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 */ 88