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re PR libgomp/51249 (semaphore implemetation for linux leaves threads blocked)

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PR libgomp/51249
	* config/linux/sem.h: Rewrite.
	* config/linux/sem.c: Rewrite.

From-SVN: r181831
This commit is contained in:
Alan Modra 2011-11-30 14:13:57 +10:30
parent b87974949f
commit 3e348fccfa
3 changed files with 101 additions and 43 deletions
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6
libgomp/ChangeLog
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@ -1,3 +1,9 @@
2011-11-30 Alan Modra <amodra@gmail.com>
PR libgomp/51249
* config/linux/sem.h: Rewrite.
* config/linux/sem.c: Rewrite.
2011-11-28 Richard Henderson <rth@redhat.com>
* libgomp.h (enum memmodel): New.

60
libgomp/config/linux/sem.c
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@ -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 <rth@redhat.com>.
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);
}

72
libgomp/config/linux/sem.h
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@ -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 <rth@redhat.com>.
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 <limits.h> /* For INT_MIN */
typedef int gomp_sem_t;
#define SEM_WAIT INT_MIN
#define SEM_INC 1
static inline void gomp_sem_init (gomp_sem_t *sem, int value)
{
*sem = value;
}
extern void gomp_sem_wait_slow (gomp_sem_t *);
static inline void gomp_sem_wait (gomp_sem_t *sem)
{
if (!__sync_bool_compare_and_swap (sem, 1, 0))
gomp_sem_wait_slow (sem);
}
extern void gomp_sem_wait_slow (gomp_sem_t *, int);
extern void gomp_sem_post_slow (gomp_sem_t *);
static inline void gomp_sem_post (gomp_sem_t *sem)
static inline void
gomp_sem_init (gomp_sem_t *sem, int value)
{
if (!__sync_bool_compare_and_swap (sem, 0, 1))
*sem = value * SEM_INC;
}
static inline void
gomp_sem_destroy (gomp_sem_t *sem)
{
}
static inline void
gomp_sem_wait (gomp_sem_t *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_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);
}
static inline void gomp_sem_destroy (gomp_sem_t *sem)
{
}
#endif /* GOMP_SEM_H */