b0e01fc73d4d8d4a6cbec6e669958fe80fa94b5e
[lttng-tools.git] / src / common / futex.cpp
1 /*
2 * Copyright (C) 2011 EfficiOS Inc.
3 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
4 *
5 * SPDX-License-Identifier: GPL-2.0-only
6 *
7 */
8
9 #define _LGPL_SOURCE
10 #include "futex.hpp"
11
12 #include <common/common.hpp>
13
14 #include <limits.h>
15 #include <unistd.h>
16 #include <urcu.h>
17 #include <urcu/futex.h>
18
19 /*
20 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
21 * "nto1" added to all function signature.
22 *
23 * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
24 * git tree for a detail example of this scheme being used. futex_async() is
25 * the urcu wrapper over the futex() sycall.
26 *
27 * There is also a formal verification available in the git tree.
28 *
29 * branch: formal-model
30 * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
31 *
32 * Ref: git://git.lttng.org/userspace-rcu.git
33 */
34
35 /*
36 * Update futex according to active or not. This scheme is used to wake every
37 * libust waiting on the shared memory map futex hence the INT_MAX used in the
38 * futex() call. If active, we set the value and wake everyone else we indicate
39 * that we are gone (cleanup() case).
40 */
41 void futex_wait_update(int32_t *futex, int active)
42 {
43 if (active) {
44 uatomic_set(futex, 1);
45 if (futex_async(futex, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 0) < 0) {
46 PERROR("futex_async");
47 abort();
48 }
49 } else {
50 uatomic_set(futex, 0);
51 }
52
53 DBG("Futex wait update active %d", active);
54 }
55
56 /*
57 * Prepare futex.
58 */
59 void futex_nto1_prepare(int32_t *futex)
60 {
61 uatomic_set(futex, -1);
62 cmm_smp_mb();
63
64 DBG("Futex n to 1 prepare done");
65 }
66
67 /*
68 * Wait futex.
69 */
70 void futex_nto1_wait(int32_t *futex)
71 {
72 cmm_smp_mb();
73
74 while (uatomic_read(futex) == -1) {
75 if (!futex_async(futex, FUTEX_WAIT, -1, nullptr, nullptr, 0)) {
76 /*
77 * Prior queued wakeups queued by unrelated code
78 * using the same address can cause futex wait to
79 * return 0 even through the futex value is still
80 * -1 (spurious wakeups). Check the value again
81 * in user-space to validate whether it really
82 * differs from -1.
83 */
84 continue;
85 }
86 switch (errno) {
87 case EAGAIN:
88 /* Value already changed. */
89 goto end;
90 case EINTR:
91 /* Retry if interrupted by signal. */
92 break; /* Get out of switch. Check again. */
93 default:
94 /* Unexpected error. */
95 PERROR("futex_async");
96 abort();
97 }
98 }
99 end:
100 DBG("Futex n to 1 wait done");
101 }
102
103 /*
104 * Wake 1 futex.
105 */
106 void futex_nto1_wake(int32_t *futex)
107 {
108 if (caa_unlikely(uatomic_read(futex) != -1))
109 goto end;
110 uatomic_set(futex, 0);
111 if (futex_async(futex, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
112 PERROR("futex_async");
113 abort();
114 }
115 end:
116 DBG("Futex n to 1 wake done");
117 }
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