On Tue, Jan 26, 2021 at 04:56:00PM +0100, Greg Kurz wrote: > On Mon, 25 Jan 2021 13:01:10 -0500 > Vivek Goyal <vgo...@redhat.com> wrote: > > > When we are shutting down virtqueues, virtio_loop() receives a message > > VHOST_USER_GET_VRING_BASE from master. We acquire ->vu_dispatch_rwlock > > and get into the process of shutting down virtqueue. In one of the > > final steps, we are waiting for fv_queue_thread() to exit/finish and > > wait with ->vu_dispatch_rwlock held. > > > > But it is possible that fv_queue_thread() itself is waiting to get > > ->vu_dispatch_rwlock (With --thread-pool=0 option). If requests > > are being processed by fv_queue_worker(), then fv_queue_worker() > > can wait for the ->vu_dispatch_rwlock, and fv_queue_thread() will > > wait for fv_queue_worker() before thread pool can be stopped. > > > > IOW, if guest is shutdown uncleanly (some sort of emergency reboot), > > it is possible that virtiofsd is processing a fs request and > > qemu initiates device shutdown sequence. In that case there seem > > to be two options. Either abort the existing request completely or > > let existing request finish. > > > > This patch is taking second approach. That is drop the ->vu_dispatch_rwlock > > temporarily so that fv_queue_thread() can finish and deadlock does not > > happen. > > > > ->vu_dispatch_rwlock provides mutual exclusion between virtio_loop() > > (handling vhost-user protocol messages) and fv_queue_thread() (handling > > fuse filesystem requests). Rational seems to be that protocol message > > might change queue memory mappings, so we don't want both to proceed > > at the same time. > > > > In this case queue is shutting down, so I hope it is fine for > > fv_queue_thread() to send response back while virtio_loop() is still > > waiting (and not handling > > It looks this lacks a \n after "fine for"
Hi Greg, Will fix. > > > any further vho-user protocol messages). > > > > IOW, assumption here is that while virto_loop is blocked processing > > VHOST_USER_GET_VRING_BASE message, it is still ok to send back the > > response on vq by fv_queue_thread(). > > > > Signed-off-by: Vivek Goyal <vgo...@redhat.com> > > --- > > tools/virtiofsd/fuse_virtio.c | 14 ++++++++++++++ > > 1 file changed, 14 insertions(+) > > > > diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c > > index 9577eaa68d..6805d8ba01 100644 > > --- a/tools/virtiofsd/fuse_virtio.c > > +++ b/tools/virtiofsd/fuse_virtio.c > > @@ -813,11 +813,20 @@ static void fv_queue_cleanup_thread(struct fv_VuDev > > *vud, int qidx) > > fuse_log(FUSE_LOG_ERR, "Eventfd_write for queue %d: %s\n", > > qidx, strerror(errno)); > > } > > + > > + /* > > + * Drop ->vu_dispath_rwlock and reacquire. We are about to wait for > > + * for fv_queue_thread() and that might require ->vu_dispatch_rwlock > > + * to finish. > > + */ > > + pthread_rwlock_unlock(&vud->vu_dispatch_rwlock); > > ret = pthread_join(ourqi->thread, NULL); > > if (ret) { > > fuse_log(FUSE_LOG_ERR, "%s: Failed to join thread idx %d err %d\n", > > __func__, qidx, ret); > > } > > + pthread_rwlock_wrlock(&vud->vu_dispatch_rwlock); > > + > > So this is assuming that fv_queue_cleanup_thread() is called with > vu_dispatch_rwlock already taken for writing, but there are no > clear evidence in the code why it should care for the locking at > all in the first place. > > On the contrary, one of its two callers is a vhost-user callback, > in which we can reasonably have this assumption, while we can > have the opposite assumption for the other one in virtio_loop(). > > This makes me think that the drop/reacquire trick should only > be done in fv_queue_set_started(), instead of... I think this sounds reasonable. I will drop lock/re-acquire in fv_queue_set_started() around the call to fv_queue_cleanup_thread(). > > > pthread_mutex_destroy(&ourqi->vq_lock); > > close(ourqi->kill_fd); > > ourqi->kick_fd = -1; > > @@ -952,7 +961,11 @@ int virtio_loop(struct fuse_session *se) > > /* > > * Make sure all fv_queue_thread()s quit on exit, as we're about to > > * free virtio dev and fuse session, no one should access them anymore. > > + * Hold ->vu_dispatch_rwlock in write mode as fv_queue_cleanup_thread() > > + * assumes mutex is locked and unlocks/re-locks it. > > */ > > + > > + pthread_rwlock_wrlock(&se->virtio_dev->vu_dispatch_rwlock); > > > ... artificially introducing another critical section here. > > The issue isn't even specific to vu_dispatch_rwlock actually : > fv_queue_cleanup_thread() shouldn't be called with any lock > held because it might sleep in pthread_join() and cause a > deadlock all the same. So I'd rather document that instead : > drop all locks before calling fv_queue_cleanup_thread(). Sounds good. Will do. > > Also, since pthread_rwlock_wrlock() can fail, I think we should > always check it's return value, at least with an assert() like > already done elsewhere. Will check return code of pthread_rwlock_wrlock() and probably use assert(). Vivek > > > for (int i = 0; i < se->virtio_dev->nqueues; i++) { > > if (!se->virtio_dev->qi[i]) { > > continue; > > @@ -961,6 +974,7 @@ int virtio_loop(struct fuse_session *se) > > fuse_log(FUSE_LOG_INFO, "%s: Stopping queue %d thread\n", > > __func__, i); > > fv_queue_cleanup_thread(se->virtio_dev, i); > > } > > + pthread_rwlock_unlock(&se->virtio_dev->vu_dispatch_rwlock); > > > > fuse_log(FUSE_LOG_INFO, "%s: Exit\n", __func__); > > >