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1 | #ifndef _URCU_STATIC_LFSTACK_H |
2 | #define _URCU_STATIC_LFSTACK_H | |
3 | ||
4 | /* | |
5 | * urcu/static/lfstack.h | |
6 | * | |
7 | * Userspace RCU library - Lock-Free Stack | |
8 | * | |
9 | * Copyright 2010-2012 - 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(struct cds_lfs_node *node) | |
38 | { | |
39 | } | |
40 | ||
41 | static inline | |
42 | void _cds_lfs_init(struct cds_lfs_stack *s) | |
43 | { | |
44 | s->head = NULL; | |
45 | } | |
46 | ||
47 | /* | |
48 | * cds_lfs_push: push a node into the stack. | |
49 | * | |
50 | * Does not require any synchronization with other push nor pop. | |
51 | * | |
52 | * Lock-free stack push is not subject to ABA problem, so no need to | |
53 | * take the RCU read-side lock. Even if "head" changes between two | |
54 | * uatomic_cmpxchg() invocations here (being popped, and then pushed | |
55 | * again by one or more concurrent threads), the second | |
56 | * uatomic_cmpxchg() invocation only cares about pushing a new entry at | |
57 | * the head of the stack, ensuring consistency by making sure the new | |
58 | * node->next is the same pointer value as the value replaced as head. | |
59 | * It does not care about the content of the actual next node, so it can | |
60 | * very well be reallocated between the two uatomic_cmpxchg(). | |
61 | * | |
62 | * We take the approach of expecting the stack to be usually empty, so | |
63 | * we first try an initial uatomic_cmpxchg() on a NULL old_head, and | |
64 | * retry if the old head was non-NULL (the value read by the first | |
65 | * uatomic_cmpxchg() is used as old head for the following loop). The | |
66 | * upside of this scheme is to minimize the amount of cacheline traffic, | |
67 | * always performing an exclusive cacheline access, rather than doing | |
68 | * non-exclusive followed by exclusive cacheline access (which would be | |
69 | * required if we first read the old head value). This design decision | |
70 | * might be revisited after more throrough benchmarking on various | |
71 | * platforms. | |
72 | * | |
73 | * Returns 0 if the stack was empty prior to adding the node. | |
74 | * Returns non-zero otherwise. | |
75 | */ | |
76 | static inline | |
77 | int _cds_lfs_push(struct cds_lfs_stack *s, | |
78 | struct cds_lfs_node *node) | |
79 | { | |
80 | struct cds_lfs_node *head = NULL; | |
81 | ||
82 | for (;;) { | |
83 | struct cds_lfs_node *old_head = head; | |
84 | ||
85 | /* | |
86 | * node->next is still private at this point, no need to | |
87 | * perform a _CMM_STORE_SHARED(). | |
88 | */ | |
89 | node->next = head; | |
90 | /* | |
91 | * uatomic_cmpxchg() implicit memory barrier orders earlier | |
92 | * stores to node before publication. | |
93 | */ | |
94 | head = uatomic_cmpxchg(&s->head, old_head, node); | |
95 | if (old_head == head) | |
96 | break; | |
97 | } | |
98 | return (int) !!((unsigned long) head); | |
99 | } | |
100 | ||
101 | /* | |
102 | * cds_lfs_pop: pop a node from the stack. | |
103 | * | |
104 | * Returns NULL if stack is empty. | |
105 | * | |
106 | * cds_lfs_pop needs to be synchronized using one of the following | |
107 | * techniques: | |
108 | * | |
109 | * 1) Calling cds_lfs_pop under rcu read lock critical section. The | |
110 | * caller must wait for a grace period to pass before freeing the | |
111 | * returned node or modifying the cds_lfs_node structure. | |
112 | * 2) Using mutual exclusion (e.g. mutexes) to protect cds_lfs_pop | |
113 | * callers. | |
114 | * 3) Ensuring that only ONE thread can call cds_lfs_pop(). | |
115 | * (multi-provider/single-consumer scheme). | |
116 | */ | |
117 | static inline | |
118 | struct cds_lfs_node *_cds_lfs_pop(struct cds_lfs_stack *s) | |
119 | { | |
120 | for (;;) { | |
121 | struct cds_lfs_node *head; | |
122 | ||
123 | head = _CMM_LOAD_SHARED(s->head); | |
124 | if (head) { | |
125 | struct cds_lfs_node *next; | |
126 | ||
127 | /* | |
128 | * Read head before head->next. Matches the | |
129 | * implicit memory barrier before | |
130 | * uatomic_cmpxchg() in cds_lfs_push. | |
131 | */ | |
132 | cmm_smp_read_barrier_depends(); | |
133 | next = _CMM_LOAD_SHARED(head->next); | |
134 | if (uatomic_cmpxchg(&s->head, head, next) == head) { | |
135 | return head; | |
136 | } else { | |
137 | /* Concurrent modification. Retry. */ | |
138 | continue; | |
139 | } | |
140 | } else { | |
141 | /* Empty stack */ | |
142 | return NULL; | |
143 | } | |
144 | } | |
145 | } | |
146 | ||
147 | #ifdef __cplusplus | |
148 | } | |
149 | #endif | |
150 | ||
151 | #endif /* _URCU_STATIC_LFSTACK_H */ |