From 3e348fccfa971cf81fe9fcf3489bf011979957e3 Mon Sep 17 00:00:00 2001 From: Alan Modra Date: Wed, 30 Nov 2011 14:13:57 +1030 Subject: [PATCH] re PR libgomp/51249 (semaphore implemetation for linux leaves threads blocked) PR libgomp/51249 * config/linux/sem.h: Rewrite. * config/linux/sem.c: Rewrite. From-SVN: r181831 --- libgomp/ChangeLog | 12 ++++++-- libgomp/config/linux/sem.c | 60 ++++++++++++++++++++++++++------------ libgomp/config/linux/sem.h | 58 +++++++++++++++++++++++++++--------- 3 files changed, 94 insertions(+), 36 deletions(-) diff --git a/libgomp/ChangeLog b/libgomp/ChangeLog index e24813884db..e9148f6375f 100644 --- a/libgomp/ChangeLog +++ b/libgomp/ChangeLog @@ -1,3 +1,9 @@ +2011-11-30 Alan Modra + + PR libgomp/51249 + * config/linux/sem.h: Rewrite. + * config/linux/sem.c: Rewrite. + 2011-11-28 Richard Henderson * libgomp.h (enum memmodel): New. @@ -316,7 +322,7 @@ Tobias Burnus PR fortran/32049 - * configure.ac: + * configure.ac: * configure: Regenerate. 2010-10-06 Marcus Shawcroft @@ -1080,7 +1086,7 @@ (gomp_new_thread_pool, gomp_free_pool_helper, gomp_free_thread): New functions. (gomp_team_start): Create new pool if current thread doesn't have - one. Use pool fields instead of global gomp_* variables. + one. Use pool fields instead of global gomp_* variables. Initialize thread_pool field for new threads. Clear single_count. Change last argument from ws to team, don't create new team, set ts.work_share to &team->work_shares[0] and clear @@ -1312,7 +1318,7 @@ inlines. * config/posix/bar.c (gomp_barrier_init): Clear generation field. (gomp_barrier_wait_end): Change second argument to - gomp_barrier_state_t. + gomp_barrier_state_t. (gomp_team_barrier_wait, gomp_team_barrier_wait_end, gomp_team_barrier_wake): New functions. * config/linux/mutex.c: Include wait.h instead of libgomp.h and diff --git a/libgomp/config/linux/sem.c b/libgomp/config/linux/sem.c index ea981024c5d..3f2fc998819 100644 --- a/libgomp/config/linux/sem.c +++ b/libgomp/config/linux/sem.c @@ -1,4 +1,4 @@ -/* Copyright (C) 2005, 2008, 2009 Free Software Foundation, Inc. +/* Copyright (C) 2005, 2008, 2009, 2011 Free Software Foundation, Inc. Contributed by Richard Henderson . This file is part of the GNU OpenMP Library (libgomp). @@ -28,34 +28,56 @@ #include "wait.h" - void -gomp_sem_wait_slow (gomp_sem_t *sem) +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) { - int val = __sync_val_compare_and_swap (sem, 0, -1); - if (val > 0) + 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 (__sync_bool_compare_and_swap (sem, val, val - 1)) - return; + 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; } - do_wait (sem, -1); } } void gomp_sem_post_slow (gomp_sem_t *sem) { - int old, tmp = *sem, wake; - - do - { - old = tmp; - wake = old > 0 ? old + 1 : 1; - tmp = __sync_val_compare_and_swap (sem, old, wake); - } - while (old != tmp); - - futex_wake (sem, wake); + futex_wake (sem, 1); } diff --git a/libgomp/config/linux/sem.h b/libgomp/config/linux/sem.h index 9b0f0f82bac..9bf480ded36 100644 --- a/libgomp/config/linux/sem.h +++ b/libgomp/config/linux/sem.h @@ -1,4 +1,4 @@ -/* Copyright (C) 2005, 2009 Free Software Foundation, Inc. +/* Copyright (C) 2005, 2009, 2011 Free Software Foundation, Inc. Contributed by Richard Henderson . This file is part of the GNU OpenMP Library (libgomp). @@ -24,34 +24,64 @@ /* 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. */ + counting semaphore implementation uses atomic instructions and the + futex syscall, and a single 32-bit int to store semaphore state. + The low 31 bits are the count, the top bit is a flag set when some + threads may be waiting. */ #ifndef GOMP_SEM_H #define GOMP_SEM_H 1 +#include /* For INT_MIN */ + typedef int gomp_sem_t; +#define SEM_WAIT INT_MIN +#define SEM_INC 1 + +extern void gomp_sem_wait_slow (gomp_sem_t *, int); +extern void gomp_sem_post_slow (gomp_sem_t *); -static inline void gomp_sem_init (gomp_sem_t *sem, int value) +static inline void +gomp_sem_init (gomp_sem_t *sem, int value) { - *sem = value; + *sem = value * SEM_INC; } -extern void gomp_sem_wait_slow (gomp_sem_t *); -static inline void gomp_sem_wait (gomp_sem_t *sem) +static inline void +gomp_sem_destroy (gomp_sem_t *sem) { - if (!__sync_bool_compare_and_swap (sem, 1, 0)) - gomp_sem_wait_slow (sem); } -extern void gomp_sem_post_slow (gomp_sem_t *); -static inline void gomp_sem_post (gomp_sem_t *sem) +static inline void +gomp_sem_wait (gomp_sem_t *sem) { - if (!__sync_bool_compare_and_swap (sem, 0, 1)) - gomp_sem_post_slow (sem); + int count = *sem; + + while ((count & ~SEM_WAIT) != 0) + if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true, + MEMMODEL_ACQUIRE, MEMMODEL_RELAXED)) + return; + gomp_sem_wait_slow (sem, count); } -static inline void gomp_sem_destroy (gomp_sem_t *sem) +static inline void +gomp_sem_post (gomp_sem_t *sem) { -} + int count = *sem; + + /* Clear SEM_WAIT here so that if there are no more waiting threads + we transition back to the uncontended state that does not make + futex syscalls. If there are waiting threads then when one is + awoken it will set SEM_WAIT again, so other waiting threads are + woken on a future gomp_sem_post. Furthermore, the awoken thread + will wake other threads in case gomp_sem_post was called again + before it had time to set SEM_WAIT. */ + while (!__atomic_compare_exchange_n (sem, &count, + (count + SEM_INC) & ~SEM_WAIT, true, + MEMMODEL_RELEASE, MEMMODEL_RELAXED)) + continue; + if (__builtin_expect (count & SEM_WAIT, 0)) + gomp_sem_post_slow (sem); +} #endif /* GOMP_SEM_H */ -- 2.30.2