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 | ||
c9e313bc | 9 | #include "error.hpp" |
28ab034a JG |
10 | #include "waiter.hpp" |
11 | ||
287a512f | 12 | #include <poll.h> |
28ab034a JG |
13 | #include <urcu/futex.h> |
14 | #include <urcu/uatomic.h> | |
287a512f JG |
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). */ | |
28ab034a | 23 | WAITER_WAITING = 0, |
287a512f | 24 | /* non-zero are used as masks. */ |
28ab034a JG |
25 | WAITER_WOKEN_UP = (1 << 0), |
26 | WAITER_RUNNING = (1 << 1), | |
27 | WAITER_TEARDOWN = (1 << 2), | |
287a512f JG |
28 | }; |
29 | ||
287a512f JG |
30 | void lttng_waiter_init(struct lttng_waiter *waiter) |
31 | { | |
32 | cds_wfs_node_init(&waiter->wait_queue_node); | |
33 | uatomic_set(&waiter->state, WAITER_WAITING); | |
34 | cmm_smp_mb(); | |
35 | } | |
36 | ||
37 | /* | |
38 | * User must init "waiter" before passing its memory to waker thread. | |
39 | */ | |
287a512f JG |
40 | void lttng_waiter_wait(struct lttng_waiter *waiter) |
41 | { | |
42 | unsigned int i; | |
43 | ||
44 | DBG("Beginning of waiter wait period"); | |
45 | /* Load and test condition before read state */ | |
46 | cmm_smp_rmb(); | |
47 | for (i = 0; i < WAIT_ATTEMPTS; i++) { | |
48 | if (uatomic_read(&waiter->state) != WAITER_WAITING) { | |
49 | goto skip_futex_wait; | |
50 | } | |
51 | caa_cpu_relax(); | |
52 | } | |
6e5438dc | 53 | while (uatomic_read(&waiter->state) == WAITER_WAITING) { |
cd9adb8b JG |
54 | if (!futex_noasync( |
55 | &waiter->state, FUTEX_WAIT, WAITER_WAITING, nullptr, nullptr, 0)) { | |
6e5438dc MD |
56 | /* |
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. | |
63 | */ | |
64 | continue; | |
65 | } | |
287a512f | 66 | switch (errno) { |
6e5438dc | 67 | case EAGAIN: |
287a512f JG |
68 | /* Value already changed. */ |
69 | goto skip_futex_wait; | |
70 | case EINTR: | |
71 | /* Retry if interrupted by signal. */ | |
28ab034a | 72 | break; /* Get out of switch. Check again. */ |
287a512f JG |
73 | default: |
74 | /* Unexpected error. */ | |
75 | PERROR("futex_noasync"); | |
76 | abort(); | |
77 | } | |
78 | } | |
79 | skip_futex_wait: | |
80 | ||
81 | /* Tell waker thread than we are running. */ | |
82 | uatomic_or(&waiter->state, WAITER_RUNNING); | |
83 | ||
84 | /* | |
85 | * Wait until waker thread lets us know it's ok to tear down | |
86 | * memory allocated for struct lttng_waiter. | |
87 | */ | |
88 | for (i = 0; i < WAIT_ATTEMPTS; i++) { | |
89 | if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) { | |
90 | break; | |
91 | } | |
92 | caa_cpu_relax(); | |
93 | } | |
94 | while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) { | |
cd9adb8b | 95 | poll(nullptr, 0, 10); |
287a512f | 96 | } |
a0377dfe | 97 | LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN); |
287a512f JG |
98 | DBG("End of waiter wait period"); |
99 | } | |
100 | ||
101 | /* | |
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. | |
105 | */ | |
287a512f JG |
106 | void lttng_waiter_wake_up(struct lttng_waiter *waiter) |
107 | { | |
108 | cmm_smp_mb(); | |
a0377dfe | 109 | LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING); |
287a512f JG |
110 | uatomic_set(&waiter->state, WAITER_WOKEN_UP); |
111 | if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) { | |
cd9adb8b | 112 | if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) { |
287a512f JG |
113 | PERROR("futex_noasync"); |
114 | abort(); | |
115 | } | |
116 | } | |
117 | /* Allow teardown of struct urcu_wait memory. */ | |
118 | uatomic_or(&waiter->state, WAITER_TEARDOWN); | |
119 | } |