Cygwin: Removed useless includes
[lttng-tools.git] / src / bin / lttng-sessiond / 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 *
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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.
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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
d14d33bf 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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12 * GNU General Public License for more details.
13 *
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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.
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17 */
18
19#define _GNU_SOURCE
0fdd1e2c 20#include <limits.h>
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21#include <unistd.h>
22#include <urcu.h>
23#include <urcu/futex.h>
24
db758600 25#include <common/error.h>
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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
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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 */
51void futex_wait_update(int32_t *futex, int active)
52{
53 if (active) {
54 uatomic_set(futex, 1);
55 futex_async(futex, FUTEX_WAKE,
56 INT_MAX, NULL, NULL, 0);
57 } else {
58 uatomic_set(futex, 0);
59 }
60
61 DBG("Futex wait update active %d", active);
62}
63
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64/*
65 * Prepare futex.
66 */
67void futex_nto1_prepare(int32_t *futex)
68{
69 uatomic_set(futex, -1);
70 cmm_smp_mb();
71
72 DBG("Futex n to 1 prepare done");
73}
74
75/*
76 * Wait futex.
77 */
78void futex_nto1_wait(int32_t *futex)
79{
80 cmm_smp_mb();
81
82 if (uatomic_read(futex) == -1) {
83 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
84 }
85
86 DBG("Futex n to 1 wait done");
87}
88
89/*
90 * Wake 1 futex.
91 */
92void futex_nto1_wake(int32_t *futex)
93{
6e59ae26 94 if (caa_unlikely(uatomic_read(futex) == -1)) {
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95 uatomic_set(futex, 0);
96 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
97 }
98
99 DBG("Futex n to 1 wake done");
100}
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