Update lttng kernel modules names
[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 *
5 * This program is free software; you can redistribute it and/or
6 * modify it under the terms of the GNU General Public License
7 * as published by the Free Software Foundation; only version 2
8 * of the License.
9 *
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
14 *
15 * You should have received a copy of the GNU General Public License
16 * along with this program; if not, write to the Free Software
17 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
18 */
19
20#define _GNU_SOURCE
0fdd1e2c 21#include <limits.h>
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22#include <sys/syscall.h>
23#include <unistd.h>
24#include <urcu.h>
25#include <urcu/futex.h>
26
db758600 27#include <common/error.h>
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28
29#include "futex.h"
30
31/*
32 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
33 * "nto1" added to all function signature.
34 *
35 * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
36 * git tree for a detail example of this scheme being used. futex_async() is
37 * the urcu wrapper over the futex() sycall.
38 *
39 * There is also a formal verification available in the git tree.
40 *
41 * branch: formal-model
42 * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
43 *
44 * Ref: git://git.lttng.org/userspace-rcu.git
45 */
46
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47/*
48 * Update futex according to active or not. This scheme is used to wake every
49 * libust waiting on the shared memory map futex hence the INT_MAX used in the
50 * futex() call. If active, we set the value and wake everyone else we indicate
51 * that we are gone (cleanup() case).
52 */
53void futex_wait_update(int32_t *futex, int active)
54{
55 if (active) {
56 uatomic_set(futex, 1);
57 futex_async(futex, FUTEX_WAKE,
58 INT_MAX, NULL, NULL, 0);
59 } else {
60 uatomic_set(futex, 0);
61 }
62
63 DBG("Futex wait update active %d", active);
64}
65
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66/*
67 * Prepare futex.
68 */
69void futex_nto1_prepare(int32_t *futex)
70{
71 uatomic_set(futex, -1);
72 cmm_smp_mb();
73
74 DBG("Futex n to 1 prepare done");
75}
76
77/*
78 * Wait futex.
79 */
80void futex_nto1_wait(int32_t *futex)
81{
82 cmm_smp_mb();
83
84 if (uatomic_read(futex) == -1) {
85 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
86 }
87
88 DBG("Futex n to 1 wait done");
89}
90
91/*
92 * Wake 1 futex.
93 */
94void futex_nto1_wake(int32_t *futex)
95{
6e59ae26 96 if (caa_unlikely(uatomic_read(futex) == -1)) {
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97 uatomic_set(futex, 0);
98 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
99 }
100
101 DBG("Futex n to 1 wake done");
102}
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