libgomp/config/linux/sem.h

   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
  37 typedef int gomp_sem_t;
  38 #define SEM_WAIT INT_MIN
  39 #define SEM_INC 1
  40
  41 extern void gomp_sem_wait_slow (gomp_sem_t *, int);
  42 extern void gomp_sem_post_slow (gomp_sem_t *);
  43
  44 static inline void
  45 gomp_sem_init (gomp_sem_t *sem, int value)
  46 {
  47   *sem = value * SEM_INC;
  48 }
  49
  50 static inline void
  51 gomp_sem_destroy (gomp_sem_t *sem)
  52 {
  53 }
  54
  55 static inline void
  56 gomp_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
  67 static inline void
  68 gomp_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 */