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

#ifndef _URCU_WFQUEUE_STATIC_H
#define _URCU_WFQUEUE_STATIC_H

/*
 * wfqueue-static.h
 *
 * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
 *
 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See wfqueue.h for linking
 * dynamically with the userspace rcu library.
 *
 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.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 <urcu/compiler.h>
#include <urcu/uatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

/*
 * Queue with wait-free enqueue/blocking dequeue.
 * This implementation adds a dummy head node when the queue is empty to ensure
 * we can always update the queue locklessly.
 *
 * Inspired from half-wait-free/half-blocking queue implementation done by
 * Paul E. McKenney.
 */

#define WFQ_ADAPT_ATTEMPTS		10	/* Retry if being set */
#define WFQ_WAIT			10	/* Wait 10 ms if being set */

static inline void _cds_wfq_node_init(struct cds_wfq_node *node)
{
	node->next = NULL;
}

static inline void _cds_wfq_init(struct cds_wfq_queue *q)
{
	int ret;

	_cds_wfq_node_init(&q->dummy);
	/* Set queue head and tail */
	q->head = &q->dummy;
	q->tail = &q->dummy.next;
	ret = pthread_mutex_init(&q->lock, NULL);
	assert(!ret);
}

static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
				    struct cds_wfq_node *node)
{
	struct cds_wfq_node **old_tail;

	/*
	 * uatomic_xchg() implicit memory barrier orders earlier stores to data
	 * structure containing node and setting node->next to NULL before
	 * publication.
	 */
	old_tail = uatomic_xchg(&q->tail, node);
	/*
	 * At this point, dequeuers see a NULL old_tail->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, node);
}

/*
 * It is valid to reuse and free a dequeued node immediately.
 *
 * 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.
 */
static inline struct cds_wfq_node *
___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
{
	struct cds_wfq_node *node, *next;
	int attempt = 0;

	/*
	 * Queue is empty if it only contains the dummy node.
	 */
	if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
		return NULL;
	node = q->head;

	/*
	 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
	 */
	while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
		if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
			poll(NULL, 0, WFQ_WAIT);	/* Wait for 10ms */
			attempt = 0;
		} else
			caa_cpu_relax();
	}
	/*
	 * Move queue head forward.
	 */
	q->head = next;
	/*
	 * Requeue dummy node if we just dequeued it.
	 */
	if (node == &q->dummy) {
		_cds_wfq_node_init(node);
		_cds_wfq_enqueue(q, node);
		return ___cds_wfq_dequeue_blocking(q);
	}
	return node;
}

static inline struct cds_wfq_node *
_cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
{
	struct cds_wfq_node *retnode;
	int ret;

	ret = pthread_mutex_lock(&q->lock);
	assert(!ret);
	retnode = ___cds_wfq_dequeue_blocking(q);
	ret = pthread_mutex_unlock(&q->lock);
	assert(!ret);
	return retnode;
}

#ifdef __cplusplus
}
#endif

#endif /* _URCU_WFQUEUE_STATIC_H */
Commit	Line	Data
4d001e96 MD	1	#ifndef _URCU_WFQUEUE_STATIC_H
	2	#define _URCU_WFQUEUE_STATIC_H
	3
	4	/*
	5	* wfqueue-static.h
	6	*
	7	* Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
	8	*
	9	* TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See wfqueue.h for linking
	10	* dynamically with the userspace rcu library.
	11	*
	12	* Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	13	*
	14	* This library is free software; you can redistribute it and/or
	15	* modify it under the terms of the GNU Lesser General Public
	16	* License as published by the Free Software Foundation; either
	17	* version 2.1 of the License, or (at your option) any later version.
	18	*
	19	* This library is distributed in the hope that it will be useful,
	20	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	21	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
	22	* Lesser General Public License for more details.
	23	*
	24	* You should have received a copy of the GNU Lesser General Public
	25	* License along with this library; if not, write to the Free Software
	26	* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
	27	*/
	28
	29	#include <pthread.h>
	30	#include <assert.h>
b57aee66	31	#include <poll.h>
4d001e96	32	#include <urcu/compiler.h>
a2e7bf9c	33	#include <urcu/uatomic.h>
4d001e96 MD	34
	35	#ifdef __cplusplus
	36	extern "C" {
	37	#endif
	38
	39	/*
	40	* Queue with wait-free enqueue/blocking dequeue.
	41	* This implementation adds a dummy head node when the queue is empty to ensure
	42	* we can always update the queue locklessly.
	43	*
	44	* Inspired from half-wait-free/half-blocking queue implementation done by
	45	* Paul E. McKenney.
	46	*/
	47
	48	#define WFQ_ADAPT_ATTEMPTS 10 /* Retry if being set */
	49	#define WFQ_WAIT 10 /* Wait 10 ms if being set */
	50
b57aee66	51	static inline void _cds_wfq_node_init(struct cds_wfq_node *node)
4d001e96 MD	52	{
	53	node->next = NULL;
	54	}
	55
b57aee66	56	static inline void _cds_wfq_init(struct cds_wfq_queue *q)
4d001e96 MD	57	{
	58	int ret;
	59
16aa9ee8	60	_cds_wfq_node_init(&q->dummy);
4d001e96 MD	61	/* Set queue head and tail */
	62	q->head = &q->dummy;
	63	q->tail = &q->dummy.next;
	64	ret = pthread_mutex_init(&q->lock, NULL);
	65	assert(!ret);
	66	}
	67
b57aee66 PM	68	static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
b57aee66 PM	69	struct cds_wfq_node *node)
4d001e96	70	{
16aa9ee8	71	struct cds_wfq_node **old_tail;
4d001e96 MD	72
	73	/*
	74	* uatomic_xchg() implicit memory barrier orders earlier stores to data
	75	* structure containing node and setting node->next to NULL before
	76	* publication.
	77	*/
	78	old_tail = uatomic_xchg(&q->tail, node);
	79	/*
	80	* At this point, dequeuers see a NULL old_tail->next, which indicates
	81	* that the queue is being appended to. The following store will append
	82	* "node" to the queue from a dequeuer perspective.
	83	*/
6cf3827c	84	CMM_STORE_SHARED(*old_tail, node);
4d001e96 MD	85	}
	86
	87	/*
	88	* It is valid to reuse and free a dequeued node immediately.
	89	*
	90	* No need to go on a waitqueue here, as there is no possible state in which the
	91	* list could cause dequeue to busy-loop needlessly while waiting for another
	92	* thread to be scheduled. The queue appears empty until tail->next is set by
	93	* enqueue.
	94	*/
b57aee66	95	static inline struct cds_wfq_node *
16aa9ee8	96	___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
4d001e96	97	{
16aa9ee8	98	struct cds_wfq_node node, next;
4d001e96 MD	99	int attempt = 0;
	100
	101	/*
	102	* Queue is empty if it only contains the dummy node.
	103	*/
6cf3827c	104	if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
4d001e96 MD	105	return NULL;
	106	node = q->head;
	107
	108	/*
	109	* Adaptative busy-looping waiting for enqueuer to complete enqueue.
	110	*/
6cf3827c	111	while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
4d001e96 MD	112	if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
	113	poll(NULL, 0, WFQ_WAIT); /* Wait for 10ms */
	114	attempt = 0;
	115	} else
06f22bdb	116	caa_cpu_relax();
4d001e96 MD	117	}
	118	/*
	119	* Move queue head forward.
	120	*/
	121	q->head = next;
	122	/*
	123	* Requeue dummy node if we just dequeued it.
	124	*/
	125	if (node == &q->dummy) {
16aa9ee8 DG	126	_cds_wfq_node_init(node);
	127	_cds_wfq_enqueue(q, node);
	128	return ___cds_wfq_dequeue_blocking(q);
4d001e96 MD	129	}
	130	return node;
	131	}
	132
b57aee66	133	static inline struct cds_wfq_node *
16aa9ee8	134	_cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
4d001e96	135	{
16aa9ee8	136	struct cds_wfq_node *retnode;
4d001e96 MD	137	int ret;
	138
	139	ret = pthread_mutex_lock(&q->lock);
	140	assert(!ret);
16aa9ee8	141	retnode = ___cds_wfq_dequeue_blocking(q);
4d001e96 MD	142	ret = pthread_mutex_unlock(&q->lock);
	143	assert(!ret);
	144	return retnode;
	145	}
	146
	147	#ifdef __cplusplus
	148	}
	149	#endif
	150
	151	#endif /* _URCU_WFQUEUE_STATIC_H */