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virtiofsd: prevent fv_queue_thread() vs virtio_loop() races 

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 <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2019-07-17 16:05:57 +01:00 committed by Dr. David Alan Gilbert
parent 620e9d8d9c
commit e7b337326d
1 changed files with 33 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
tools/virtiofsd/fuse_virtio.c
View File

@ -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;
}