1 // SPDX-FileCopyrightText: 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
3 // SPDX-License-Identifier: LGPL-2.1-or-later
9 * Userspace RCU library wait/wakeup management
12 #include <urcu/assert.h>
13 #include <urcu/uatomic.h>
14 #include <urcu/wfstack.h>
15 #include <urcu/futex.h>
19 * Number of busy-loop attempts before waiting on futex for grace period
22 #define URCU_WAIT_ATTEMPTS 1000
24 enum urcu_wait_state
{
25 /* URCU_WAIT_WAITING is compared directly (futex compares it). */
26 URCU_WAIT_WAITING
= 0,
27 /* non-zero are used as masks. */
28 URCU_WAIT_WAKEUP
= (1 << 0),
29 URCU_WAIT_RUNNING
= (1 << 1),
30 URCU_WAIT_TEARDOWN
= (1 << 2),
33 struct urcu_wait_node
{
34 struct cds_wfs_node node
;
35 int32_t state
; /* enum urcu_wait_state */
38 #define URCU_WAIT_NODE_INIT(name, _state) \
39 { .node = { .next = NULL }, .state = _state }
41 #define DEFINE_URCU_WAIT_NODE(name, state) \
42 struct urcu_wait_node name = URCU_WAIT_NODE_INIT(name, state)
44 #define DECLARE_URCU_WAIT_NODE(name) \
45 struct urcu_wait_node name
47 struct urcu_wait_queue
{
48 struct cds_wfs_stack stack
;
51 #define URCU_WAIT_QUEUE_HEAD_INIT(name) \
54 .head = CDS_WFS_END, \
55 .lock = PTHREAD_MUTEX_INITIALIZER, \
59 #define DECLARE_URCU_WAIT_QUEUE(name) \
60 struct urcu_wait_queue name
62 #define DEFINE_URCU_WAIT_QUEUE(name) \
63 struct urcu_wait_queue name = URCU_WAIT_QUEUE_HEAD_INIT(name)
66 struct cds_wfs_head
*head
;
70 * Add ourself atomically to a wait queue. Return 0 if queue was
71 * previously empty, else return 1.
72 * A full memory barrier is issued before being added to the wait queue.
75 bool urcu_wait_add(struct urcu_wait_queue
*queue
,
76 struct urcu_wait_node
*node
)
78 return cds_wfs_push(&queue
->stack
, &node
->node
);
82 * Atomically move all waiters from wait queue into our local struct
86 void urcu_move_waiters(struct urcu_waiters
*waiters
,
87 struct urcu_wait_queue
*queue
)
89 waiters
->head
= __cds_wfs_pop_all(&queue
->stack
);
93 void urcu_wait_set_state(struct urcu_wait_node
*node
,
94 enum urcu_wait_state state
)
100 void urcu_wait_node_init(struct urcu_wait_node
*node
,
101 enum urcu_wait_state state
)
103 urcu_wait_set_state(node
, state
);
104 cds_wfs_node_init(&node
->node
);
108 * Note: urcu_adaptative_wake_up needs "value" to stay allocated
109 * throughout its execution. In this scheme, the waiter owns the node
110 * memory, and we only allow it to free this memory when it receives the
111 * URCU_WAIT_TEARDOWN flag.
114 void urcu_adaptative_wake_up(struct urcu_wait_node
*wait
)
116 urcu_posix_assert(uatomic_read(&wait
->state
) == URCU_WAIT_WAITING
);
117 uatomic_store(&wait
->state
, URCU_WAIT_WAKEUP
, CMM_RELEASE
);
118 if (!(uatomic_read(&wait
->state
) & URCU_WAIT_RUNNING
)) {
119 if (futex_noasync(&wait
->state
, FUTEX_WAKE
, 1,
123 /* Allow teardown of struct urcu_wait memory. */
124 uatomic_or_mo(&wait
->state
, URCU_WAIT_TEARDOWN
, CMM_RELEASE
);
128 * Caller must initialize "value" to URCU_WAIT_WAITING before passing its
129 * memory to waker thread.
132 void urcu_adaptative_busy_wait(struct urcu_wait_node
*wait
)
136 /* Load and test condition before read state */
138 for (i
= 0; i
< URCU_WAIT_ATTEMPTS
; i
++) {
139 if (uatomic_load(&wait
->state
, CMM_ACQUIRE
) != URCU_WAIT_WAITING
)
140 goto skip_futex_wait
;
143 while (uatomic_load(&wait
->state
, CMM_ACQUIRE
) == URCU_WAIT_WAITING
) {
144 if (!futex_noasync(&wait
->state
, FUTEX_WAIT
, URCU_WAIT_WAITING
, NULL
, NULL
, 0)) {
146 * Prior queued wakeups queued by unrelated code
147 * using the same address can cause futex wait to
148 * return 0 even through the futex value is still
149 * URCU_WAIT_WAITING (spurious wakeups). Check
150 * the value again in user-space to validate
151 * whether it really differs from
158 /* Value already changed. */
159 goto skip_futex_wait
;
161 /* Retry if interrupted by signal. */
162 break; /* Get out of switch. Check again. */
164 /* Unexpected error. */
170 /* Tell waker thread than we are running. */
171 uatomic_or(&wait
->state
, URCU_WAIT_RUNNING
);
174 * Wait until waker thread lets us know it's ok to tear down
175 * memory allocated for struct urcu_wait.
177 for (i
= 0; i
< URCU_WAIT_ATTEMPTS
; i
++) {
178 if (uatomic_load(&wait
->state
, CMM_RELAXED
) & URCU_WAIT_TEARDOWN
)
182 while (!(uatomic_load(&wait
->state
, CMM_ACQUIRE
) & URCU_WAIT_TEARDOWN
))
184 urcu_posix_assert(uatomic_read(&wait
->state
) & URCU_WAIT_TEARDOWN
);
188 void urcu_wake_all_waiters(struct urcu_waiters
*waiters
)
190 struct cds_wfs_node
*iter
, *iter_n
;
192 /* Wake all waiters in our stack head */
193 cds_wfs_for_each_blocking_safe(waiters
->head
, iter
, iter_n
) {
194 struct urcu_wait_node
*wait_node
=
195 caa_container_of(iter
, struct urcu_wait_node
, node
);
197 /* Don't wake already running threads */
198 if (uatomic_load(&wait_node
->state
, CMM_RELAXED
) & URCU_WAIT_RUNNING
)
200 urcu_adaptative_wake_up(wait_node
);
204 #endif /* _URCU_WAIT_H */
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