7 * Userspace RCU library wait/wakeup management
9 * Copyright (c) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
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.
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
26 #include <urcu/assert.h>
27 #include <urcu/uatomic.h>
28 #include <urcu/wfstack.h>
32 * Number of busy-loop attempts before waiting on futex for grace period
35 #define URCU_WAIT_ATTEMPTS 1000
37 enum urcu_wait_state
{
38 /* URCU_WAIT_WAITING is compared directly (futex compares it). */
39 URCU_WAIT_WAITING
= 0,
40 /* non-zero are used as masks. */
41 URCU_WAIT_WAKEUP
= (1 << 0),
42 URCU_WAIT_RUNNING
= (1 << 1),
43 URCU_WAIT_TEARDOWN
= (1 << 2),
46 struct urcu_wait_node
{
47 struct cds_wfs_node node
;
48 int32_t state
; /* enum urcu_wait_state */
51 #define URCU_WAIT_NODE_INIT(name, _state) \
54 #define DEFINE_URCU_WAIT_NODE(name, state) \
55 struct urcu_wait_node name = URCU_WAIT_NODE_INIT(name, state)
57 #define DECLARE_URCU_WAIT_NODE(name) \
58 struct urcu_wait_node name
60 struct urcu_wait_queue
{
61 struct cds_wfs_stack stack
;
64 #define URCU_WAIT_QUEUE_HEAD_INIT(name) \
67 .head = CDS_WFS_END, \
68 .lock = PTHREAD_MUTEX_INITIALIZER, \
72 #define DECLARE_URCU_WAIT_QUEUE(name) \
73 struct urcu_wait_queue name
75 #define DEFINE_URCU_WAIT_QUEUE(name) \
76 struct urcu_wait_queue name = URCU_WAIT_QUEUE_HEAD_INIT(name)
79 struct cds_wfs_head
*head
;
83 * Add ourself atomically to a wait queue. Return 0 if queue was
84 * previously empty, else return 1.
85 * A full memory barrier is issued before being added to the wait queue.
88 bool urcu_wait_add(struct urcu_wait_queue
*queue
,
89 struct urcu_wait_node
*node
)
91 return cds_wfs_push(&queue
->stack
, &node
->node
);
95 * Atomically move all waiters from wait queue into our local struct
99 void urcu_move_waiters(struct urcu_waiters
*waiters
,
100 struct urcu_wait_queue
*queue
)
102 waiters
->head
= __cds_wfs_pop_all(&queue
->stack
);
106 void urcu_wait_set_state(struct urcu_wait_node
*node
,
107 enum urcu_wait_state state
)
113 void urcu_wait_node_init(struct urcu_wait_node
*node
,
114 enum urcu_wait_state state
)
116 urcu_wait_set_state(node
, state
);
117 cds_wfs_node_init(&node
->node
);
121 * Note: urcu_adaptative_wake_up needs "value" to stay allocated
122 * throughout its execution. In this scheme, the waiter owns the node
123 * memory, and we only allow it to free this memory when it receives the
124 * URCU_WAIT_TEARDOWN flag.
127 void urcu_adaptative_wake_up(struct urcu_wait_node
*wait
)
130 urcu_posix_assert(uatomic_read(&wait
->state
) == URCU_WAIT_WAITING
);
131 uatomic_set(&wait
->state
, URCU_WAIT_WAKEUP
);
132 if (!(uatomic_read(&wait
->state
) & URCU_WAIT_RUNNING
)) {
133 if (futex_noasync(&wait
->state
, FUTEX_WAKE
, 1,
137 /* Allow teardown of struct urcu_wait memory. */
138 uatomic_or(&wait
->state
, URCU_WAIT_TEARDOWN
);
142 * Caller must initialize "value" to URCU_WAIT_WAITING before passing its
143 * memory to waker thread.
146 void urcu_adaptative_busy_wait(struct urcu_wait_node
*wait
)
150 /* Load and test condition before read state */
152 for (i
= 0; i
< URCU_WAIT_ATTEMPTS
; i
++) {
153 if (uatomic_read(&wait
->state
) != URCU_WAIT_WAITING
)
154 goto skip_futex_wait
;
157 while (uatomic_read(&wait
->state
) == URCU_WAIT_WAITING
) {
158 if (!futex_noasync(&wait
->state
, FUTEX_WAIT
, URCU_WAIT_WAITING
, NULL
, NULL
, 0)) {
160 * Prior queued wakeups queued by unrelated code
161 * using the same address can cause futex wait to
162 * return 0 even through the futex value is still
163 * URCU_WAIT_WAITING (spurious wakeups). Check
164 * the value again in user-space to validate
165 * whether it really differs from
172 /* Value already changed. */
173 goto skip_futex_wait
;
175 /* Retry if interrupted by signal. */
176 break; /* Get out of switch. Check again. */
178 /* Unexpected error. */
184 /* Tell waker thread than we are running. */
185 uatomic_or(&wait
->state
, URCU_WAIT_RUNNING
);
188 * Wait until waker thread lets us know it's ok to tear down
189 * memory allocated for struct urcu_wait.
191 for (i
= 0; i
< URCU_WAIT_ATTEMPTS
; i
++) {
192 if (uatomic_read(&wait
->state
) & URCU_WAIT_TEARDOWN
)
196 while (!(uatomic_read(&wait
->state
) & URCU_WAIT_TEARDOWN
))
198 urcu_posix_assert(uatomic_read(&wait
->state
) & URCU_WAIT_TEARDOWN
);
202 void urcu_wake_all_waiters(struct urcu_waiters
*waiters
)
204 struct cds_wfs_node
*iter
, *iter_n
;
206 /* Wake all waiters in our stack head */
207 cds_wfs_for_each_blocking_safe(waiters
->head
, iter
, iter_n
) {
208 struct urcu_wait_node
*wait_node
=
209 caa_container_of(iter
, struct urcu_wait_node
, node
);
211 /* Don't wake already running threads */
212 if (wait_node
->state
& URCU_WAIT_RUNNING
)
214 urcu_adaptative_wake_up(wait_node
);
218 #endif /* _URCU_WAIT_H */
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