semaphore: fix a hangup problem under load on NetBSD hosts.
Fix following bugs in "fallback implementation of counting semaphores
with mutex+condvar" added in c166cb72f1676855816340666c3b618beef4b976:
- 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>
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 4489abf..4de133e 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -170,12 +170,11 @@
#if defined(__APPLE__) || defined(__NetBSD__)
pthread_mutex_lock(&sem->lock);
- if (sem->count == INT_MAX) {
+ if (sem->count == UINT_MAX) {
rc = EINVAL;
- } else if (sem->count++ < 0) {
- rc = pthread_cond_signal(&sem->cond);
} else {
- rc = 0;
+ sem->count++;
+ rc = pthread_cond_signal(&sem->cond);
}
pthread_mutex_unlock(&sem->lock);
if (rc != 0) {
@@ -207,19 +206,21 @@
struct timespec ts;
#if defined(__APPLE__) || defined(__NetBSD__)
+ rc = 0;
compute_abs_deadline(&ts, ms);
pthread_mutex_lock(&sem->lock);
- --sem->count;
- while (sem->count < 0) {
+ while (sem->count == 0) {
rc = pthread_cond_timedwait(&sem->cond, &sem->lock, &ts);
if (rc == ETIMEDOUT) {
- ++sem->count;
break;
}
if (rc != 0) {
error_exit(rc, __func__);
}
}
+ if (rc != ETIMEDOUT) {
+ --sem->count;
+ }
pthread_mutex_unlock(&sem->lock);
return (rc == ETIMEDOUT ? -1 : 0);
#else
@@ -249,16 +250,19 @@
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;
+#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 {
rc = sem_wait(&sem->sem);
} while (rc == -1 && errno == EINTR);
