sem.h revision 1.11 
1/* Copyright (C) 2005-2020 Free Software Foundation, Inc.
2   Contributed by Richard Henderson <rth@redhat.com>.
3
4   This file is part of the GNU Offloading and Multi Processing Library
5   (libgomp).
6
7   Libgomp is free software; you can redistribute it and/or modify it
8   under the terms of the GNU General Public License as published by
9   the Free Software Foundation; either version 3, or (at your option)
10   any later version.
11
12   Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
13   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
14   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
15   more details.
16
17   Under Section 7 of GPL version 3, you are granted additional
18   permissions described in the GCC Runtime Library Exception, version
19   3.1, as published by the Free Software Foundation.
20
21   You should have received a copy of the GNU General Public License and
22   a copy of the GCC Runtime Library Exception along with this program;
23   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
24   <http://www.gnu.org/licenses/>.  */
25
26/* This is a Linux specific implementation of a semaphore synchronization
27   mechanism for libgomp.  This type is private to the library.  This
28   counting semaphore implementation uses atomic instructions and the
29   futex syscall, and a single 32-bit int to store semaphore state.
30   The low 31 bits are the count, the top bit is a flag set when some
31   threads may be waiting.  */
32
33#ifndef GOMP_SEM_H
34#define GOMP_SEM_H 1
35
36typedef int gomp_sem_t;
37#define SEM_WAIT (-__INT_MAX__ - 1)
38#define SEM_INC 1
39
40extern void gomp_sem_wait_slow (gomp_sem_t *, int);
41extern void gomp_sem_post_slow (gomp_sem_t *);
42
43static inline void
44gomp_sem_init (gomp_sem_t *sem, int value)
45{
46  *sem = value * SEM_INC;
47}
48
49static inline void
50gomp_sem_destroy (gomp_sem_t *sem)
51{
52}
53
54static inline void
55gomp_sem_wait (gomp_sem_t *sem)
56{
57  int count = *sem;
58
59  while ((count & ~SEM_WAIT) != 0)
60    if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true,
61				     MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
62      return;
63  gomp_sem_wait_slow (sem, count);
64}
65
66static inline void
67gomp_sem_post (gomp_sem_t *sem)
68{
69  int count = *sem;
70
71  /* Clear SEM_WAIT here so that if there are no more waiting threads
72     we transition back to the uncontended state that does not make
73     futex syscalls.  If there are waiting threads then when one is
74     awoken it will set SEM_WAIT again, so other waiting threads are
75     woken on a future gomp_sem_post.  Furthermore, the awoken thread
76     will wake other threads in case gomp_sem_post was called again
77     before it had time to set SEM_WAIT.  */
78  while (!__atomic_compare_exchange_n (sem, &count,
79				       (count + SEM_INC) & ~SEM_WAIT, true,
80				       MEMMODEL_RELEASE, MEMMODEL_RELAXED))
81    continue;
82
83  if (__builtin_expect (count & SEM_WAIT, 0))
84    gomp_sem_post_slow (sem);
85}
86#endif /* GOMP_SEM_H */
87