ifdef C, wait-free stack adaptative pop wait
[urcu.git] / urcu / rcuwfqueue.h
CommitLineData
0e2e9380
MD
1#ifndef _URCU_RCUWFQUEUE_H
2#define _URCU_RCUWFQUEUE_H
3
ee566d46
MD
4/*
5 * rcuwfqueue.h
6 *
7 * Userspace RCU library - RCU 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 <urcu/urcu_ref.h>
27#include <assert.h>
28
0e2e9380
MD
29#ifdef __cplusplus
30extern "C" {
31#endif
32
ee566d46
MD
33#if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
34#error "Dynamic loader LGPL wrappers not implemented yet"
35#endif
36
37/*
38 * RCU queue with wait-free enqueue/blocking dequeue using reference counting.
39 * Enqueue and dequeue operations hold a RCU read lock to deal with cmpxchg ABA
40 * problem. This implementation keeps a dummy head node to ensure we can always
41 * update the queue locklessly. Given that this is a queue, the dummy head node
42 * must always advance as we dequeue entries. Therefore, we keep a reference
43 * count on each entry we are dequeueing, so they can be kept as dummy head node
44 * until the next dequeue, at which point their reference count will be
45 * decremented.
46 */
47
48#define URCU_WFQ_PERMANENT_REF 128
49
50struct rcu_wfq_node {
51 struct rcu_wfq_node *next;
52 struct urcu_ref ref;
53};
54
55struct rcu_wfq_queue {
56 struct rcu_wfq_node *head, *tail;
57 struct rcu_wfq_node init; /* Dummy initialization node */
58};
59
60void rcu_wfq_node_init(struct rcu_wfq_node *node)
61{
62 node->next = NULL;
63 urcu_ref_init(&node->ref);
64}
65
66void rcu_wfq_init(struct rcu_wfq_queue *q)
67{
68 rcu_wfq_node_init(&q->init);
69 /* Make sure the initial node is never freed. */
70 urcu_ref_set(&q->init.ref, URCU_WFQ_PERMANENT_REF);
71 /* Set queue end */
72 q->head = q->tail = &q->init;
73}
74
75void rcu_wfq_enqueue(struct rcu_wfq_queue *q, struct rcu_wfq_node *node)
76{
77 struct rcu_wfq_node *old_tail;
78
79 urcu_ref_get(&node->ref);
80 /*
81 * uatomic_xchg() implicit memory barrier orders earlier stores to node
82 * (setting it to NULL and incrementing the refcount) before
83 * publication.
84 */
85 old_tail = uatomic_xchg(&q->tail, node);
86 /*
87 * At this point, dequeuers see a NULL old_tail->next, which indicates
88 * end of queue. The following store will append "node" to the queue
89 * from a dequeuer perspective.
90 */
91 STORE_SHARED(old_tail->next, node);
92}
93
94/*
95 * The entry returned by dequeue must be taken care of by doing a urcu_ref_put,
96 * which calls the release primitive when the reference count drops to zero. A
97 * grace period must be waited before performing the actual memory reclamation
98 * in the release primitive. The wfq node returned by dequeue must not be
99 * modified/re-used/freed until the reference count reaches zero and a grace
100 * period has elapsed (after the refcount reached 0).
101 *
0e2e9380
MD
102 * No need to go on a waitqueue here, as there is no possible state in which the
103 * list could cause dequeue to busy-loop needlessly while waiting for another
104 * thread to be scheduled. The queue appears empty until tail->next is set by
105 * enqueue.
ee566d46
MD
106 */
107struct rcu_wfq_node *
108rcu_wfq_dequeue_blocking(struct rcu_wfq_queue *q,
109 void (*release)(struct urcu_ref *))
110{
111 for (;;) {
112 struct rcu_wfq_node *head, *next;
113
114 rcu_read_lock();
115 head = rcu_dereference(q->head);
116 next = rcu_dereference(head->next);
117 if (next) {
118 if (uatomic_cmpxchg(&q->head, head, next) == head) {
119 rcu_read_unlock();
120 urcu_ref_put(&head->ref, release);
121 return next;
122 } else {
123 /* Concurrently pushed, retry */
124 rcu_read_unlock();
125 continue;
126 }
127 } else {
128 /* Empty */
129 rcu_read_unlock();
130 return NULL;
131 }
132 }
133}
0e2e9380
MD
134
135#ifdef __cplusplus
136}
137#endif
138
139#endif /* _URCU_RCUWFQUEUE_H */
This page took 0.026795 seconds and 4 git commands to generate.