/* Copyright (C) 2005-2018 Free Software Foundation, Inc. Contributed by Richard Henderson . This file is part of the GNU Offloading and Multi Processing Library (libgomp). Libgomp is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version. Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. Under Section 7 of GPL version 3, you are granted additional permissions described in the GCC Runtime Library Exception, version 3.1, as published by the Free Software Foundation. You should have received a copy of the GNU General Public License and a copy of the GCC Runtime Library Exception along with this program; see the files COPYING3 and COPYING.RUNTIME respectively. If not, see . */ /* This is a Linux specific implementation of a semaphore synchronization mechanism for libgomp. This type is private to the library. This implementation uses atomic instructions and the futex syscall. */ #include "wait.h" void gomp_sem_wait_slow (gomp_sem_t *sem, int count) { /* First loop spins a while. */ while (count == 0) if (do_spin (sem, 0) /* Spin timeout, nothing changed. Set waiting flag. */ && __atomic_compare_exchange_n (sem, &count, SEM_WAIT, false, MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) { futex_wait (sem, SEM_WAIT); count = *sem; break; } /* Something changed. If it wasn't the wait flag, we're good to go. */ else if (__builtin_expect (((count = *sem) & SEM_WAIT) == 0 && count != 0, 1)) { if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, false, MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) return; } /* Second loop waits until semaphore is posted. We always exit this loop with wait flag set, so next post will awaken a thread. */ while (1) { unsigned int wake = count & ~SEM_WAIT; int newval = SEM_WAIT; if (wake != 0) newval |= wake - SEM_INC; if (__atomic_compare_exchange_n (sem, &count, newval, false, MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) { if (wake != 0) { /* If we can wake more threads, do so now. */ if (wake > SEM_INC) gomp_sem_post_slow (sem); break; } do_wait (sem, SEM_WAIT); count = *sem; } } } void gomp_sem_post_slow (gomp_sem_t *sem) { futex_wake (sem, 1); }