cc21fac53fa64f318a94420f92fe9883072e3ca1
[urcu.git] / urcu / static / wfqueue.h
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 <poll.h>
32 #include <urcu/compiler.h>
33 #include <urcu/uatomic.h>
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
51 static inline void _cds_wfq_node_init(struct cds_wfq_node *node)
52 {
53 node->next = NULL;
54 }
55
56 static inline void _cds_wfq_init(struct cds_wfq_queue *q)
57 {
58 int ret;
59
60 _cds_wfq_node_init(&q->dummy);
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
68 static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
69 struct cds_wfq_node *node)
70 {
71 struct cds_wfq_node **old_tail;
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->next);
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 */
84 CMM_STORE_SHARED(*old_tail, node);
85 }
86
87 /*
88 * Waiting for enqueuer to complete enqueue and return the next node
89 */
90 static inline struct cds_wfq_node *
91 ___cds_wfq_node_sync_next(struct cds_wfq_node *node)
92 {
93 struct cds_wfq_node *next;
94 int attempt = 0;
95
96 /*
97 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
98 */
99 while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
100 if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
101 (void) poll(NULL, 0, WFQ_WAIT); /* Wait for 10ms */
102 attempt = 0;
103 } else
104 caa_cpu_relax();
105 }
106
107 return next;
108 }
109
110 /*
111 * It is valid to reuse and free a dequeued node immediately.
112 *
113 * No need to go on a waitqueue here, as there is no possible state in which the
114 * list could cause dequeue to busy-loop needlessly while waiting for another
115 * thread to be scheduled. The queue appears empty until tail->next is set by
116 * enqueue.
117 */
118 static inline struct cds_wfq_node *
119 ___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
120 {
121 struct cds_wfq_node *node, *next;
122
123 /*
124 * Queue is empty if it only contains the dummy node.
125 */
126 if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
127 return NULL;
128 node = q->head;
129
130 next = ___cds_wfq_node_sync_next(node);
131
132 /*
133 * Move queue head forward.
134 */
135 q->head = next;
136 /*
137 * Requeue dummy node if we just dequeued it.
138 */
139 if (node == &q->dummy) {
140 _cds_wfq_node_init(node);
141 _cds_wfq_enqueue(q, node);
142 return ___cds_wfq_dequeue_blocking(q);
143 }
144 return node;
145 }
146
147 static inline struct cds_wfq_node *
148 _cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
149 {
150 struct cds_wfq_node *retnode;
151 int ret;
152
153 ret = pthread_mutex_lock(&q->lock);
154 assert(!ret);
155 retnode = ___cds_wfq_dequeue_blocking(q);
156 ret = pthread_mutex_unlock(&q->lock);
157 assert(!ret);
158 return retnode;
159 }
160
161 #ifdef __cplusplus
162 }
163 #endif
164
165 #endif /* _URCU_WFQUEUE_STATIC_H */
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