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1 | #ifndef _URCU_RCUWFQUEUE_H |
2 | #define _URCU_RCUWFQUEUE_H | |
3 | ||
ee566d46 MD |
4 | /* |
5 | * rcuwfqueue.h | |
6 | * | |
7 | * Userspace RCU library - RCU Queue with Wait-Free Enqueue/Blocking Dequeue | |
8 | * | |
9 | * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com> | |
10 | * | |
11 | * This library is free software; you can redistribute it and/or | |
12 | * modify it under the terms of the GNU Lesser General Public | |
13 | * License as published by the Free Software Foundation; either | |
14 | * version 2.1 of the License, or (at your option) any later version. | |
15 | * | |
16 | * This library is distributed in the hope that it will be useful, | |
17 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
18 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
19 | * Lesser General Public License for more details. | |
20 | * | |
21 | * You should have received a copy of the GNU Lesser General Public | |
22 | * License along with this library; if not, write to the Free Software | |
23 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA | |
24 | */ | |
25 | ||
26 | #include <urcu/urcu_ref.h> | |
27 | #include <assert.h> | |
28 | ||
0e2e9380 MD |
29 | #ifdef __cplusplus |
30 | extern "C" { | |
31 | #endif | |
32 | ||
ee566d46 MD |
33 | #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE)) |
34 | #error "Dynamic loader LGPL wrappers not implemented yet" | |
35 | #endif | |
36 | ||
37 | /* | |
38 | * RCU queue with wait-free enqueue/blocking dequeue using reference counting. | |
39 | * Enqueue and dequeue operations hold a RCU read lock to deal with cmpxchg ABA | |
40 | * problem. This implementation keeps a dummy head node to ensure we can always | |
41 | * update the queue locklessly. Given that this is a queue, the dummy head node | |
42 | * must always advance as we dequeue entries. Therefore, we keep a reference | |
43 | * count on each entry we are dequeueing, so they can be kept as dummy head node | |
44 | * until the next dequeue, at which point their reference count will be | |
45 | * decremented. | |
46 | */ | |
47 | ||
48 | #define URCU_WFQ_PERMANENT_REF 128 | |
49 | ||
50 | struct rcu_wfq_node { | |
51 | struct rcu_wfq_node *next; | |
52 | struct urcu_ref ref; | |
53 | }; | |
54 | ||
55 | struct rcu_wfq_queue { | |
56 | struct rcu_wfq_node *head, *tail; | |
57 | struct rcu_wfq_node init; /* Dummy initialization node */ | |
58 | }; | |
59 | ||
60 | void rcu_wfq_node_init(struct rcu_wfq_node *node) | |
61 | { | |
62 | node->next = NULL; | |
63 | urcu_ref_init(&node->ref); | |
64 | } | |
65 | ||
66 | void rcu_wfq_init(struct rcu_wfq_queue *q) | |
67 | { | |
68 | rcu_wfq_node_init(&q->init); | |
69 | /* Make sure the initial node is never freed. */ | |
70 | urcu_ref_set(&q->init.ref, URCU_WFQ_PERMANENT_REF); | |
71 | /* Set queue end */ | |
72 | q->head = q->tail = &q->init; | |
73 | } | |
74 | ||
75 | void rcu_wfq_enqueue(struct rcu_wfq_queue *q, struct rcu_wfq_node *node) | |
76 | { | |
77 | struct rcu_wfq_node *old_tail; | |
78 | ||
79 | urcu_ref_get(&node->ref); | |
80 | /* | |
81 | * uatomic_xchg() implicit memory barrier orders earlier stores to node | |
82 | * (setting it to NULL and incrementing the refcount) before | |
83 | * publication. | |
84 | */ | |
85 | old_tail = uatomic_xchg(&q->tail, node); | |
86 | /* | |
87 | * At this point, dequeuers see a NULL old_tail->next, which indicates | |
88 | * end of queue. The following store will append "node" to the queue | |
89 | * from a dequeuer perspective. | |
90 | */ | |
91 | STORE_SHARED(old_tail->next, node); | |
92 | } | |
93 | ||
94 | /* | |
95 | * The entry returned by dequeue must be taken care of by doing a urcu_ref_put, | |
96 | * which calls the release primitive when the reference count drops to zero. A | |
97 | * grace period must be waited before performing the actual memory reclamation | |
98 | * in the release primitive. The wfq node returned by dequeue must not be | |
99 | * modified/re-used/freed until the reference count reaches zero and a grace | |
100 | * period has elapsed (after the refcount reached 0). | |
101 | * | |
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102 | * No need to go on a waitqueue here, as there is no possible state in which the |
103 | * list could cause dequeue to busy-loop needlessly while waiting for another | |
104 | * thread to be scheduled. The queue appears empty until tail->next is set by | |
105 | * enqueue. | |
ee566d46 MD |
106 | */ |
107 | struct rcu_wfq_node * | |
108 | rcu_wfq_dequeue_blocking(struct rcu_wfq_queue *q, | |
109 | void (*release)(struct urcu_ref *)) | |
110 | { | |
111 | for (;;) { | |
112 | struct rcu_wfq_node *head, *next; | |
113 | ||
114 | rcu_read_lock(); | |
115 | head = rcu_dereference(q->head); | |
116 | next = rcu_dereference(head->next); | |
117 | if (next) { | |
118 | if (uatomic_cmpxchg(&q->head, head, next) == head) { | |
119 | rcu_read_unlock(); | |
120 | urcu_ref_put(&head->ref, release); | |
121 | return next; | |
122 | } else { | |
123 | /* Concurrently pushed, retry */ | |
124 | rcu_read_unlock(); | |
125 | continue; | |
126 | } | |
127 | } else { | |
128 | /* Empty */ | |
129 | rcu_read_unlock(); | |
130 | return NULL; | |
131 | } | |
132 | } | |
133 | } | |
0e2e9380 MD |
134 | |
135 | #ifdef __cplusplus | |
136 | } | |
137 | #endif | |
138 | ||
139 | #endif /* _URCU_RCUWFQUEUE_H */ |