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semaphore: fix a hangup problem under load on NetBSD hosts.

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Fix following bugs in "fallback implementation of counting semaphores
with mutex+condvar" added in c166cb72f1:
 - waiting threads are not restarted properly if more than one threads
   are waiting unblock signals in qemu_sem_timedwait()
 - possible missing pthread_cond_signal(3) calls when waiting threads
   are returned by ETIMEDOUT
 - fix an uninitialized variable
The problem is analyzed by and fix is provided by Noriyuki Soda.

Also put additional cleanup suggested by Laszlo Ersek:
 - make QemuSemaphore.count unsigned (it won't be negative)
 - check a return value of in pthread_cond_wait() in qemu_sem_wait()

Signed-off-by: Izumi Tsutsui <tsutsui@ceres.dti.ne.jp>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1372841894-10634-1-git-send-email-tsutsui@ceres.dti.ne.jp
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Izumi Tsutsui 2013-07-03 17:58:14 +09:00 committed by Anthony Liguori
parent e1d0fb378a
commit 79761c6681
2 changed files with 20 additions and 16 deletions
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2
include/qemu/thread-posix.h
View File

@ -15,7 +15,7 @@ struct QemuSemaphore {
#if defined(__APPLE__) || defined(__NetBSD__) #if defined(__APPLE__) || defined(__NetBSD__)
pthread_mutex_t lock; pthread_mutex_t lock;
pthread_cond_t cond; pthread_cond_t cond;
int count; unsigned int count;
#else #else
sem_t sem; sem_t sem;
#endif #endif

34
util/qemu-thread-posix.c
View File

@ -170,12 +170,11 @@ void qemu_sem_post(QemuSemaphore *sem)
#if defined(__APPLE__) || defined(__NetBSD__) #if defined(__APPLE__) || defined(__NetBSD__)
pthread_mutex_lock(&sem->lock); pthread_mutex_lock(&sem->lock);
if (sem->count == INT_MAX) { if (sem->count == UINT_MAX) {
rc = EINVAL; rc = EINVAL;
} else if (sem->count++ < 0) {
rc = pthread_cond_signal(&sem->cond);
} else { } else {
rc = 0; sem->count++;
rc = pthread_cond_signal(&sem->cond);
} }
pthread_mutex_unlock(&sem->lock); pthread_mutex_unlock(&sem->lock);
if (rc != 0) { if (rc != 0) {
@ -207,19 +206,21 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms)
struct timespec ts; struct timespec ts;
#if defined(__APPLE__) || defined(__NetBSD__) #if defined(__APPLE__) || defined(__NetBSD__)
rc = 0;
compute_abs_deadline(&ts, ms); compute_abs_deadline(&ts, ms);
pthread_mutex_lock(&sem->lock); pthread_mutex_lock(&sem->lock);
--sem->count; while (sem->count == 0) {
while (sem->count < 0) {
rc = pthread_cond_timedwait(&sem->cond, &sem->lock, &ts); rc = pthread_cond_timedwait(&sem->cond, &sem->lock, &ts);
if (rc == ETIMEDOUT) { if (rc == ETIMEDOUT) {
++sem->count;
break; break;
} }
if (rc != 0) { if (rc != 0) {
error_exit(rc, __func__); error_exit(rc, __func__);
} }
} }
if (rc != ETIMEDOUT) {
--sem->count;
}
pthread_mutex_unlock(&sem->lock); pthread_mutex_unlock(&sem->lock);
return (rc == ETIMEDOUT ? -1 : 0); return (rc == ETIMEDOUT ? -1 : 0);
#else #else
@ -249,16 +250,19 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms)
void qemu_sem_wait(QemuSemaphore *sem) void qemu_sem_wait(QemuSemaphore *sem)
{ {
#if defined(__APPLE__) || defined(__NetBSD__)
pthread_mutex_lock(&sem->lock);
--sem->count;
while (sem->count < 0) {
pthread_cond_wait(&sem->cond, &sem->lock);
}
pthread_mutex_unlock(&sem->lock);
#else
int rc; int rc;
#if defined(__APPLE__) || defined(__NetBSD__)
pthread_mutex_lock(&sem->lock);
while (sem->count == 0) {
rc = pthread_cond_wait(&sem->cond, &sem->lock);
if (rc != 0) {
error_exit(rc, __func__);
}
}
--sem->count;
pthread_mutex_unlock(&sem->lock);
#else
do { do {
rc = sem_wait(&sem->sem); rc = sem_wait(&sem->sem);
} while (rc == -1 && errno == EINTR); } while (rc == -1 && errno == EINTR);