Commit | Line | Data |
---|---|---|
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 | ||
21 | enum 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 | ||
30 | LTTNG_HIDDEN | |
31 | void 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 | */ | |
41 | LTTNG_HIDDEN | |
42 | void 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 | } | |
1824a395 MD |
55 | while (uatomic_read(&waiter->state) == WAITER_WAITING) { |
56 | if (!futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING, NULL, NULL, 0)) { | |
57 | /* | |
58 | * Prior queued wakeups queued by unrelated code | |
59 | * using the same address can cause futex wait to | |
60 | * return 0 even through the futex value is still | |
61 | * WAITER_WAITING (spurious wakeups). Check | |
62 | * the value again in user-space to validate | |
63 | * whether it really differs from WAITER_WAITING. | |
64 | */ | |
65 | continue; | |
66 | } | |
287a512f | 67 | switch (errno) { |
1824a395 | 68 | case EAGAIN: |
287a512f JG |
69 | /* Value already changed. */ |
70 | goto skip_futex_wait; | |
71 | case EINTR: | |
72 | /* Retry if interrupted by signal. */ | |
1824a395 | 73 | break; /* Get out of switch. Check again. */ |
287a512f JG |
74 | default: |
75 | /* Unexpected error. */ | |
76 | PERROR("futex_noasync"); | |
77 | abort(); | |
78 | } | |
79 | } | |
80 | skip_futex_wait: | |
81 | ||
82 | /* Tell waker thread than we are running. */ | |
83 | uatomic_or(&waiter->state, WAITER_RUNNING); | |
84 | ||
85 | /* | |
86 | * Wait until waker thread lets us know it's ok to tear down | |
87 | * memory allocated for struct lttng_waiter. | |
88 | */ | |
89 | for (i = 0; i < WAIT_ATTEMPTS; i++) { | |
90 | if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) { | |
91 | break; | |
92 | } | |
93 | caa_cpu_relax(); | |
94 | } | |
95 | while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) { | |
96 | poll(NULL, 0, 10); | |
97 | } | |
98 | assert(uatomic_read(&waiter->state) & WAITER_TEARDOWN); | |
99 | DBG("End of waiter wait period"); | |
100 | } | |
101 | ||
102 | /* | |
103 | * Note: lttng_waiter_wake needs waiter to stay allocated throughout its | |
104 | * execution. In this scheme, the waiter owns the node memory, and we only allow | |
105 | * it to free this memory when it sees the WAITER_TEARDOWN flag. | |
106 | */ | |
107 | LTTNG_HIDDEN | |
108 | void lttng_waiter_wake_up(struct lttng_waiter *waiter) | |
109 | { | |
110 | cmm_smp_mb(); | |
111 | assert(uatomic_read(&waiter->state) == WAITER_WAITING); | |
112 | uatomic_set(&waiter->state, WAITER_WOKEN_UP); | |
113 | if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) { | |
114 | if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, | |
115 | NULL, NULL, 0) < 0) { | |
116 | PERROR("futex_noasync"); | |
117 | abort(); | |
118 | } | |
119 | } | |
120 | /* Allow teardown of struct urcu_wait memory. */ | |
121 | uatomic_or(&waiter->state, WAITER_TEARDOWN); | |
122 | } |