cleanup: spelling fixes in comments
[urcu.git] / include / urcu / static / rculfstack.h
1 #ifndef _URCU_RCULFSTACK_STATIC_H
2 #define _URCU_RCULFSTACK_STATIC_H
3
4 /*
5 * rculfstack-static.h
6 *
7 * Userspace RCU library - Lock-Free RCU Stack
8 *
9 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
10 *
11 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfstack.h for linking
12 * dynamically with the userspace rcu library.
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 <urcu/uatomic.h>
30 #include <urcu-pointer.h>
31
32 #ifdef __cplusplus
33 extern "C" {
34 #endif
35
36 static inline
37 void _cds_lfs_node_init_rcu(struct cds_lfs_node_rcu *node __attribute__((unused)))
38 {
39 }
40
41 static inline
42 void _cds_lfs_init_rcu(struct cds_lfs_stack_rcu *s)
43 {
44 s->head = NULL;
45 }
46
47 /*
48 * Lock-free stack push is not subject to ABA problem, so no need to
49 * take the RCU read-side lock. Even if "head" changes between two
50 * uatomic_cmpxchg() invocations here (being popped, and then pushed
51 * again by one or more concurrent threads), the second
52 * uatomic_cmpxchg() invocation only cares about pushing a new entry at
53 * the head of the stack, ensuring consistency by making sure the new
54 * node->next is the same pointer value as the value replaced as head.
55 * It does not care about the content of the actual next node, so it can
56 * very well be reallocated between the two uatomic_cmpxchg().
57 *
58 * We take the approach of expecting the stack to be usually empty, so
59 * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
60 * retry if the old head was non-NULL (the value read by the first
61 * uatomic_cmpxchg() is used as old head for the following loop). The
62 * upside of this scheme is to minimize the amount of cacheline traffic,
63 * always performing an exclusive cacheline access, rather than doing
64 * non-exclusive followed by exclusive cacheline access (which would be
65 * required if we first read the old head value). This design decision
66 * might be revisited after more thorough benchmarking on various
67 * platforms.
68 *
69 * Returns 0 if the stack was empty prior to adding the node.
70 * Returns non-zero otherwise.
71 */
72 static inline
73 int _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s,
74 struct cds_lfs_node_rcu *node)
75 {
76 struct cds_lfs_node_rcu *head = NULL;
77
78 for (;;) {
79 struct cds_lfs_node_rcu *old_head = head;
80
81 node->next = head;
82 /*
83 * uatomic_cmpxchg() implicit memory barrier orders earlier
84 * stores to node before publication.
85 */
86 head = uatomic_cmpxchg(&s->head, old_head, node);
87 if (old_head == head)
88 break;
89 }
90 return (int) !!((unsigned long) head);
91 }
92
93 /*
94 * Should be called under rcu read-side lock.
95 *
96 * The caller must wait for a grace period to pass before freeing the returned
97 * node or modifying the cds_lfs_node_rcu structure.
98 * Returns NULL if stack is empty.
99 */
100 static inline
101 struct cds_lfs_node_rcu *
102 _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s)
103 {
104 for (;;) {
105 struct cds_lfs_node_rcu *head;
106
107 head = rcu_dereference(s->head);
108 if (head) {
109 struct cds_lfs_node_rcu *next = rcu_dereference(head->next);
110
111 if (uatomic_cmpxchg(&s->head, head, next) == head) {
112 return head;
113 } else {
114 /* Concurrent modification. Retry. */
115 continue;
116 }
117 } else {
118 /* Empty stack */
119 return NULL;
120 }
121 }
122 }
123
124 #ifdef __cplusplus
125 }
126 #endif
127
128 #endif /* _URCU_RCULFSTACK_STATIC_H */
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