[urcu.git] / urcu / static / wfcqueue.h

#ifndef _URCU_WFCQUEUE_STATIC_H
#define _URCU_WFCQUEUE_STATIC_H

/*
 * urcu/static/wfcqueue.h
 *
 * Userspace RCU library - Concurrent Queue with Wait-Free Enqueue/Blocking Dequeue
 *
 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu/wfcqueue.h for
 * linking dynamically with the userspace rcu library.
 *
 * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 * Copyright 2011-2012 - Lai Jiangshan <laijs@cn.fujitsu.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
 */

#include <pthread.h>
#include <assert.h>
#include <poll.h>
#include <stdbool.h>
#include <urcu/compiler.h>
#include <urcu/uatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

/*
 * Concurrent queue with wait-free enqueue/blocking dequeue.
 *
 * This queue has been designed and implemented collaboratively by
 * Mathieu Desnoyers and Lai Jiangshan. Inspired from
 * half-wait-free/half-blocking queue implementation done by Paul E.
 * McKenney.
 *
 * Mutual exclusion of cds_wfcq_* / __cds_wfcq_* API
 *
 * Synchronization table:
 *
 * External synchronization techniques described in the API below is
 * required between pairs marked with "X". No external synchronization
 * required between pairs marked with "-".
 *
 * Legend:
 * [1] cds_wfcq_enqueue
 * [2] __cds_wfcq_splice (destination queue)
 * [3] __cds_wfcq_dequeue
 * [4] __cds_wfcq_splice (source queue)
 * [5] __cds_wfcq_first
 * [6] __cds_wfcq_next
 *
 *     [1] [2] [3] [4] [5] [6]
 * [1]  -   -   -   -   -   -
 * [2]  -   -   -   -   -   -
 * [3]  -   -   X   X   X   X
 * [4]  -   -   X   -   X   X
 * [5]  -   -   X   X   -   -
 * [6]  -   -   X   X   -   -
 *
 * Mutual exclusion can be ensured by holding cds_wfcq_dequeue_lock().
 *
 * For convenience, cds_wfcq_dequeue_blocking() and
 * cds_wfcq_splice_blocking() hold the dequeue lock.
 *
 * Besides locking, mutual exclusion of dequeue, splice and iteration
 * can be ensured by performing all of those operations from a single
 * thread, without requiring any lock.
 */

#define WFCQ_ADAPT_ATTEMPTS             10      /* Retry if being set */
#define WFCQ_WAIT                       10      /* Wait 10 ms if being set */

/*
 * cds_wfcq_node_init: initialize wait-free queue node.
 */
static inline void _cds_wfcq_node_init(struct cds_wfcq_node *node)
{
        node->next = NULL;
}

/*
 * cds_wfcq_init: initialize wait-free queue.
 */
static inline void _cds_wfcq_init(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        int ret;

        /* Set queue head and tail */
        _cds_wfcq_node_init(&head->node);
        tail->p = &head->node;
        ret = pthread_mutex_init(&head->lock, NULL);
        assert(!ret);
}

/*
 * cds_wfcq_empty: return whether wait-free queue is empty.
 *
 * No memory barrier is issued. No mutual exclusion is required.
 *
 * We perform the test on head->node.next to check if the queue is
 * possibly empty, but we confirm this by checking if the tail pointer
 * points to the head node because the tail pointer is the linearisation
 * point of the enqueuers. Just checking the head next pointer could
 * make a queue appear empty if an enqueuer is preempted for a long time
 * between xchg() and setting the previous node's next pointer.
 */
static inline bool _cds_wfcq_empty(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        /*
         * Queue is empty if no node is pointed by head->node.next nor
         * tail->p. Even though the tail->p check is sufficient to find
         * out of the queue is empty, we first check head->node.next as a
         * common case to ensure that dequeuers do not frequently access
         * enqueuer's tail->p cache line.
         */
        return CMM_LOAD_SHARED(head->node.next) == NULL
                && CMM_LOAD_SHARED(tail->p) == &head->node;
}

static inline void _cds_wfcq_dequeue_lock(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        int ret;

        ret = pthread_mutex_lock(&head->lock);
        assert(!ret);
}

static inline void _cds_wfcq_dequeue_unlock(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        int ret;

        ret = pthread_mutex_unlock(&head->lock);
        assert(!ret);
}

static inline bool ___cds_wfcq_append(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                struct cds_wfcq_node *new_head,
                struct cds_wfcq_node *new_tail)
{
        struct cds_wfcq_node *old_tail;

        /*
         * Implicit memory barrier before uatomic_xchg() orders earlier
         * stores to data structure containing node and setting
         * node->next to NULL before publication.
         */
        old_tail = uatomic_xchg(&tail->p, new_tail);

        /*
         * Implicit memory barrier after uatomic_xchg() orders store to
         * q->tail before store to old_tail->next.
         *
         * At this point, dequeuers see a NULL tail->p->next, which
         * indicates that the queue is being appended to. The following
         * store will append "node" to the queue from a dequeuer
         * perspective.
         */
        CMM_STORE_SHARED(old_tail->next, new_head);
        /*
         * Return false if queue was empty prior to adding the node,
         * else return true.
         */
        return old_tail != &head->node;
}

/*
 * cds_wfcq_enqueue: enqueue a node into a wait-free queue.
 *
 * Issues a full memory barrier before enqueue. No mutual exclusion is
 * required.
 *
 * Returns false if the queue was empty prior to adding the node.
 * Returns true otherwise.
 */
static inline bool _cds_wfcq_enqueue(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                struct cds_wfcq_node *new_tail)
{
        return ___cds_wfcq_append(head, tail, new_tail, new_tail);
}

/*
 * ___cds_wfcq_busy_wait: adaptative busy-wait.
 *
 * Returns 1 if nonblocking and needs to block, 0 otherwise.
 */
static inline bool
___cds_wfcq_busy_wait(int *attempt, int blocking)
{
        if (!blocking)
                return 1;
        if (++(*attempt) >= WFCQ_ADAPT_ATTEMPTS) {
                poll(NULL, 0, WFCQ_WAIT);       /* Wait for 10ms */
                *attempt = 0;
        } else {
                caa_cpu_relax();
        }
        return 0;
}

/*
 * Waiting for enqueuer to complete enqueue and return the next node.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_node_sync_next(struct cds_wfcq_node *node, int blocking)
{
        struct cds_wfcq_node *next;
        int attempt = 0;

        /*
         * Adaptative busy-looping waiting for enqueuer to complete enqueue.
         */
        while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
                if (___cds_wfcq_busy_wait(&attempt, blocking))
                        return CDS_WFCQ_WOULDBLOCK;
        }

        return next;
}

static inline struct cds_wfcq_node *
___cds_wfcq_first(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                int blocking)
{
        struct cds_wfcq_node *node;

        if (_cds_wfcq_empty(head, tail))
                return NULL;
        node = ___cds_wfcq_node_sync_next(&head->node, blocking);
        /* Load head->node.next before loading node's content */
        cmm_smp_read_barrier_depends();
        return node;
}

/*
 * __cds_wfcq_first_blocking: get first node of a queue, without dequeuing.
 *
 * Content written into the node before enqueue is guaranteed to be
 * consistent, but no other memory ordering is ensured.
 * Dequeue/splice/iteration mutual exclusion should be ensured by the
 * caller.
 *
 * Used by for-like iteration macros in urcu/wfqueue.h:
 * __cds_wfcq_for_each_blocking()
 * __cds_wfcq_for_each_blocking_safe()
 *
 * Returns NULL if queue is empty, first node otherwise.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_first_blocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        return ___cds_wfcq_first(head, tail, 1);
}


/*
 * __cds_wfcq_first_nonblocking: get first node of a queue, without dequeuing.
 *
 * Same as __cds_wfcq_first_blocking, but returns CDS_WFCQ_WOULDBLOCK if
 * it needs to block.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_first_nonblocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        return ___cds_wfcq_first(head, tail, 0);
}

static inline struct cds_wfcq_node *
___cds_wfcq_next(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                struct cds_wfcq_node *node,
                int blocking)
{
        struct cds_wfcq_node *next;

        /*
         * Even though the following tail->p check is sufficient to find
         * out if we reached the end of the queue, we first check
         * node->next as a common case to ensure that iteration on nodes
         * do not frequently access enqueuer's tail->p cache line.
         */
        if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
                /* Load node->next before tail->p */
                cmm_smp_rmb();
                if (CMM_LOAD_SHARED(tail->p) == node)
                        return NULL;
                next = ___cds_wfcq_node_sync_next(node, blocking);
        }
        /* Load node->next before loading next's content */
        cmm_smp_read_barrier_depends();
        return next;
}

/*
 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
 *
 * Content written into the node before enqueue is guaranteed to be
 * consistent, but no other memory ordering is ensured.
 * Dequeue/splice/iteration mutual exclusion should be ensured by the
 * caller.
 *
 * Used by for-like iteration macros in urcu/wfqueue.h:
 * __cds_wfcq_for_each_blocking()
 * __cds_wfcq_for_each_blocking_safe()
 *
 * Returns NULL if reached end of queue, non-NULL next queue node
 * otherwise.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_next_blocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                struct cds_wfcq_node *node)
{
        return ___cds_wfcq_next(head, tail, node, 1);
}

/*
 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
 *
 * Same as __cds_wfcq_next_blocking, but returns CDS_WFCQ_WOULDBLOCK if
 * it needs to block.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_next_nonblocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                struct cds_wfcq_node *node)
{
        return ___cds_wfcq_next(head, tail, node, 0);
}

static inline struct cds_wfcq_node *
___cds_wfcq_dequeue(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail,
                int blocking)
{
        struct cds_wfcq_node *node, *next;

        if (_cds_wfcq_empty(head, tail))
                return NULL;

        node = ___cds_wfcq_node_sync_next(&head->node, blocking);
        if (!blocking && node == CDS_WFCQ_WOULDBLOCK)
                return CDS_WFCQ_WOULDBLOCK;

        if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
                /*
                 * @node is probably the only node in the queue.
                 * Try to move the tail to &q->head.
                 * q->head.next is set to NULL here, and stays
                 * NULL if the cmpxchg succeeds. Should the
                 * cmpxchg fail due to a concurrent enqueue, the
                 * q->head.next will be set to the next node.
                 * The implicit memory barrier before
                 * uatomic_cmpxchg() orders load node->next
                 * before loading q->tail.
                 * The implicit memory barrier before uatomic_cmpxchg
                 * orders load q->head.next before loading node's
                 * content.
                 */
                _cds_wfcq_node_init(&head->node);
                if (uatomic_cmpxchg(&tail->p, node, &head->node) == node)
                        return node;
                next = ___cds_wfcq_node_sync_next(node, blocking);
                /*
                 * In nonblocking mode, if we would need to block to
                 * get node's next, set the head next node pointer
                 * (currently NULL) back to its original value.
                 */
                if (!blocking && next == CDS_WFCQ_WOULDBLOCK) {
                        head->node.next = node;
                        return CDS_WFCQ_WOULDBLOCK;
                }
        }

        /*
         * Move queue head forward.
         */
        head->node.next = next;

        /* Load q->head.next before loading node's content */
        cmm_smp_read_barrier_depends();
        return node;
}

/*
 * __cds_wfcq_dequeue_blocking: dequeue a node from the queue.
 *
 * Content written into the node before enqueue is guaranteed to be
 * consistent, but no other memory ordering is ensured.
 * It is valid to reuse and free a dequeued node immediately.
 * Dequeue/splice/iteration mutual exclusion should be ensured by the
 * caller.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        return ___cds_wfcq_dequeue(head, tail, 1);
}

/*
 * __cds_wfcq_dequeue_nonblocking: dequeue a node from a wait-free queue.
 *
 * Same as __cds_wfcq_dequeue_blocking, but returns CDS_WFCQ_WOULDBLOCK
 * if it needs to block.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_dequeue_nonblocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        return ___cds_wfcq_dequeue(head, tail, 0);
}

/*
 * __cds_wfcq_splice: enqueue all src_q nodes at the end of dest_q.
 *
 * Dequeue all nodes from src_q.
 * dest_q must be already initialized.
 * Mutual exclusion for src_q should be ensured by the caller as
 * specified in the "Synchronisation table".
 * Returns enum cds_wfcq_ret which indicates the state of the src or
 * dest queue.
 */
static inline enum cds_wfcq_ret
___cds_wfcq_splice(
                struct cds_wfcq_head *dest_q_head,
                struct cds_wfcq_tail *dest_q_tail,
                struct cds_wfcq_head *src_q_head,
                struct cds_wfcq_tail *src_q_tail,
                int blocking)
{
        struct cds_wfcq_node *head, *tail;
        int attempt = 0;

        /*
         * Initial emptiness check to speed up cases where queue is
         * empty: only require loads to check if queue is empty.
         */
        if (_cds_wfcq_empty(src_q_head, src_q_tail))
                return CDS_WFCQ_RET_SRC_EMPTY;

        for (;;) {
                /*
                 * Open-coded _cds_wfcq_empty() by testing result of
                 * uatomic_xchg, as well as tail pointer vs head node
                 * address.
                 */
                head = uatomic_xchg(&src_q_head->node.next, NULL);
                if (head)
                        break;  /* non-empty */
                if (CMM_LOAD_SHARED(src_q_tail->p) == &src_q_head->node)
                        return CDS_WFCQ_RET_SRC_EMPTY;
                if (___cds_wfcq_busy_wait(&attempt, blocking))
                        return CDS_WFCQ_RET_WOULDBLOCK;
        }

        /*
         * Memory barrier implied before uatomic_xchg() orders store to
         * src_q->head before store to src_q->tail. This is required by
         * concurrent enqueue on src_q, which exchanges the tail before
         * updating the previous tail's next pointer.
         */
        tail = uatomic_xchg(&src_q_tail->p, &src_q_head->node);

        /*
         * Append the spliced content of src_q into dest_q. Does not
         * require mutual exclusion on dest_q (wait-free).
         */
        if (___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail))
                return CDS_WFCQ_RET_DEST_NON_EMPTY;
        else
                return CDS_WFCQ_RET_DEST_EMPTY;
}

/*
 * __cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
 *
 * Dequeue all nodes from src_q.
 * dest_q must be already initialized.
 * Mutual exclusion for src_q should be ensured by the caller as
 * specified in the "Synchronisation table".
 * Returns enum cds_wfcq_ret which indicates the state of the src or
 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
 */
static inline enum cds_wfcq_ret
___cds_wfcq_splice_blocking(
                struct cds_wfcq_head *dest_q_head,
                struct cds_wfcq_tail *dest_q_tail,
                struct cds_wfcq_head *src_q_head,
                struct cds_wfcq_tail *src_q_tail)
{
        return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
                src_q_head, src_q_tail, 1);
}

/*
 * __cds_wfcq_splice_nonblocking: enqueue all src_q nodes at the end of dest_q.
 *
 * Same as __cds_wfcq_splice_blocking, but returns
 * CDS_WFCQ_RET_WOULDBLOCK if it needs to block.
 */
static inline enum cds_wfcq_ret
___cds_wfcq_splice_nonblocking(
                struct cds_wfcq_head *dest_q_head,
                struct cds_wfcq_tail *dest_q_tail,
                struct cds_wfcq_head *src_q_head,
                struct cds_wfcq_tail *src_q_tail)
{
        return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
                src_q_head, src_q_tail, 0);
}

/*
 * cds_wfcq_dequeue_blocking: dequeue a node from a wait-free queue.
 *
 * Content written into the node before enqueue is guaranteed to be
 * consistent, but no other memory ordering is ensured.
 * Mutual exlusion with cds_wfcq_splice_blocking and dequeue lock is
 * ensured.
 * It is valid to reuse and free a dequeued node immediately.
 */
static inline struct cds_wfcq_node *
_cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
                struct cds_wfcq_tail *tail)
{
        struct cds_wfcq_node *retval;

        _cds_wfcq_dequeue_lock(head, tail);
        retval = ___cds_wfcq_dequeue_blocking(head, tail);
        _cds_wfcq_dequeue_unlock(head, tail);
        return retval;
}

/*
 * cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
 *
 * Dequeue all nodes from src_q.
 * dest_q must be already initialized.
 * Content written into the node before enqueue is guaranteed to be
 * consistent, but no other memory ordering is ensured.
 * Mutual exlusion with cds_wfcq_dequeue_blocking and dequeue lock is
 * ensured.
 * Returns enum cds_wfcq_ret which indicates the state of the src or
 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
 */
static inline enum cds_wfcq_ret
_cds_wfcq_splice_blocking(
                struct cds_wfcq_head *dest_q_head,
                struct cds_wfcq_tail *dest_q_tail,
                struct cds_wfcq_head *src_q_head,
                struct cds_wfcq_tail *src_q_tail)
{
        enum cds_wfcq_ret ret;

        _cds_wfcq_dequeue_lock(src_q_head, src_q_tail);
        ret = ___cds_wfcq_splice_blocking(dest_q_head, dest_q_tail,
                        src_q_head, src_q_tail);
        _cds_wfcq_dequeue_unlock(src_q_head, src_q_tail);
        return ret;
}

#ifdef __cplusplus
}
#endif

#endif /* _URCU_WFCQUEUE_STATIC_H */