                                uatomic_dec(&crdp->futex);
                                /*
                                 * Decrement futex before reading
                                 * call_rcu list.
                                 */
                                cmm_smp_mb();
                        } else {
                                poll(NULL, 0, 10);
                        }
                } else {
                        poll(NULL, 0, 10);
                }
                rcu_thread_online();
        }
        if (!rt) {
                /*
                 * Read call_rcu list before write futex.
                 */
                cmm_smp_mb();
                uatomic_set(&crdp->futex, 0);
        }
        uatomic_or(&crdp->flags, URCU_CALL_RCU_STOPPED);
        rcu_unregister_thread();
        return NULL;
}

/*
 * Create both a call_rcu thread and the corresponding call_rcu_data
 * structure, linking the structure in as specified.  Caller must hold
 * call_rcu_mutex.
 */

static void call_rcu_data_init(struct call_rcu_data **crdpp,
                               unsigned long flags,
                               int cpu_affinity)
{
        struct call_rcu_data *crdp;
        int ret;

        crdp = malloc(sizeof(*crdp));
        if (crdp == NULL)
                urcu_die(errno);
        memset(crdp, '\0', sizeof(*crdp));
        cds_wfcq_init(&crdp->cbs_head, &crdp->cbs_tail);
        crdp->qlen = 0;
        crdp->futex = 0;
        crdp->flags = flags;
        cds_list_add(&crdp->list, &call_rcu_data_list);
        crdp->cpu_affinity = cpu_affinity;
        cmm_smp_mb();  /* Structure initialized before pointer is planted. */
        *crdpp = crdp;
        ret = pthread_create(&crdp->tid, NULL, call_rcu_thread, crdp);
        if (ret)
                urcu_die(ret);
}

/*
 * Return a pointer to the call_rcu_data structure for the specified
 * CPU, returning NULL if there is none.  We cannot automatically
 * created it because the platform we are running on might not define
 * urcu_sched_getcpu().
 *
 * The call to this function and use of the returned call_rcu_data
 * should be protected by RCU read-side lock.
 */

struct call_rcu_data *get_cpu_call_rcu_data(int cpu)
{
        static int warned = 0;
        struct call_rcu_data **pcpu_crdp;

        pcpu_crdp = rcu_dereference(per_cpu_call_rcu_data);
        if (pcpu_crdp == NULL)
                return NULL;
        if (!warned && maxcpus > 0 && (cpu < 0 || maxcpus <= cpu)) {
                fprintf(stderr, "[error] liburcu: get CPU # out of range\n");
                warned = 1;
        }
        if (cpu < 0 || maxcpus <= cpu)
                return NULL;
        return rcu_dereference(pcpu_crdp[cpu]);
}

/*
 * Return the tid corresponding to the call_rcu thread whose
 * call_rcu_data structure is specified.
 */

pthread_t get_call_rcu_thread(struct call_rcu_data *crdp)
{
        return crdp->tid;
}

/*
 * Create a call_rcu_data structure (with thread) and return a pointer.
 */

static struct call_rcu_data *__create_call_rcu_data(unsigned long flags,
                                                    int cpu_affinity)
{
        struct call_rcu_data *crdp;

        call_rcu_data_init(&crdp, flags, cpu_affinity);
        return crdp;
}

struct call_rcu_data *create_call_rcu_data(unsigned long flags,
                                           int cpu_affinity)
{
        struct call_rcu_data *crdp;

        call_rcu_lock(&call_rcu_mutex);
        crdp = __create_call_rcu_data(flags, cpu_affinity);
        call_rcu_unlock(&call_rcu_mutex);
        return crdp;
}

/*
 * Set the specified CPU to use the specified call_rcu_data structure.
 *
 * Use NULL to remove a CPU's call_rcu_data structure, but it is
 * the caller's responsibility to dispose of the removed structure.
 * Use get_cpu_call_rcu_data() to obtain a pointer to the old structure
 * (prior to NULLing it out, of course).
 *
 * The caller must wait for a grace-period to pass between return from
 * set_cpu_call_rcu_data() and call to call_rcu_data_free() passing the
 * previous call rcu data as argument.
 */

int set_cpu_call_rcu_data(int cpu, struct call_rcu_data *crdp)
{
        static int warned = 0;

        call_rcu_lock(&call_rcu_mutex);
        alloc_cpu_call_rcu_data();
        if (cpu < 0 || maxcpus <= cpu) {
                if (!warned) {
                        fprintf(stderr, "[error] liburcu: set CPU # out of range\n");
                        warned = 1;
                }
                call_rcu_unlock(&call_rcu_mutex);
                errno = EINVAL;
                return -EINVAL;
        }

        if (per_cpu_call_rcu_data == NULL) {
                call_rcu_unlock(&call_rcu_mutex);
                errno = ENOMEM;
                return -ENOMEM;
        }

        if (per_cpu_call_rcu_data[cpu] != NULL && crdp != NULL) {
                call_rcu_unlock(&call_rcu_mutex);
                errno = EEXIST;
                return -EEXIST;
        }

        rcu_set_pointer(&per_cpu_call_rcu_data[cpu], crdp);
        call_rcu_unlock(&call_rcu_mutex);
        return 0;
}

/*
 * Return a pointer to the default call_rcu_data structure, creating
 * one if need be.  Because we never free call_rcu_data structures,
 * we don't need to be in an RCU read-side critical section.
 */

struct call_rcu_data *get_default_call_rcu_data(void)
{
        if (default_call_rcu_data != NULL)
                return rcu_dereference(default_call_rcu_data);
        call_rcu_lock(&call_rcu_mutex);
        if (default_call_rcu_data != NULL) {
                call_rcu_unlock(&call_rcu_mutex);
                return default_call_rcu_data;
        }
        call_rcu_data_init(&default_call_rcu_data, 0, -1);
        call_rcu_unlock(&call_rcu_mutex);
        return default_call_rcu_data;
}

/*
 * Return the call_rcu_data structure that applies to the currently
 * running thread.  Any call_rcu_data structure assigned specifically
 * to this thread has first priority, followed by any call_rcu_data
 * structure assigned to the CPU on which the thread is running,
 * followed by the default call_rcu_data structure.  If there is not
 * yet a default call_rcu_data structure, one will be created.
 *
 * Calls to this function and use of the returned call_rcu_data should
 * be protected by RCU read-side lock.
 */
struct call_rcu_data *get_call_rcu_data(void)
{
        struct call_rcu_data *crd;

        if (URCU_TLS(thread_call_rcu_data) != NULL)
                return URCU_TLS(thread_call_rcu_data);

        if (maxcpus > 0) {
                crd = get_cpu_call_rcu_data(urcu_sched_getcpu());
                if (crd)
                        return crd;
        }

        return get_default_call_rcu_data();
}

/*
 * Return a pointer to this task's call_rcu_data if there is one.
 */

struct call_rcu_data *get_thread_call_rcu_data(void)
{
        return URCU_TLS(thread_call_rcu_data);
}

/*
 * Set this task's call_rcu_data structure as specified, regardless
 * of whether or not this task already had one.  (This allows switching
 * to and from real-time call_rcu threads, for example.)
 *
 * Use NULL to remove a thread's call_rcu_data structure, but it is
 * the caller's responsibility to dispose of the removed structure.
 * Use get_thread_call_rcu_data() to obtain a pointer to the old structure
 * (prior to NULLing it out, of course).
 */

void set_thread_call_rcu_data(struct call_rcu_data *crdp)
{
        URCU_TLS(thread_call_rcu_data) = crdp;
}

/*
 * Create a separate call_rcu thread for each CPU.  This does not
 * replace a pre-existing call_rcu thread -- use the set_cpu_call_rcu_data()
 * function if you want that behavior. Should be paired with
 * free_all_cpu_call_rcu_data() to teardown these call_rcu worker
 * threads.
 */

int create_all_cpu_call_rcu_data(unsigned long flags)
{
        int i;
        struct call_rcu_data *crdp;
        int ret;

        call_rcu_lock(&call_rcu_mutex);
        alloc_cpu_call_rcu_data();
        call_rcu_unlock(&call_rcu_mutex);
        if (maxcpus <= 0) {
                errno = EINVAL;
                return -EINVAL;
        }
        if (per_cpu_call_rcu_data == NULL) {
                errno = ENOMEM;
                return -ENOMEM;
        }
        for (i = 0; i < maxcpus; i++) {
                call_rcu_lock(&call_rcu_mutex);
                if (get_cpu_call_rcu_data(i)) {
                        call_rcu_unlock(&call_rcu_mutex);
                        continue;
                }
                crdp = __create_call_rcu_data(flags, i);
                if (crdp == NULL) {
                        call_rcu_unlock(&call_rcu_mutex);
                        errno = ENOMEM;
                        return -ENOMEM;
                }
                call_rcu_unlock(&call_rcu_mutex);
                if ((ret = set_cpu_call_rcu_data(i, crdp)) != 0) {
                        call_rcu_data_free(crdp);

                        /* it has been created by other thread */
                        if (ret == -EEXIST)
                                continue;

                        return ret;
                }
        }
        return 0;
}

/*
 * Wake up the call_rcu thread corresponding to the specified
 * call_rcu_data structure.
 */
static void wake_call_rcu_thread(struct call_rcu_data *crdp)
{
        if (!(_CMM_LOAD_SHARED(crdp->flags) & URCU_CALL_RCU_RT))
                call_rcu_wake_up(crdp);
}

static void _call_rcu(struct rcu_head *head,
                      void (*func)(struct rcu_head *head),
                      struct call_rcu_data *crdp)
{
        cds_wfcq_node_init(&head->next);
        head->func = func;
        cds_wfcq_enqueue(&crdp->cbs_head, &crdp->cbs_tail, &head->next);
        uatomic_inc(&crdp->qlen);
        wake_call_rcu_thread(crdp);
}

/*
 * Schedule a function to be invoked after a following grace period.
 * This is the only function that must be called -- the others are
 * only present to allow applications to tune their use of RCU for
 * maximum performance.
 *
 * Note that unless a call_rcu thread has not already been created,
 * the first invocation of call_rcu() will create one.  So, if you
 * need the first invocation of call_rcu() to be fast, make sure
 * to create a call_rcu thread first.  One way to accomplish this is
 * "get_call_rcu_data();", and another is create_all_cpu_call_rcu_data().
 *
 * call_rcu must be called by registered RCU read-side threads.
 */
void call_rcu(struct rcu_head *head,
              void (*func)(struct rcu_head *head))
{
        struct call_rcu_data *crdp;

        /* Holding rcu read-side lock across use of per-cpu crdp */
        rcu_read_lock();
        crdp = get_call_rcu_data();
        _call_rcu(head, func, crdp);
        rcu_read_unlock();
}

/*
 * Free up the specified call_rcu_data structure, terminating the
 * associated call_rcu thread.  The caller must have previously
 * removed the call_rcu_data structure from per-thread or per-CPU
 * usage.  For example, set_cpu_call_rcu_data(cpu, NULL) for per-CPU
 * call_rcu_data structures or set_thread_call_rcu_data(NULL) for
 * per-thread call_rcu_data structures.
 *
 * We silently refuse to free up the default call_rcu_data structure
 * because that is where we put any leftover callbacks.  Note that
 * the possibility of self-spawning callbacks makes it impossible
 * to execute all the callbacks in finite time without putting any
 * newly spawned callbacks somewhere else.  The "somewhere else" of
 * last resort is the default call_rcu_data structure.
 *
 * We also silently refuse to free NULL pointers.  This simplifies
 * the calling code.
 *
 * The caller must wait for a grace-period to pass between return from
 * set_cpu_call_rcu_data() and call to call_rcu_data_free() passing the
 * previous call rcu data as argument.
 *
 * Note: introducing __cds_wfcq_splice_blocking() in this function fixed
 * a list corruption bug in the 0.7.x series. The equivalent fix
 * appeared in 0.6.8 for the stable-0.6 branch.
 */
void call_rcu_data_free(struct call_rcu_data *crdp)
{
        if (crdp == NULL || crdp == default_call_rcu_data) {
                return;
        }
        if ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) {
                uatomic_or(&crdp->flags, URCU_CALL_RCU_STOP);
                wake_call_rcu_thread(crdp);
                while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0)
                        poll(NULL, 0, 1);
        }
        if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) {
                /* Create default call rcu data if need be */
                (void) get_default_call_rcu_data();
                __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head,
                        &default_call_rcu_data->cbs_tail,
                        &crdp->cbs_head, &crdp->cbs_tail);
                uatomic_add(&default_call_rcu_data->qlen,
                            uatomic_read(&crdp->qlen));
                wake_call_rcu_thread(default_call_rcu_data);
        }

        call_rcu_lock(&call_rcu_mutex);
        cds_list_del(&crdp->list);
        call_rcu_unlock(&call_rcu_mutex);

        free(crdp);
}

/*
 * Clean up all the per-CPU call_rcu threads.
 */
void free_all_cpu_call_rcu_data(void)
{
        int cpu;
        struct call_rcu_data **crdp;
        static int warned = 0;

        if (maxcpus <= 0)
                return;

        crdp = malloc(sizeof(*crdp) * maxcpus);
        if (!crdp) {
                if (!warned) {
                        fprintf(stderr, "[error] liburcu: unable to allocate per-CPU pointer array\n");
                }
                warned = 1;
                return;
        }

        for (cpu = 0; cpu < maxcpus; cpu++) {
                crdp[cpu] = get_cpu_call_rcu_data(cpu);
                if (crdp[cpu] == NULL)
                        continue;
                set_cpu_call_rcu_data(cpu, NULL);
        }
        /*
         * Wait for call_rcu sites acting as RCU readers of the
         * call_rcu_data to become quiescent.
         */
        synchronize_rcu();
        for (cpu = 0; cpu < maxcpus; cpu++) {
                if (crdp[cpu] == NULL)
                        continue;
                call_rcu_data_free(crdp[cpu]);
        }
        free(crdp);
}

static
void _rcu_barrier_complete(struct rcu_head *head)
{
        struct call_rcu_completion_work *work;
        struct call_rcu_completion *completion;

        work = caa_container_of(head, struct call_rcu_completion_work, head);
        completion = work->completion;
        uatomic_dec(&completion->barrier_count);
        call_rcu_completion_wake_up(completion);
        free(work);
}

/*
 * Wait for all in-flight call_rcu callbacks to complete execution.
 */
void rcu_barrier(void)
{
        struct call_rcu_data *crdp;
        struct call_rcu_completion completion;
        int count = 0;
        int was_online;

        /* Put in offline state in QSBR. */
        was_online = rcu_read_ongoing();
        if (was_online)
                rcu_thread_offline();
        /*
         * Calling a rcu_barrier() within a RCU read-side critical
         * section is an error.
         */
        if (rcu_read_ongoing()) {
                static int warned = 0;

                if (!warned) {
                        fprintf(stderr, "[error] liburcu: rcu_barrier() called from within RCU read-side critical section.\n");
                }
                warned = 1;
                goto online;
        }

        call_rcu_lock(&call_rcu_mutex);
        cds_list_for_each_entry(crdp, &call_rcu_data_list, list)
                count++;

        completion.barrier_count = count;

        cds_list_for_each_entry(crdp, &call_rcu_data_list, list) {
                struct call_rcu_completion_work *work;

                work = calloc(sizeof(*work), 1);
                if (!work)
                        urcu_die(errno);
                work->completion = &completion;
                _call_rcu(&work->head, _rcu_barrier_complete, crdp);
        }
        call_rcu_unlock(&call_rcu_mutex);

        /* Wait for them */
        for (;;) {
                uatomic_dec(&completion.futex);
                /* Decrement futex before reading barrier_count */
                cmm_smp_mb();
                if (!uatomic_read(&completion.barrier_count))
                        break;
                call_rcu_completion_wait(&completion);
        }
online:
        if (was_online)
                rcu_thread_online();
}

/*
 * Acquire the call_rcu_mutex in order to ensure that the child sees
 * all of the call_rcu() data structures in a consistent state. Ensure
 * that all call_rcu threads are in a quiescent state across fork.
 * Suitable for pthread_atfork() and friends.
 */
void call_rcu_before_fork(void)
{
        struct call_rcu_data *crdp;

        call_rcu_lock(&call_rcu_mutex);

        cds_list_for_each_entry(crdp, &call_rcu_data_list, list) {
                uatomic_or(&crdp->flags, URCU_CALL_RCU_PAUSE);
                cmm_smp_mb__after_uatomic_or();
                wake_call_rcu_thread(crdp);
        }
        cds_list_for_each_entry(crdp, &call_rcu_data_list, list) {
                while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSED) == 0)
                        poll(NULL, 0, 1);
        }
}

/*
 * Clean up call_rcu data structures in the parent of a successful fork()
 * that is not followed by exec() in the child.  Suitable for
 * pthread_atfork() and friends.
 */
void call_rcu_after_fork_parent(void)
{
        struct call_rcu_data *crdp;

        cds_list_for_each_entry(crdp, &call_rcu_data_list, list)
                uatomic_and(&crdp->flags, ~URCU_CALL_RCU_PAUSE);
        call_rcu_unlock(&call_rcu_mutex);
}

/*
 * Clean up call_rcu data structures in the child of a successful fork()
 * that is not followed by exec().  Suitable for pthread_atfork() and
 * friends.
 */
void call_rcu_after_fork_child(void)
{
        struct call_rcu_data *crdp, *next;

        /* Release the mutex. */
        call_rcu_unlock(&call_rcu_mutex);

        /* Do nothing when call_rcu() has not been used */
        if (cds_list_empty(&call_rcu_data_list))
                return;

        /*
         * Allocate a new default call_rcu_data structure in order
         * to get a working call_rcu thread to go with it.
         */
        default_call_rcu_data = NULL;
        (void)get_default_call_rcu_data();

        /* Cleanup call_rcu_data pointers before use */
        maxcpus_reset();
        free(per_cpu_call_rcu_data);
        rcu_set_pointer(&per_cpu_call_rcu_data, NULL);
        URCU_TLS(thread_call_rcu_data) = NULL;

        /*
         * Dispose of all of the rest of the call_rcu_data structures.
         * Leftover call_rcu callbacks will be merged into the new
         * default call_rcu thread queue.
         */
        cds_list_for_each_entry_safe(crdp, next, &call_rcu_data_list, list) {
                if (crdp == default_call_rcu_data)
                        continue;
                uatomic_set(&crdp->flags, URCU_CALL_RCU_STOPPED);
                call_rcu_data_free(crdp);
        }
}