Message ID | 1372784047-12468-1-git-send-email-tsutsui@ceres.dti.ne.jp |
---|---|
State | New |
Headers | show |
On 07/02/13 18:54, Izumi Tsutsui wrote: > 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> > --- > > v2: > - make QemuSemaphore.count unsigned (it won't be negative) > - also eliminate checks for negative count values > - check a return value of in pthread_cond_wait() in qemu_sem_wait() I've compared this patch against v1. I can see one problem near the end: > > include/qemu/thread-posix.h | 2 +- > util/qemu-thread-posix.c | 26 +++++++++++++++----------- > 2 files changed, 16 insertions(+), 12 deletions(-) > > diff --git a/include/qemu/thread-posix.h b/include/qemu/thread-posix.h > index 0f30dcc..361566a 100644 > --- a/include/qemu/thread-posix.h > +++ b/include/qemu/thread-posix.h > @@ -15,7 +15,7 @@ struct QemuSemaphore { > #if defined(__APPLE__) || defined(__NetBSD__) > pthread_mutex_t lock; > pthread_cond_t cond; > - int count; > + unsigned int count; > #else > sem_t sem; > #endif > diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c > index 4489abf..44f6f30 100644 > --- a/util/qemu-thread-posix.c > +++ b/util/qemu-thread-posix.c > @@ -170,12 +170,11 @@ void qemu_sem_post(QemuSemaphore *sem) > > #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 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms) > 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 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms) > > void qemu_sem_wait(QemuSemaphore *sem) > { > + int rc; > + > #if defined(__APPLE__) || defined(__NetBSD__) > pthread_mutex_lock(&sem->lock); > - --sem->count; > - while (sem->count < 0) { > + while (sem->count == 0) { > pthread_cond_wait(&sem->cond, &sem->lock); > + if (rc != 0) { > + error_exit(rc, __func__); > + } You forgot to store the retval of pthread_cond_wait() in "rc", the (rc!=0) check refers to an indeterminate value. Otherwise it's good IMO. Thanks! Laszlo > } > + --sem->count; > pthread_mutex_unlock(&sem->lock); > #else > - int rc; > - > do { > rc = sem_wait(&sem->sem); > } while (rc == -1 && errno == EINTR); >
Laszlo Ersek wrote: > On 07/02/13 18:54, Izumi Tsutsui wrote: : > > @@ -249,16 +250,19 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms) > > > > void qemu_sem_wait(QemuSemaphore *sem) > > { > > + int rc; > > + > > #if defined(__APPLE__) || defined(__NetBSD__) > > pthread_mutex_lock(&sem->lock); > > - --sem->count; > > - while (sem->count < 0) { > > + while (sem->count == 0) { > > pthread_cond_wait(&sem->cond, &sem->lock); > > + if (rc != 0) { > > + error_exit(rc, __func__); > > + } > > You forgot to store the retval of pthread_cond_wait() in "rc", the > (rc!=0) check refers to an indeterminate value. Yes, I was a bit hasty and probably missed gcc warnings. (there is no qemu_sem_wait() callers currently) I'll post v3 one. Thanks, --- Izumi Tsutsui
diff --git a/include/qemu/thread-posix.h b/include/qemu/thread-posix.h index 0f30dcc..361566a 100644 --- a/include/qemu/thread-posix.h +++ b/include/qemu/thread-posix.h @@ -15,7 +15,7 @@ struct QemuSemaphore { #if defined(__APPLE__) || defined(__NetBSD__) pthread_mutex_t lock; pthread_cond_t cond; - int count; + unsigned int count; #else sem_t sem; #endif diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index 4489abf..44f6f30 100644 --- a/util/qemu-thread-posix.c +++ b/util/qemu-thread-posix.c @@ -170,12 +170,11 @@ void qemu_sem_post(QemuSemaphore *sem) #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 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms) 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 @@ int qemu_sem_timedwait(QemuSemaphore *sem, int ms) void qemu_sem_wait(QemuSemaphore *sem) { + int rc; + #if defined(__APPLE__) || defined(__NetBSD__) pthread_mutex_lock(&sem->lock); - --sem->count; - while (sem->count < 0) { + while (sem->count == 0) { pthread_cond_wait(&sem->cond, &sem->lock); + if (rc != 0) { + error_exit(rc, __func__); + } } + --sem->count; pthread_mutex_unlock(&sem->lock); #else - int rc; - do { rc = sem_wait(&sem->sem); } while (rc == -1 && errno == EINTR);