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a5eb5ffc MD |
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 | } |