From e7b337326d594b71b07cd6dbb332c49c122c80a4 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Wed, 17 Jul 2019 16:05:57 +0100 Subject: [PATCH] virtiofsd: prevent fv_queue_thread() vs virtio_loop() races MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit We call into libvhost-user from the virtqueue handler thread and the vhost-user message processing thread without a lock. There is nothing protecting the virtqueue handler thread if the vhost-user message processing thread changes the virtqueue or memory table while it is running. This patch introduces a read-write lock. Virtqueue handler threads are readers. The vhost-user message processing thread is a writer. This will allow concurrency for multiqueue in the future while protecting against fv_queue_thread() vs virtio_loop() races. Note that the critical sections could be made smaller but it would be more invasive and require libvhost-user changes. Let's start simple and improve performance later, if necessary. Another option would be an RCU-style approach with lighter-weight primitives. Signed-off-by: Stefan Hajnoczi Reviewed-by: Daniel P. Berrangé Signed-off-by: Dr. David Alan Gilbert --- tools/virtiofsd/fuse_virtio.c | 34 +++++++++++++++++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c index fb8d6d1379..f6242f9338 100644 --- a/tools/virtiofsd/fuse_virtio.c +++ b/tools/virtiofsd/fuse_virtio.c @@ -58,6 +58,18 @@ struct fv_VuDev { VuDev dev; struct fuse_session *se; + /* + * Either handle virtqueues or vhost-user protocol messages. Don't do + * both at the same time since that could lead to race conditions if + * virtqueues or memory tables change while another thread is accessing + * them. + * + * The assumptions are: + * 1. fv_queue_thread() reads/writes to virtqueues and only reads VuDev. + * 2. virtio_loop() reads/writes virtqueues and VuDev. + */ + pthread_rwlock_t vu_dispatch_rwlock; + /* * The following pair of fields are only accessed in the main * virtio_loop @@ -415,6 +427,8 @@ static void *fv_queue_thread(void *opaque) qi->qidx, qi->kick_fd); while (1) { struct pollfd pf[2]; + int ret; + pf[0].fd = qi->kick_fd; pf[0].events = POLLIN; pf[0].revents = 0; @@ -461,6 +475,9 @@ static void *fv_queue_thread(void *opaque) fuse_log(FUSE_LOG_ERR, "Eventfd_read for queue: %m\n"); break; } + /* Mutual exclusion with virtio_loop() */ + ret = pthread_rwlock_rdlock(&qi->virtio_dev->vu_dispatch_rwlock); + assert(ret == 0); /* there is no possible error case */ /* out is from guest, in is too guest */ unsigned int in_bytes, out_bytes; vu_queue_get_avail_bytes(dev, q, &in_bytes, &out_bytes, ~0, ~0); @@ -469,6 +486,7 @@ static void *fv_queue_thread(void *opaque) "%s: Queue %d gave evalue: %zx available: in: %u out: %u\n", __func__, qi->qidx, (size_t)evalue, in_bytes, out_bytes); + while (1) { bool allocated_bufv = false; struct fuse_bufvec bufv; @@ -597,6 +615,8 @@ static void *fv_queue_thread(void *opaque) free(elem); elem = NULL; } + + pthread_rwlock_unlock(&qi->virtio_dev->vu_dispatch_rwlock); } out: pthread_mutex_destroy(&ch.lock); @@ -711,6 +731,8 @@ int virtio_loop(struct fuse_session *se) while (!fuse_session_exited(se)) { struct pollfd pf[1]; + bool ok; + int ret; pf[0].fd = se->vu_socketfd; pf[0].events = POLLIN; pf[0].revents = 0; @@ -735,7 +757,15 @@ int virtio_loop(struct fuse_session *se) } assert(pf[0].revents & POLLIN); fuse_log(FUSE_LOG_DEBUG, "%s: Got VU event\n", __func__); - if (!vu_dispatch(&se->virtio_dev->dev)) { + /* Mutual exclusion with fv_queue_thread() */ + ret = pthread_rwlock_wrlock(&se->virtio_dev->vu_dispatch_rwlock); + assert(ret == 0); /* there is no possible error case */ + + ok = vu_dispatch(&se->virtio_dev->dev); + + pthread_rwlock_unlock(&se->virtio_dev->vu_dispatch_rwlock); + + if (!ok) { fuse_log(FUSE_LOG_ERR, "%s: vu_dispatch failed\n", __func__); break; } @@ -877,6 +907,7 @@ int virtio_session_mount(struct fuse_session *se) se->vu_socketfd = data_sock; se->virtio_dev->se = se; + pthread_rwlock_init(&se->virtio_dev->vu_dispatch_rwlock, NULL); vu_init(&se->virtio_dev->dev, 2, se->vu_socketfd, fv_panic, fv_set_watch, fv_remove_watch, &fv_iface); @@ -892,6 +923,7 @@ void virtio_session_close(struct fuse_session *se) } free(se->virtio_dev->qi); + pthread_rwlock_destroy(&se->virtio_dev->vu_dispatch_rwlock); free(se->virtio_dev); se->virtio_dev = NULL; }