Wait-free queue: wait-free enqueue, blocking dequeue without RCU dependency
[urcu.git] / urcu / wfqueue.h
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1#ifndef _URCU_WFQUEUE_H
2#define _URCU_WFQUEUE_H
3
4/*
5 * wfqueue.h
6 *
7 * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
8 *
9 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
10 *
11 * This library is free software; you can redistribute it and/or
12 * modify it under the terms of the GNU Lesser General Public
13 * License as published by the Free Software Foundation; either
14 * version 2.1 of the License, or (at your option) any later version.
15 *
16 * This library is distributed in the hope that it will be useful,
17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
19 * Lesser General Public License for more details.
20 *
21 * You should have received a copy of the GNU Lesser General Public
22 * License along with this library; if not, write to the Free Software
23 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
24 */
25
26#include <pthread.h>
27#include <assert.h>
28#include <urcu/compiler.h>
29
30#ifdef __cplusplus
31extern "C" {
32#endif
33
34#if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
35#error "Dynamic loader LGPL wrappers not implemented yet"
36#endif
37
38/*
39 * Queue with wait-free enqueue/blocking dequeue.
40 * This implementation adds a dummy head node when the queue is empty to ensure
41 * we can always update the queue locklessly.
42 *
43 * Inspired from half-wait-free/half-blocking queue implementation done by
44 * Paul E. McKenney.
45 */
46
47struct wfq_node {
48 struct wfq_node *next;
49};
50
51struct wfq_queue {
52 struct wfq_node *head, **tail;
53 struct wfq_node dummy; /* Dummy node */
54 pthread_mutex_t lock;
55};
56
57void wfq_node_init(struct wfq_node *node)
58{
59 node->next = NULL;
60}
61
62void wfq_init(struct wfq_queue *q)
63{
64 int ret;
65
66 wfq_node_init(&q->dummy);
67 /* Set queue head and tail */
68 q->head = &q->dummy;
69 q->tail = &q->dummy.next;
70 ret = pthread_mutex_init(&q->lock, NULL);
71 assert(!ret);
72}
73
74void wfq_enqueue(struct wfq_queue *q, struct wfq_node *node)
75{
76 struct wfq_node **old_tail;
77
78 /*
79 * uatomic_xchg() implicit memory barrier orders earlier stores to data
80 * structure containing node and setting node->next to NULL before
81 * publication.
82 */
83 old_tail = uatomic_xchg(&q->tail, node);
84 /*
85 * At this point, dequeuers see a NULL old_tail->next, which indicates
86 * that the queue is being appended to. The following store will append
87 * "node" to the queue from a dequeuer perspective.
88 */
89 STORE_SHARED(*old_tail, node);
90}
91
92/*
93 * It is valid to reuse and free a dequeued node immediately.
94 *
95 * No need to go on a waitqueue here, as there is no possible state in which the
96 * list could cause dequeue to busy-loop needlessly while waiting for another
97 * thread to be scheduled. The queue appears empty until tail->next is set by
98 * enqueue.
99 */
100struct wfq_node *
101__wfq_dequeue_blocking(struct wfq_queue *q)
102{
103 struct wfq_node *node, *next;
104 int busy_wait = 16;
105
106 /*
107 * Queue is empty if it only contains the dummy node.
108 */
109 if (q->head == &q->dummy && LOAD_SHARED(q->tail) == &q->dummy.next)
110 return NULL;
111 node = q->head;
112
113 /*
114 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
115 */
116 while ((next = LOAD_SHARED(node->next)) == NULL) {
117 if (busy_wait > 0) {
118 cpu_relax();
119 busy_wait--;
120 } else
121 poll(NULL, 0, 1); /* Wait for 1ms */
122 }
123 /*
124 * Move queue head forward.
125 */
126 q->head = next;
127 /*
128 * Requeue dummy node if we just dequeued it.
129 */
130 if (node == &q->dummy) {
131 wfq_node_init(node);
132 wfq_enqueue(q, node);
133 return __wfq_dequeue_blocking(q);
134 }
135 return node;
136}
137
138struct wfq_node *
139wfq_dequeue_blocking(struct wfq_queue *q)
140{
141 struct wfq_node *retnode;
142 int ret;
143
144 ret = pthread_mutex_lock(&q->lock);
145 assert(!ret);
146 retnode = __wfq_dequeue_blocking(q);
147 ret = pthread_mutex_unlock(&q->lock);
148 assert(!ret);
149 return retnode;
150}
151
152#ifdef __cplusplus
153}
154#endif
155
156#endif /* _URCU_WFQUEUE_H */
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