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099e26bd DG |
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 | |
21 | #include <sys/syscall.h> | |
22 | #include <unistd.h> | |
23 | #include <urcu.h> | |
24 | #include <urcu/futex.h> | |
25 | ||
26 | #include <lttngerr.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 | * Prepare futex. | |
48 | */ | |
49 | void futex_nto1_prepare(int32_t *futex) | |
50 | { | |
51 | uatomic_set(futex, -1); | |
52 | cmm_smp_mb(); | |
53 | ||
54 | DBG("Futex n to 1 prepare done"); | |
55 | } | |
56 | ||
57 | /* | |
58 | * Wait futex. | |
59 | */ | |
60 | void futex_nto1_wait(int32_t *futex) | |
61 | { | |
62 | cmm_smp_mb(); | |
63 | ||
64 | if (uatomic_read(futex) == -1) { | |
65 | futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0); | |
66 | } | |
67 | ||
68 | DBG("Futex n to 1 wait done"); | |
69 | } | |
70 | ||
71 | /* | |
72 | * Wake 1 futex. | |
73 | */ | |
74 | void futex_nto1_wake(int32_t *futex) | |
75 | { | |
76 | if (unlikely(uatomic_read(futex) == -1)) { | |
77 | uatomic_set(futex, 0); | |
78 | futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0); | |
79 | } | |
80 | ||
81 | DBG("Futex n to 1 wake done"); | |
82 | } |