Fix: lttng: enable-channel: leak of popt arguments
[lttng-tools.git] / src / common / futex.cpp
1 /*
2 * Copyright (C) 2011 EfficiOS Inc.
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.hpp>
16
17 #include "futex.hpp"
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 */
41 void futex_wait_update(int32_t *futex, int active)
42 {
43 if (active) {
44 uatomic_set(futex, 1);
45 if (futex_async(futex, FUTEX_WAKE,
46 INT_MAX, NULL, NULL, 0) < 0) {
47 PERROR("futex_async");
48 abort();
49 }
50 } else {
51 uatomic_set(futex, 0);
52 }
53
54 DBG("Futex wait update active %d", active);
55 }
56
57 /*
58 * Prepare futex.
59 */
60 void futex_nto1_prepare(int32_t *futex)
61 {
62 uatomic_set(futex, -1);
63 cmm_smp_mb();
64
65 DBG("Futex n to 1 prepare done");
66 }
67
68 /*
69 * Wait futex.
70 */
71 void futex_nto1_wait(int32_t *futex)
72 {
73 cmm_smp_mb();
74
75 if (uatomic_read(futex) != -1)
76 goto end;
77 while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
78 switch (errno) {
79 case EWOULDBLOCK:
80 /* Value already changed. */
81 goto end;
82 case EINTR:
83 /* Retry if interrupted by signal. */
84 break; /* Get out of switch. */
85 default:
86 /* Unexpected error. */
87 PERROR("futex_async");
88 abort();
89 }
90 }
91 end:
92 DBG("Futex n to 1 wait done");
93 }
94
95 /*
96 * Wake 1 futex.
97 */
98 void futex_nto1_wake(int32_t *futex)
99 {
100 if (caa_unlikely(uatomic_read(futex) != -1))
101 goto end;
102 uatomic_set(futex, 0);
103 if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
104 PERROR("futex_async");
105 abort();
106 }
107 end:
108 DBG("Futex n to 1 wake done");
109 }
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