sem.h revision 1.3 
1/* Copyright (C) 2005-2013 Free Software Foundation, Inc.
2   Contributed by Richard Henderson <rth@redhat.com>.
3
4   This file is part of the GNU OpenMP Library (libgomp).
5
6   Libgomp is free software; you can redistribute it and/or modify it
7   under the terms of the GNU General Public License as published by
8   the Free Software Foundation; either version 3, or (at your option)
9   any later version.
10
11   Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
12   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
13   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
14   more details.
15
16   Under Section 7 of GPL version 3, you are granted additional
17   permissions described in the GCC Runtime Library Exception, version
18   3.1, as published by the Free Software Foundation.
19
20   You should have received a copy of the GNU General Public License and
21   a copy of the GCC Runtime Library Exception along with this program;
22   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
23   <http://www.gnu.org/licenses/>.  */
24
25/* This is a Linux specific implementation of a semaphore synchronization
26   mechanism for libgomp.  This type is private to the library.  This
27   counting semaphore implementation uses atomic instructions and the
28   futex syscall, and a single 32-bit int to store semaphore state.
29   The low 31 bits are the count, the top bit is a flag set when some
30   threads may be waiting.  */
31
32#ifndef GOMP_SEM_H
33#define GOMP_SEM_H 1
34
35#include <limits.h> /* For INT_MIN */
36
37typedef int gomp_sem_t;
38#define SEM_WAIT INT_MIN
39#define SEM_INC 1
40
41extern void gomp_sem_wait_slow (gomp_sem_t *, int);
42extern void gomp_sem_post_slow (gomp_sem_t *);
43
44static inline void
45gomp_sem_init (gomp_sem_t *sem, int value)
46{
47  *sem = value * SEM_INC;
48}
49
50static inline void
51gomp_sem_destroy (gomp_sem_t *sem)
52{
53}
54
55static inline void
56gomp_sem_wait (gomp_sem_t *sem)
57{
58  int count = *sem;
59
60  while ((count & ~SEM_WAIT) != 0)
61    if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true,
62				     MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
63      return;
64  gomp_sem_wait_slow (sem, count);
65}
66
67static inline void
68gomp_sem_post (gomp_sem_t *sem)
69{
70  int count = *sem;
71
72  /* Clear SEM_WAIT here so that if there are no more waiting threads
73     we transition back to the uncontended state that does not make
74     futex syscalls.  If there are waiting threads then when one is
75     awoken it will set SEM_WAIT again, so other waiting threads are
76     woken on a future gomp_sem_post.  Furthermore, the awoken thread
77     will wake other threads in case gomp_sem_post was called again
78     before it had time to set SEM_WAIT.  */
79  while (!__atomic_compare_exchange_n (sem, &count,
80				       (count + SEM_INC) & ~SEM_WAIT, true,
81				       MEMMODEL_RELEASE, MEMMODEL_RELAXED))
82    continue;
83
84  if (__builtin_expect (count & SEM_WAIT, 0))
85    gomp_sem_post_slow (sem);
86}
87#endif /* GOMP_SEM_H */
88