Commit 79761c66 authored by Izumi Tsutsui's avatar Izumi Tsutsui Committed by Anthony Liguori
Browse files

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



Fix following bugs in "fallback implementation of counting semaphores
with mutex+condvar" added in c166cb72:
 - 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: default avatarIzumi Tsutsui <tsutsui@ceres.dti.ne.jp>
Reviewed-by: default avatarLaszlo Ersek <lersek@redhat.com>
Message-id: 1372841894-10634-1-git-send-email-tsutsui@ceres.dti.ne.jp
Signed-off-by: default avatarAnthony Liguori <aliguori@us.ibm.com>
parent e1d0fb37

include/qemu/thread-posix.h

+1 −1
Original line number Diff line number Diff line
@@ -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

util/qemu-thread-posix.c

+16 −12
Original line number Diff line number Diff line
@@ -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) {

        pthread_cond_wait(&sem->cond, &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

    int rc;



    do {

        rc = sem_wait(&sem->sem);

    } while (rc == -1 && errno == EINTR);