1/* sem_post -- post to a POSIX semaphore. Generic futex-using version.
2 Copyright (C) 2003-2023 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library 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 GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <atomic.h>
20#include <errno.h>
21#include <sysdep.h>
22#include <lowlevellock.h> /* lll_futex* used by the old code. */
23#include <futex-internal.h>
24#include <internaltypes.h>
25#include <semaphore.h>
26
27#include <shlib-compat.h>
28
29
30/* See sem_wait for an explanation of the algorithm. */
31int
32__new_sem_post (sem_t *sem)
33{
34 struct new_sem *isem = (struct new_sem *) sem;
35 int private = isem->private;
36
37#if __HAVE_64B_ATOMICS
38 /* Add a token to the semaphore. We use release MO to make sure that a
39 thread acquiring this token synchronizes with us and other threads that
40 added tokens before (the release sequence includes atomic RMW operations
41 by other threads). */
42 /* TODO Use atomic_fetch_add to make it scale better than a CAS loop? */
43 uint64_t d = atomic_load_relaxed (&isem->data);
44 do
45 {
46 if ((d & SEM_VALUE_MASK) == SEM_VALUE_MAX)
47 {
48 __set_errno (EOVERFLOW);
49 return -1;
50 }
51 }
52 while (!atomic_compare_exchange_weak_release (&isem->data, &d, d + 1));
53
54 /* If there is any potentially blocked waiter, wake one of them. */
55 if ((d >> SEM_NWAITERS_SHIFT) > 0)
56 futex_wake (((unsigned int *) &isem->data) + SEM_VALUE_OFFSET, 1, private);
57#else
58 /* Add a token to the semaphore. Similar to 64b version. */
59 unsigned int v = atomic_load_relaxed (&isem->value);
60 do
61 {
62 if ((v >> SEM_VALUE_SHIFT) == SEM_VALUE_MAX)
63 {
64 __set_errno (EOVERFLOW);
65 return -1;
66 }
67 }
68 while (!atomic_compare_exchange_weak_release
69 (&isem->value, &v, v + (1 << SEM_VALUE_SHIFT)));
70
71 /* If there is any potentially blocked waiter, wake one of them. */
72 if ((v & SEM_NWAITERS_MASK) != 0)
73 futex_wake (&isem->value, 1, private);
74#endif
75
76 return 0;
77}
78versioned_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_34);
79
80#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
81compat_symbol (libpthread, __new_sem_post, sem_post, GLIBC_2_1);
82#endif
83
84#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
85int
86attribute_compat_text_section
87__old_sem_post (sem_t *sem)
88{
89 unsigned int *futex = (unsigned int *) sem;
90
91 /* We must need to synchronize with consumers of this token, so the atomic
92 increment must have release MO semantics. */
93 atomic_write_barrier ();
94 atomic_fetch_add_release (futex, 1);
95 /* We always have to assume it is a shared semaphore. */
96 futex_wake (futex, 1, LLL_SHARED);
97 return 0;
98}
99compat_symbol (libpthread, __old_sem_post, sem_post, GLIBC_2_0);
100#endif
101