Add wait-free push/blocking pop stack
[urcu.git] / urcu / rcuwfstack.h
1 /*
2 * rcuwfstack.h
3 *
4 * Userspace RCU library - RCU Stack with Wait-Free push, Blocking pop.
5 *
6 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
7 *
8 * This library is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU Lesser General Public
10 * License as published by the Free Software Foundation; either
11 * version 2.1 of the License, or (at your option) any later version.
12 *
13 * This library is distributed in the hope that it will be useful,
14 * but WITHOUT ANY WARRANTY; without even the implied warranty of
15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 * Lesser General Public License for more details.
17 *
18 * You should have received a copy of the GNU Lesser General Public
19 * License along with this library; if not, write to the Free Software
20 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
21 */
22
23 #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
24 #error "Dynamic loader LGPL wrappers not implemented yet"
25 #endif
26
27 struct rcu_wfs_node {
28 struct rcu_wfs_node *next;
29 };
30
31 struct rcu_wfs_stack {
32 struct rcu_wfs_node *head;
33 struct rcu_wfs_node end;
34 };
35
36 void rcu_wfs_node_init(struct rcu_wfs_node *node)
37 {
38 node->next = NULL;
39 }
40
41 void rcu_wfs_init(struct rcu_wfs_stack *s)
42 {
43 s->head = &s->end;
44 rcu_wfs_node_init(&s->end);
45 }
46
47 void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node)
48 {
49 struct rcu_wfs_node *old_head;
50
51 /*
52 * uatomic_xchg() implicit memory barrier orders earlier stores to node
53 * (setting it to NULL) before publication.
54 */
55 old_head = uatomic_xchg(&s->head, node);
56 /*
57 * At this point, dequeuers see a NULL node->next, they should busy-wait
58 * until node->next is set to old_head.
59 */
60 STORE_SHARED(node->next, old_head);
61 }
62
63 /*
64 * The caller must wait for a grace period before freeing the returned node.
65 * Returns NULL if stack is empty.
66 *
67 * cmpxchg is protected from ABA races by holding a RCU read lock between
68 * s->head read and cmpxchg modifying s->head and requiring that dequeuers wait
69 * for a grace period before freeing the returned node.
70 *
71 * TODO: implement adaptative busy-wait and wait/wakeup scheme rather than busy
72 * loops. Better for UP.
73 */
74 struct rcu_wfs_node *
75 rcu_wfs_pop(struct rcu_wfs_stack *s)
76 {
77 rcu_read_lock();
78 for (;;) {
79 struct rcu_wfs_node *head = rcu_dereference(s->head);
80
81 if (head != &s->end) {
82 struct rcu_wfs_node *next = rcu_dereference(head->next);
83
84 /* Retry while head is being set by push(). */
85 if (!next)
86 continue;
87
88 if (uatomic_cmpxchg(&s->head, head, next) == head) {
89 rcu_read_unlock();
90 return head;
91 } else {
92 /* Concurrent modification. Retry. */
93 continue;
94 }
95 } else {
96 /* Empty stack */
97 rcu_read_unlock();
98 return NULL;
99 }
100 }
101 }
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