event-notifier: implement `lttng_trigger_needs_tracer_notifier()` function
[lttng-tools.git] / src / common / waiter.c
CommitLineData
287a512f 1/*
ab5be9fa
MJ
2 * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
3 * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
287a512f 4 *
ab5be9fa 5 * SPDX-License-Identifier: LGPL-2.1-only
287a512f 6 *
287a512f
JG
7 */
8
9#include "waiter.h"
10#include <urcu/uatomic.h>
11#include <urcu/futex.h>
12#include <assert.h>
13#include "error.h"
14#include <poll.h>
15
16/*
17 * Number of busy-loop attempts before waiting on futex.
18 */
19#define WAIT_ATTEMPTS 1000
20
21enum waiter_state {
22 /* WAITER_WAITING is compared directly (futex compares it). */
23 WAITER_WAITING = 0,
24 /* non-zero are used as masks. */
25 WAITER_WOKEN_UP = (1 << 0),
26 WAITER_RUNNING = (1 << 1),
27 WAITER_TEARDOWN = (1 << 2),
28};
29
30LTTNG_HIDDEN
31void lttng_waiter_init(struct lttng_waiter *waiter)
32{
33 cds_wfs_node_init(&waiter->wait_queue_node);
34 uatomic_set(&waiter->state, WAITER_WAITING);
35 cmm_smp_mb();
36}
37
38/*
39 * User must init "waiter" before passing its memory to waker thread.
40 */
41LTTNG_HIDDEN
42void lttng_waiter_wait(struct lttng_waiter *waiter)
43{
44 unsigned int i;
45
46 DBG("Beginning of waiter wait period");
47 /* Load and test condition before read state */
48 cmm_smp_rmb();
49 for (i = 0; i < WAIT_ATTEMPTS; i++) {
50 if (uatomic_read(&waiter->state) != WAITER_WAITING) {
51 goto skip_futex_wait;
52 }
53 caa_cpu_relax();
54 }
55 while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
56 NULL, NULL, 0)) {
57 switch (errno) {
58 case EWOULDBLOCK:
59 /* Value already changed. */
60 goto skip_futex_wait;
61 case EINTR:
62 /* Retry if interrupted by signal. */
63 break; /* Get out of switch. */
64 default:
65 /* Unexpected error. */
66 PERROR("futex_noasync");
67 abort();
68 }
69 }
70skip_futex_wait:
71
72 /* Tell waker thread than we are running. */
73 uatomic_or(&waiter->state, WAITER_RUNNING);
74
75 /*
76 * Wait until waker thread lets us know it's ok to tear down
77 * memory allocated for struct lttng_waiter.
78 */
79 for (i = 0; i < WAIT_ATTEMPTS; i++) {
80 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
81 break;
82 }
83 caa_cpu_relax();
84 }
85 while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
86 poll(NULL, 0, 10);
87 }
88 assert(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
89 DBG("End of waiter wait period");
90}
91
92/*
93 * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
94 * execution. In this scheme, the waiter owns the node memory, and we only allow
95 * it to free this memory when it sees the WAITER_TEARDOWN flag.
96 */
97LTTNG_HIDDEN
98void lttng_waiter_wake_up(struct lttng_waiter *waiter)
99{
100 cmm_smp_mb();
101 assert(uatomic_read(&waiter->state) == WAITER_WAITING);
102 uatomic_set(&waiter->state, WAITER_WOKEN_UP);
103 if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
104 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
105 NULL, NULL, 0) < 0) {
106 PERROR("futex_noasync");
107 abort();
108 }
109 }
110 /* Allow teardown of struct urcu_wait memory. */
111 uatomic_or(&waiter->state, WAITER_TEARDOWN);
112}
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