Sync lttng-modules ABI version in internal kernel-ioctl.h
[lttng-tools.git] / src / common / futex.c
... / ...
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1/*
2 * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
3 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
4 *
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License, version 2 only,
7 * as published by the Free Software Foundation.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * You should have received a copy of the GNU General Public License along
15 * with this program; if not, write to the Free Software Foundation, Inc.,
16 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
17 */
18
19#define _LGPL_SOURCE
20#include <limits.h>
21#include <unistd.h>
22#include <urcu.h>
23#include <urcu/futex.h>
24
25#include <common/common.h>
26
27#include "futex.h"
28
29/*
30 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
31 * "nto1" added to all function signature.
32 *
33 * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
34 * git tree for a detail example of this scheme being used. futex_async() is
35 * the urcu wrapper over the futex() sycall.
36 *
37 * There is also a formal verification available in the git tree.
38 *
39 * branch: formal-model
40 * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
41 *
42 * Ref: git://git.lttng.org/userspace-rcu.git
43 */
44
45/*
46 * Update futex according to active or not. This scheme is used to wake every
47 * libust waiting on the shared memory map futex hence the INT_MAX used in the
48 * futex() call. If active, we set the value and wake everyone else we indicate
49 * that we are gone (cleanup() case).
50 */
51LTTNG_HIDDEN
52void futex_wait_update(int32_t *futex, int active)
53{
54 if (active) {
55 uatomic_set(futex, 1);
56 if (futex_async(futex, FUTEX_WAKE,
57 INT_MAX, NULL, NULL, 0) < 0) {
58 PERROR("futex_async");
59 abort();
60 }
61 } else {
62 uatomic_set(futex, 0);
63 }
64
65 DBG("Futex wait update active %d", active);
66}
67
68/*
69 * Prepare futex.
70 */
71LTTNG_HIDDEN
72void futex_nto1_prepare(int32_t *futex)
73{
74 uatomic_set(futex, -1);
75 cmm_smp_mb();
76
77 DBG("Futex n to 1 prepare done");
78}
79
80/*
81 * Wait futex.
82 */
83LTTNG_HIDDEN
84void futex_nto1_wait(int32_t *futex)
85{
86 cmm_smp_mb();
87
88 if (uatomic_read(futex) != -1)
89 goto end;
90 while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
91 switch (errno) {
92 case EWOULDBLOCK:
93 /* Value already changed. */
94 goto end;
95 case EINTR:
96 /* Retry if interrupted by signal. */
97 break; /* Get out of switch. */
98 default:
99 /* Unexpected error. */
100 PERROR("futex_async");
101 abort();
102 }
103 }
104end:
105 DBG("Futex n to 1 wait done");
106}
107
108/*
109 * Wake 1 futex.
110 */
111LTTNG_HIDDEN
112void futex_nto1_wake(int32_t *futex)
113{
114 if (caa_unlikely(uatomic_read(futex) != -1))
115 goto end;
116 uatomic_set(futex, 0);
117 if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
118 PERROR("futex_async");
119 abort();
120 }
121end:
122 DBG("Futex n to 1 wake done");
123}
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