1 #ifndef _URCU_RCULFQUEUE_STATIC_H
2 #define _URCU_RCULFQUEUE_STATIC_H
7 * Userspace RCU library - Lock-Free RCU Queue
9 * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
11 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfqueue.h for linking
12 * dynamically with the userspace rcu library.
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.
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.
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
29 #include <urcu-call-rcu.h>
30 #include <urcu/uatomic.h>
31 #include <urcu-pointer.h>
39 struct cds_lfq_node_rcu_dummy
{
40 struct cds_lfq_node_rcu parent
;
42 struct cds_lfq_queue_rcu
*q
;
46 * Lock-free RCU queue. Enqueue and dequeue operations hold a RCU read
47 * lock to deal with cmpxchg ABA problem. This queue is *not* circular:
48 * head points to the oldest node, tail points to the newest node.
49 * A dummy node is kept to ensure enqueue and dequeue can always proceed
50 * concurrently. Keeping a separate head and tail helps with large
51 * queues: enqueue and dequeue can proceed concurrently without
52 * wrestling for exclusive access to the same variables.
54 * Dequeue retry if it detects that it would be dequeueing the last node
55 * (it means a dummy node dequeue-requeue is in progress). This ensures
56 * that there is always at least one node in the queue.
58 * In the dequeue operation, we internally reallocate the dummy node
59 * upon dequeue/requeue and use call_rcu to free the old one after a
64 struct cds_lfq_node_rcu
*make_dummy(struct cds_lfq_queue_rcu
*q
,
65 struct cds_lfq_node_rcu
*next
)
67 struct cds_lfq_node_rcu_dummy
*dummy
;
69 dummy
= malloc(sizeof(struct cds_lfq_node_rcu_dummy
));
71 dummy
->parent
.next
= next
;
72 dummy
->parent
.dummy
= 1;
74 return &dummy
->parent
;
78 void free_dummy_cb(struct rcu_head
*head
)
80 struct cds_lfq_node_rcu_dummy
*dummy
=
81 caa_container_of(head
, struct cds_lfq_node_rcu_dummy
, head
);
86 void rcu_free_dummy(struct cds_lfq_node_rcu
*node
)
88 struct cds_lfq_node_rcu_dummy
*dummy
;
91 dummy
= caa_container_of(node
, struct cds_lfq_node_rcu_dummy
, parent
);
92 dummy
->q
->queue_call_rcu(&dummy
->head
, free_dummy_cb
);
96 void free_dummy(struct cds_lfq_node_rcu
*node
)
98 struct cds_lfq_node_rcu_dummy
*dummy
;
101 dummy
= caa_container_of(node
, struct cds_lfq_node_rcu_dummy
, parent
);
106 void _cds_lfq_node_init_rcu(struct cds_lfq_node_rcu
*node
)
113 void _cds_lfq_init_rcu(struct cds_lfq_queue_rcu
*q
,
114 void queue_call_rcu(struct rcu_head
*head
,
115 void (*func
)(struct rcu_head
*head
)))
117 q
->tail
= make_dummy(q
, NULL
);
119 q
->queue_call_rcu
= queue_call_rcu
;
123 * The queue should be emptied before calling destroy.
125 * Return 0 on success, -EPERM if queue is not empty.
128 int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu
*q
)
130 struct cds_lfq_node_rcu
*head
;
132 head
= rcu_dereference(q
->head
);
133 if (!(head
->dummy
&& head
->next
== NULL
))
134 return -EPERM
; /* not empty */
140 * Should be called under rcu read lock critical section.
143 void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu
*q
,
144 struct cds_lfq_node_rcu
*node
)
147 * uatomic_cmpxchg() implicit memory barrier orders earlier stores to
148 * node before publication.
152 struct cds_lfq_node_rcu
*tail
, *next
;
154 tail
= rcu_dereference(q
->tail
);
155 next
= uatomic_cmpxchg(&tail
->next
, NULL
, node
);
158 * Tail was at the end of queue, we successfully
159 * appended to it. Now move tail (another
160 * enqueue might beat us to it, that's fine).
162 (void) uatomic_cmpxchg(&q
->tail
, tail
, node
);
166 * Failure to append to current tail.
167 * Help moving tail further and retry.
169 (void) uatomic_cmpxchg(&q
->tail
, tail
, next
);
176 void enqueue_dummy(struct cds_lfq_queue_rcu
*q
)
178 struct cds_lfq_node_rcu
*node
;
180 /* We need to reallocate to protect from ABA. */
181 node
= make_dummy(q
, NULL
);
182 _cds_lfq_enqueue_rcu(q
, node
);
186 * Should be called under rcu read lock critical section.
188 * The caller must wait for a grace period to pass before freeing the returned
189 * node or modifying the cds_lfq_node_rcu structure.
190 * Returns NULL if queue is empty.
193 struct cds_lfq_node_rcu
*_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu
*q
)
196 struct cds_lfq_node_rcu
*head
, *next
;
198 head
= rcu_dereference(q
->head
);
199 next
= rcu_dereference(head
->next
);
200 if (head
->dummy
&& next
== NULL
)
201 return NULL
; /* empty */
203 * We never, ever allow dequeue to get to a state where
204 * the queue is empty (we need at least one node in the
205 * queue). This is ensured by checking if the head next
206 * is NULL, which means we need to enqueue a dummy node
207 * before we can hope dequeuing anything.
211 next
= rcu_dereference(head
->next
);
213 if (uatomic_cmpxchg(&q
->head
, head
, next
) != head
)
214 continue; /* Concurrently pushed. */
216 /* Free dummy after grace period. */
217 rcu_free_dummy(head
);
218 continue; /* try again */
228 #endif /* _URCU_RCULFQUEUE_STATIC_H */
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