sem.c revision 1.1.1.1.2.1
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 implementation uses atomic instructions and the futex syscall. */ 28 29#include "wait.h" 30 31void 32gomp_sem_wait_slow (gomp_sem_t *sem, int count) 33{ 34 /* First loop spins a while. */ 35 while (count == 0) 36 if (do_spin (sem, 0) 37 /* Spin timeout, nothing changed. Set waiting flag. */ 38 && __atomic_compare_exchange_n (sem, &count, SEM_WAIT, false, 39 MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) 40 { 41 futex_wait (sem, SEM_WAIT); 42 count = *sem; 43 break; 44 } 45 /* Something changed. If it wasn't the wait flag, we're good to go. */ 46 else if (__builtin_expect (((count = *sem) & SEM_WAIT) == 0 && count != 0, 47 1)) 48 { 49 if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, false, 50 MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) 51 return; 52 } 53 54 /* Second loop waits until semaphore is posted. We always exit this 55 loop with wait flag set, so next post will awaken a thread. */ 56 while (1) 57 { 58 unsigned int wake = count & ~SEM_WAIT; 59 int newval = SEM_WAIT; 60 61 if (wake != 0) 62 newval |= wake - SEM_INC; 63 if (__atomic_compare_exchange_n (sem, &count, newval, false, 64 MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) 65 { 66 if (wake != 0) 67 { 68 /* If we can wake more threads, do so now. */ 69 if (wake > SEM_INC) 70 gomp_sem_post_slow (sem); 71 break; 72 } 73 do_wait (sem, SEM_WAIT); 74 count = *sem; 75 } 76 } 77} 78 79void 80gomp_sem_post_slow (gomp_sem_t *sem) 81{ 82 futex_wake (sem, 1); 83} 84