Fix: consumerd: unbalanced subbuffer 'get' when checking operation availability
[lttng-tools.git] / src / common / futex.c
... / ...
CommitLineData
1/*
2 * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
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 <limits.h>
11#include <unistd.h>
12#include <urcu.h>
13#include <urcu/futex.h>
14
15#include <common/common.h>
16
17#include "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 */
41LTTNG_HIDDEN
42void futex_wait_update(int32_t *futex, int active)
43{
44 if (active) {
45 uatomic_set(futex, 1);
46 if (futex_async(futex, FUTEX_WAKE,
47 INT_MAX, NULL, NULL, 0) < 0) {
48 PERROR("futex_async");
49 abort();
50 }
51 } else {
52 uatomic_set(futex, 0);
53 }
54
55 DBG("Futex wait update active %d", active);
56}
57
58/*
59 * Prepare futex.
60 */
61LTTNG_HIDDEN
62void futex_nto1_prepare(int32_t *futex)
63{
64 uatomic_set(futex, -1);
65 cmm_smp_mb();
66
67 DBG("Futex n to 1 prepare done");
68}
69
70/*
71 * Wait futex.
72 */
73LTTNG_HIDDEN
74void futex_nto1_wait(int32_t *futex)
75{
76 cmm_smp_mb();
77
78 if (uatomic_read(futex) != -1)
79 goto end;
80 while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
81 switch (errno) {
82 case EWOULDBLOCK:
83 /* Value already changed. */
84 goto end;
85 case EINTR:
86 /* Retry if interrupted by signal. */
87 break; /* Get out of switch. */
88 default:
89 /* Unexpected error. */
90 PERROR("futex_async");
91 abort();
92 }
93 }
94end:
95 DBG("Futex n to 1 wait done");
96}
97
98/*
99 * Wake 1 futex.
100 */
101LTTNG_HIDDEN
102void futex_nto1_wake(int32_t *futex)
103{
104 if (caa_unlikely(uatomic_read(futex) != -1))
105 goto end;
106 uatomic_set(futex, 0);
107 if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
108 PERROR("futex_async");
109 abort();
110 }
111end:
112 DBG("Futex n to 1 wake done");
113}
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