wfcqueue documentation: hint at for_each iterators

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

#ifndef _URCU_WFCQUEUE_STATIC_H
#define _URCU_WFCQUEUE_STATIC_H

/*
 * wfcqueue-static.h
 *
 * Userspace RCU library - Concurrent Queue with Wait-Free Enqueue/Blocking Dequeue
 *
 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See 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.
 *
 * Inspired from half-wait-free/half-blocking queue implementation done by
 * Paul E. McKenney.
 *
 * Mutual exclusion of __cds_wfcq_* API
 *
 * Unless otherwise stated, the caller must ensure mutual exclusion of
 * queue update operations "dequeue" and "splice" (for source queue).
 * Queue read operations "first" and "next", which are used by
 * "for_each" iterations, need to be protected against concurrent
 * "dequeue" and "splice" (for source queue) by the caller.
 * "enqueue", "splice" (for destination queue), and "empty" are the only
 * operations that can be used without any mutual exclusion.
 * 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.
 */

#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.
 */
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 void ___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);
}

/*
 * cds_wfcq_enqueue: enqueue a node into a wait-free queue.
 *
 * Issues a full memory barrier before enqueue. No mutual exclusion is
 * required.
 */
static inline void _cds_wfcq_enqueue(struct cds_wfcq_head *head,
		struct cds_wfcq_tail *tail,
		struct cds_wfcq_node *new_tail)
{
	___cds_wfcq_append(head, tail, new_tail, new_tail);
}

/*
 * 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)
{
	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 (++attempt >= WFCQ_ADAPT_ATTEMPTS) {
			poll(NULL, 0, WFCQ_WAIT);	/* Wait for 10ms */
			attempt = 0;
		} else {
			caa_cpu_relax();
		}
	}

	return next;
}

/*
 * __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.
 * Should be called with cds_wfcq_dequeue_lock() held.
 *
 * Used by for-like iteration macros in urcu/wfqueue.h:
 * __cds_wfcq_for_each_blocking()
 * __cds_wfcq_for_each_blocking_safe()
 */
static inline struct cds_wfcq_node *
___cds_wfcq_first_blocking(struct cds_wfcq_head *head,
		struct cds_wfcq_tail *tail)
{
	struct cds_wfcq_node *node;

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

/*
 * __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.
 * Should be called with cds_wfcq_dequeue_lock() held.
 *
 * Used by for-like iteration macros in urcu/wfqueue.h:
 * __cds_wfcq_for_each_blocking()
 * __cds_wfcq_for_each_blocking_safe()
 */
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)
{
	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);
	}
	/* Load node->next before loading next's content */
	cmm_smp_read_barrier_depends();
	return next;
}

/*
 * __cds_wfcq_dequeue_blocking: dequeue a node from the queue.
 *
 * No need to go on a waitqueue here, as there is no possible state in which the
 * list could cause dequeue to busy-loop needlessly while waiting for another
 * thread to be scheduled. The queue appears empty until tail->next is set by
 * enqueue.
 *
 * 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.
 * Should be called with cds_wfcq_dequeue_lock() held.
 */
static inline struct cds_wfcq_node *
___cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
		struct cds_wfcq_tail *tail)
{
	struct cds_wfcq_node *node, *next;

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

	node = ___cds_wfcq_node_sync_next(&head->node);

	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);
	}

	/*
	 * 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_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.
 * Should be called with cds_wfcq_dequeue_lock() held on src_q.
 */
static inline void
___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)
{
	struct cds_wfcq_node *head, *tail;

	if (_cds_wfcq_empty(src_q_head, src_q_tail))
		return;

	head = ___cds_wfcq_node_sync_next(&src_q_head->node);
	_cds_wfcq_node_init(&src_q_head->node);

	/*
	 * 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).
	 */
	___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail);
}

/*
 * 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 (and only with) cds_wfcq_splice_blocking 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 (and only with) cds_wfcq_dequeue_blocking is
 * ensured.
 */
static inline void
_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)
{
	_cds_wfcq_dequeue_lock(src_q_head, src_q_tail);
	___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);
}

#ifdef __cplusplus
}
#endif

#endif /* _URCU_WFCQUEUE_STATIC_H */