1/* sem_post -- post to a POSIX semaphore. Generic futex-using version.
2 Copyright (C) 2003-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <https://www.gnu.org/licenses/>. */
19
20#include <atomic.h>
21#include <errno.h>
22#include <sysdep.h>
23#include <lowlevellock.h> /* lll_futex* used by the old code. */
24#include <futex-internal.h>
25#include <internaltypes.h>
26#include <semaphore.h>
27
28#include <shlib-compat.h>
29
30
31/* See sem_wait for an explanation of the algorithm. */
32int
33__new_sem_post (sem_t *sem)
34{
35 struct new_sem *isem = (struct new_sem *) sem;
36 int private = isem->private;
37
38#if __HAVE_64B_ATOMICS
39 /* Add a token to the semaphore. We use release MO to make sure that a
40 thread acquiring this token synchronizes with us and other threads that
41 added tokens before (the release sequence includes atomic RMW operations
42 by other threads). */
43 /* TODO Use atomic_fetch_add to make it scale better than a CAS loop? */
44 uint64_t d = atomic_load_relaxed (&isem->data);
45 do
46 {
47 if ((d & SEM_VALUE_MASK) == SEM_VALUE_MAX)
48 {
49 __set_errno (EOVERFLOW);
50 return -1;
51 }
52 }
53 while (!atomic_compare_exchange_weak_release (&isem->data, &d, d + 1));
54
55 /* If there is any potentially blocked waiter, wake one of them. */
56 if ((d >> SEM_NWAITERS_SHIFT) > 0)
57 futex_wake (((unsigned int *) &isem->data) + SEM_VALUE_OFFSET, 1, private);
58#else
59 /* Add a token to the semaphore. Similar to 64b version. */
60 unsigned int v = atomic_load_relaxed (&isem->value);
61 do
62 {
63 if ((v >> SEM_VALUE_SHIFT) == SEM_VALUE_MAX)
64 {
65 __set_errno (EOVERFLOW);
66 return -1;
67 }
68 }
69 while (!atomic_compare_exchange_weak_release
70 (&isem->value, &v, v + (1 << SEM_VALUE_SHIFT)));
71
72 /* If there is any potentially blocked waiter, wake one of them. */
73 if ((v & SEM_NWAITERS_MASK) != 0)
74 futex_wake (&isem->value, 1, private);
75#endif
76
77 return 0;
78}
79versioned_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_34);
80
81#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
82compat_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_1);
83#endif
84
85#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
86int
87attribute_compat_text_section
88__old_sem_post (sem_t *sem)
89{
90 unsigned int *futex = (unsigned int *) sem;
91
92 /* We must need to synchronize with consumers of this token, so the atomic
93 increment must have release MO semantics. */
94 atomic_write_barrier ();
95 (void) atomic_increment_val (futex);
96 /* We always have to assume it is a shared semaphore. */
97 futex_wake (futex, 1, LLL_SHARED);
98 return 0;
99}
100compat_symbol (libpthread, __old_sem_post, sem_post, GLIBC_2_0);
101#endif
102