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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 | ||
4a2f213c MD |
23 | #include <assert.h> |
24 | ||
a5eb5ffc MD |
25 | #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE)) |
26 | #error "Dynamic loader LGPL wrappers not implemented yet" | |
27 | #endif | |
28 | ||
4a2f213c MD |
29 | #define RCU_WF_STACK_END ((void *)0x1UL) |
30 | ||
a5eb5ffc MD |
31 | struct rcu_wfs_node { |
32 | struct rcu_wfs_node *next; | |
33 | }; | |
34 | ||
35 | struct rcu_wfs_stack { | |
36 | struct rcu_wfs_node *head; | |
a5eb5ffc MD |
37 | }; |
38 | ||
39 | void rcu_wfs_node_init(struct rcu_wfs_node *node) | |
40 | { | |
41 | node->next = NULL; | |
42 | } | |
43 | ||
44 | void rcu_wfs_init(struct rcu_wfs_stack *s) | |
45 | { | |
4a2f213c | 46 | s->head = RCU_WF_STACK_END; |
a5eb5ffc MD |
47 | } |
48 | ||
49 | void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node) | |
50 | { | |
51 | struct rcu_wfs_node *old_head; | |
52 | ||
4a2f213c | 53 | assert(node->next == NULL); |
a5eb5ffc MD |
54 | /* |
55 | * uatomic_xchg() implicit memory barrier orders earlier stores to node | |
56 | * (setting it to NULL) before publication. | |
57 | */ | |
58 | old_head = uatomic_xchg(&s->head, node); | |
59 | /* | |
60 | * At this point, dequeuers see a NULL node->next, they should busy-wait | |
61 | * until node->next is set to old_head. | |
62 | */ | |
63 | STORE_SHARED(node->next, old_head); | |
64 | } | |
65 | ||
66 | /* | |
4a2f213c MD |
67 | * The caller must wait for a grace period before: |
68 | * - freeing the returned node. | |
69 | * - modifying the ->next pointer of the returned node. (be careful with unions) | |
70 | * - passing the returned node back to push() on the same stack they got it | |
71 | * from. | |
72 | * | |
a5eb5ffc MD |
73 | * Returns NULL if stack is empty. |
74 | * | |
75 | * cmpxchg is protected from ABA races by holding a RCU read lock between | |
76 | * s->head read and cmpxchg modifying s->head and requiring that dequeuers wait | |
77 | * for a grace period before freeing the returned node. | |
78 | * | |
79 | * TODO: implement adaptative busy-wait and wait/wakeup scheme rather than busy | |
80 | * loops. Better for UP. | |
81 | */ | |
82 | struct rcu_wfs_node * | |
83 | rcu_wfs_pop(struct rcu_wfs_stack *s) | |
84 | { | |
a5eb5ffc | 85 | for (;;) { |
4a2f213c | 86 | struct rcu_wfs_node *head; |
a5eb5ffc | 87 | |
4a2f213c MD |
88 | rcu_read_lock(); |
89 | head = rcu_dereference(s->head); | |
90 | if (head != RCU_WF_STACK_END) { | |
a5eb5ffc MD |
91 | struct rcu_wfs_node *next = rcu_dereference(head->next); |
92 | ||
93 | /* Retry while head is being set by push(). */ | |
4a2f213c MD |
94 | if (!next) { |
95 | rcu_read_unlock(); | |
a5eb5ffc | 96 | continue; |
4a2f213c | 97 | } |
a5eb5ffc MD |
98 | if (uatomic_cmpxchg(&s->head, head, next) == head) { |
99 | rcu_read_unlock(); | |
100 | return head; | |
101 | } else { | |
102 | /* Concurrent modification. Retry. */ | |
4a2f213c | 103 | rcu_read_unlock(); |
a5eb5ffc MD |
104 | continue; |
105 | } | |
106 | } else { | |
107 | /* Empty stack */ | |
108 | rcu_read_unlock(); | |
109 | return NULL; | |
110 | } | |
111 | } | |
112 | } |