rculfqueue: LGPL-ize
[urcu.git] / urcu / wfqueue-static.h
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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>
31#include <urcu/compiler.h>
32#include <urcu/uatomic_arch.h>
33
34#ifdef __cplusplus
35extern "C" {
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
47#define WFQ_ADAPT_ATTEMPTS 10 /* Retry if being set */
48#define WFQ_WAIT 10 /* Wait 10 ms if being set */
49
50void _wfq_node_init(struct wfq_node *node)
51{
52 node->next = NULL;
53}
54
55void _wfq_init(struct wfq_queue *q)
56{
57 int ret;
58
59 _wfq_node_init(&q->dummy);
60 /* Set queue head and tail */
61 q->head = &q->dummy;
62 q->tail = &q->dummy.next;
63 ret = pthread_mutex_init(&q->lock, NULL);
64 assert(!ret);
65}
66
67void _wfq_enqueue(struct wfq_queue *q, struct wfq_node *node)
68{
69 struct wfq_node **old_tail;
70
71 /*
72 * uatomic_xchg() implicit memory barrier orders earlier stores to data
73 * structure containing node and setting node->next to NULL before
74 * publication.
75 */
76 old_tail = uatomic_xchg(&q->tail, node);
77 /*
78 * At this point, dequeuers see a NULL old_tail->next, which indicates
79 * that the queue is being appended to. The following store will append
80 * "node" to the queue from a dequeuer perspective.
81 */
82 STORE_SHARED(*old_tail, node);
83}
84
85/*
86 * It is valid to reuse and free a dequeued node immediately.
87 *
88 * No need to go on a waitqueue here, as there is no possible state in which the
89 * list could cause dequeue to busy-loop needlessly while waiting for another
90 * thread to be scheduled. The queue appears empty until tail->next is set by
91 * enqueue.
92 */
93struct wfq_node *
b0dd35e2 94___wfq_dequeue_blocking(struct wfq_queue *q)
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95{
96 struct wfq_node *node, *next;
97 int attempt = 0;
98
99 /*
100 * Queue is empty if it only contains the dummy node.
101 */
102 if (q->head == &q->dummy && LOAD_SHARED(q->tail) == &q->dummy.next)
103 return NULL;
104 node = q->head;
105
106 /*
107 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
108 */
109 while ((next = LOAD_SHARED(node->next)) == NULL) {
110 if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
111 poll(NULL, 0, WFQ_WAIT); /* Wait for 10ms */
112 attempt = 0;
113 } else
114 cpu_relax();
115 }
116 /*
117 * Move queue head forward.
118 */
119 q->head = next;
120 /*
121 * Requeue dummy node if we just dequeued it.
122 */
123 if (node == &q->dummy) {
124 _wfq_node_init(node);
125 _wfq_enqueue(q, node);
b0dd35e2 126 return ___wfq_dequeue_blocking(q);
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127 }
128 return node;
129}
130
131struct wfq_node *
132_wfq_dequeue_blocking(struct wfq_queue *q)
133{
134 struct wfq_node *retnode;
135 int ret;
136
137 ret = pthread_mutex_lock(&q->lock);
138 assert(!ret);
b0dd35e2 139 retnode = ___wfq_dequeue_blocking(q);
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140 ret = pthread_mutex_unlock(&q->lock);
141 assert(!ret);
142 return retnode;
143}
144
145#ifdef __cplusplus
146}
147#endif
148
149#endif /* _URCU_WFQUEUE_STATIC_H */
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