adfe66b1c29feb3295949253f315819c8d110d92
[lttng-tools.git] / src / bin / lttng-sessiond / futex.c
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 _GNU_SOURCE
20 #include <limits.h>
21 #include <sys/syscall.h>
22 #include <unistd.h>
23 #include <urcu.h>
24 #include <urcu/futex.h>
25
26 #include <common/error.h>
27
28 #include "futex.h"
29
30 /*
31 * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
32 * "nto1" added to all function signature.
33 *
34 * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
35 * git tree for a detail example of this scheme being used. futex_async() is
36 * the urcu wrapper over the futex() sycall.
37 *
38 * There is also a formal verification available in the git tree.
39 *
40 * branch: formal-model
41 * commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
42 *
43 * Ref: git://git.lttng.org/userspace-rcu.git
44 */
45
46 /*
47 * Update futex according to active or not. This scheme is used to wake every
48 * libust waiting on the shared memory map futex hence the INT_MAX used in the
49 * futex() call. If active, we set the value and wake everyone else we indicate
50 * that we are gone (cleanup() case).
51 */
52 void futex_wait_update(int32_t *futex, int active)
53 {
54 if (active) {
55 uatomic_set(futex, 1);
56 futex_async(futex, FUTEX_WAKE,
57 INT_MAX, NULL, NULL, 0);
58 } else {
59 uatomic_set(futex, 0);
60 }
61
62 DBG("Futex wait update active %d", active);
63 }
64
65 /*
66 * Prepare futex.
67 */
68 void futex_nto1_prepare(int32_t *futex)
69 {
70 uatomic_set(futex, -1);
71 cmm_smp_mb();
72
73 DBG("Futex n to 1 prepare done");
74 }
75
76 /*
77 * Wait futex.
78 */
79 void futex_nto1_wait(int32_t *futex)
80 {
81 cmm_smp_mb();
82
83 if (uatomic_read(futex) == -1) {
84 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
85 }
86
87 DBG("Futex n to 1 wait done");
88 }
89
90 /*
91 * Wake 1 futex.
92 */
93 void futex_nto1_wake(int32_t *futex)
94 {
95 if (caa_unlikely(uatomic_read(futex) == -1)) {
96 uatomic_set(futex, 0);
97 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
98 }
99
100 DBG("Futex n to 1 wake done");
101 }
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