2 * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
3 * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
5 * SPDX-License-Identifier: LGPL-2.1-only
13 #include <urcu/futex.h>
14 #include <urcu/uatomic.h>
17 * Number of busy-loop attempts before waiting on futex.
19 #define WAIT_ATTEMPTS 1000
22 /* WAITER_WAITING is compared directly (futex compares it). */
24 /* non-zero are used as masks. */
25 WAITER_WOKEN_UP
= (1 << 0),
26 WAITER_RUNNING
= (1 << 1),
27 WAITER_TEARDOWN
= (1 << 2),
30 void lttng_waiter_init(struct lttng_waiter
*waiter
)
32 cds_wfs_node_init(&waiter
->wait_queue_node
);
33 uatomic_set(&waiter
->state
, WAITER_WAITING
);
38 * User must init "waiter" before passing its memory to waker thread.
40 void lttng_waiter_wait(struct lttng_waiter
*waiter
)
44 DBG("Beginning of waiter wait period");
45 /* Load and test condition before read state */
47 for (i
= 0; i
< WAIT_ATTEMPTS
; i
++) {
48 if (uatomic_read(&waiter
->state
) != WAITER_WAITING
) {
53 while (uatomic_read(&waiter
->state
) == WAITER_WAITING
) {
55 &waiter
->state
, FUTEX_WAIT
, WAITER_WAITING
, nullptr, nullptr, 0)) {
57 * Prior queued wakeups queued by unrelated code
58 * using the same address can cause futex wait to
59 * return 0 even through the futex value is still
60 * WAITER_WAITING (spurious wakeups). Check
61 * the value again in user-space to validate
62 * whether it really differs from WAITER_WAITING.
68 /* Value already changed. */
71 /* Retry if interrupted by signal. */
72 break; /* Get out of switch. Check again. */
74 /* Unexpected error. */
75 PERROR("futex_noasync");
81 /* Tell waker thread than we are running. */
82 uatomic_or(&waiter
->state
, WAITER_RUNNING
);
85 * Wait until waker thread lets us know it's ok to tear down
86 * memory allocated for struct lttng_waiter.
88 for (i
= 0; i
< WAIT_ATTEMPTS
; i
++) {
89 if (uatomic_read(&waiter
->state
) & WAITER_TEARDOWN
) {
94 while (!(uatomic_read(&waiter
->state
) & WAITER_TEARDOWN
)) {
97 LTTNG_ASSERT(uatomic_read(&waiter
->state
) & WAITER_TEARDOWN
);
98 DBG("End of waiter wait period");
102 * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
103 * execution. In this scheme, the waiter owns the node memory, and we only allow
104 * it to free this memory when it sees the WAITER_TEARDOWN flag.
106 void lttng_waiter_wake_up(struct lttng_waiter
*waiter
)
109 LTTNG_ASSERT(uatomic_read(&waiter
->state
) == WAITER_WAITING
);
110 uatomic_set(&waiter
->state
, WAITER_WOKEN_UP
);
111 if (!(uatomic_read(&waiter
->state
) & WAITER_RUNNING
)) {
112 if (futex_noasync(&waiter
->state
, FUTEX_WAKE
, 1, nullptr, nullptr, 0) < 0) {
113 PERROR("futex_noasync");
117 /* Allow teardown of struct urcu_wait memory. */
118 uatomic_or(&waiter
->state
, WAITER_TEARDOWN
);
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