1 #ifndef _URCU_WFQUEUE_STATIC_H
2 #define _URCU_WFQUEUE_STATIC_H
7 * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
9 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See wfqueue.h for linking
10 * dynamically with the userspace rcu library.
12 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
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.
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
31 #include <urcu/compiler.h>
32 #include <urcu/uatomic_arch.h>
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.
43 * Inspired from half-wait-free/half-blocking queue implementation done by
47 #define WFQ_ADAPT_ATTEMPTS 10 /* Retry if being set */
48 #define WFQ_WAIT 10 /* Wait 10 ms if being set */
50 void _wfq_node_init(struct wfq_node
*node
)
55 void _wfq_init(struct wfq_queue
*q
)
59 _wfq_node_init(&q
->dummy
);
60 /* Set queue head and tail */
62 q
->tail
= &q
->dummy
.next
;
63 ret
= pthread_mutex_init(&q
->lock
, NULL
);
67 void _wfq_enqueue(struct wfq_queue
*q
, struct wfq_node
*node
)
69 struct wfq_node
**old_tail
;
72 * uatomic_xchg() implicit memory barrier orders earlier stores to data
73 * structure containing node and setting node->next to NULL before
76 old_tail
= uatomic_xchg(&q
->tail
, node
);
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.
82 STORE_SHARED(*old_tail
, node
);
86 * It is valid to reuse and free a dequeued node immediately.
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
94 ___wfq_dequeue_blocking(struct wfq_queue
*q
)
96 struct wfq_node
*node
, *next
;
100 * Queue is empty if it only contains the dummy node.
102 if (q
->head
== &q
->dummy
&& LOAD_SHARED(q
->tail
) == &q
->dummy
.next
)
107 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
109 while ((next
= LOAD_SHARED(node
->next
)) == NULL
) {
110 if (++attempt
>= WFQ_ADAPT_ATTEMPTS
) {
111 poll(NULL
, 0, WFQ_WAIT
); /* Wait for 10ms */
117 * Move queue head forward.
121 * Requeue dummy node if we just dequeued it.
123 if (node
== &q
->dummy
) {
124 _wfq_node_init(node
);
125 _wfq_enqueue(q
, node
);
126 return ___wfq_dequeue_blocking(q
);
132 _wfq_dequeue_blocking(struct wfq_queue
*q
)
134 struct wfq_node
*retnode
;
137 ret
= pthread_mutex_lock(&q
->lock
);
139 retnode
= ___wfq_dequeue_blocking(q
);
140 ret
= pthread_mutex_unlock(&q
->lock
);
149 #endif /* _URCU_WFQUEUE_STATIC_H */
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