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453629a9 MD |
1 | /* |
2 | * rculfqueue.h | |
3 | * | |
4 | * Userspace RCU library - Lock-Free RCU Queue | |
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 | #include <urcu/urcu_ref.h> | |
24 | #include <assert.h> | |
25 | ||
26 | #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE)) | |
27 | #error "Dynamic loader LGPL wrappers not implemented yet" | |
28 | #endif | |
29 | ||
30 | /* | |
31 | * Lock-free RCU queue using reference counting. Enqueue and dequeue operations | |
32 | * hold a RCU read lock to deal with cmpxchg ABA problem. This implementation | |
33 | * keeps a dummy head node to ensure we can always update the queue locklessly. | |
34 | * Given that this is a queue, the dummy head node must always advance as we | |
35 | * dequeue entries. Therefore, we keep a reference count on each entry we are | |
36 | * dequeueing, so they can be kept as dummy head node until the next dequeue, at | |
37 | * which point their reference count will be decremented. | |
38 | */ | |
39 | ||
40 | #define URCU_LFQ_PERMANENT_REF 128 | |
41 | ||
42 | struct rcu_lfq_node { | |
43 | struct rcu_lfq_node *next; | |
44 | struct urcu_ref ref; | |
45 | }; | |
46 | ||
47 | struct rcu_lfq_queue { | |
48 | struct rcu_lfq_node *head, *tail; | |
49 | struct rcu_lfq_node init; /* Dummy initialization node */ | |
50 | }; | |
51 | ||
52 | void rcu_lfq_node_init(struct rcu_lfq_node *node) | |
53 | { | |
54 | node->next = NULL; | |
55 | urcu_ref_init(&node->ref); | |
56 | } | |
57 | ||
58 | void rcu_lfq_init(struct rcu_lfq_queue *q) | |
59 | { | |
60 | rcu_lfq_node_init(&q->init); | |
61 | /* Make sure the initial node is never freed. */ | |
62 | urcu_ref_set(&q->init.ref, URCU_LFQ_PERMANENT_REF); | |
63 | q->head = q->tail = &q->init; | |
64 | } | |
65 | ||
66 | void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node) | |
67 | { | |
68 | urcu_ref_get(&node->ref); | |
69 | ||
70 | /* | |
71 | * uatomic_cmpxchg() implicit memory barrier orders earlier stores to | |
72 | * node before publication. | |
73 | */ | |
74 | ||
75 | rcu_read_lock(); | |
76 | for (;;) { | |
77 | struct rcu_lfq_node *tail = rcu_dereference(q->tail); | |
78 | struct rcu_lfq_node *next; | |
79 | ||
80 | /* | |
a5eb5ffc | 81 | * Typically expect tail->next to be NULL. |
453629a9 MD |
82 | */ |
83 | next = uatomic_cmpxchg(&tail->next, NULL, node); | |
84 | if (next == NULL) { | |
85 | /* | |
86 | * Tail was at the end of queue, we successfully | |
87 | * appended to it. | |
88 | * Now move tail (another enqueue might beat | |
89 | * us to it, that's fine). | |
90 | */ | |
91 | uatomic_cmpxchg(&q->tail, tail, node); | |
92 | rcu_read_unlock(); | |
93 | return; | |
94 | } else { | |
95 | /* | |
96 | * Failure to append to current tail. Help moving tail | |
97 | * further and retry. | |
98 | */ | |
99 | uatomic_cmpxchg(&q->tail, tail, next); | |
100 | continue; | |
101 | } | |
102 | } | |
103 | } | |
104 | ||
105 | /* | |
106 | * The entry returned by dequeue must be taken care of by doing a urcu_ref_put, | |
107 | * which calls the release primitive when the reference count drops to zero. A | |
108 | * grace period must be waited before performing the actual memory reclamation | |
109 | * in the release primitive. | |
110 | * The entry lfq node returned by dequeue must no be re-used before the | |
111 | * reference count reaches zero. | |
112 | */ | |
113 | struct rcu_lfq_node * | |
114 | rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *)) | |
115 | { | |
116 | rcu_read_lock(); | |
117 | for (;;) { | |
118 | struct rcu_lfq_node *head = rcu_dereference(q->head); | |
119 | struct rcu_lfq_node *next = rcu_dereference(head->next); | |
120 | ||
121 | if (next) { | |
122 | if (uatomic_cmpxchg(&q->head, head, next) == head) { | |
123 | rcu_read_unlock(); | |
124 | urcu_ref_put(&head->ref, release); | |
125 | return next; | |
126 | } else { | |
127 | /* Concurrently pushed, retry */ | |
128 | continue; | |
129 | } | |
130 | } else { | |
131 | /* Empty */ | |
132 | rcu_read_unlock(); | |
133 | return NULL; | |
134 | } | |
135 | } | |
136 | } |