| Message ID | 20210312092212.782255-8-groug@kaod.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | virtiofsd: Avoid potential deadlocks | expand |
* Greg Kurz (groug@kaod.org) wrote: > QEMU can stop a virtqueue by sending a VHOST_USER_GET_VRING_BASE request > to virtiofsd. As with all other vhost-user protocol messages, the thread > that runs the main event loop in virtiofsd takes the vu_dispatch lock in > write mode. This ensures that no other thread can access virtqueues or > memory tables at the same time. > > In the case of VHOST_USER_GET_VRING_BASE, the main thread basically > notifies the queue thread that it should terminate and waits for its > termination: > > main() > virtio_loop() > vu_dispatch_wrlock() > vu_dispatch() > vu_process_message() > vu_get_vring_base_exec() > fv_queue_cleanup_thread() > pthread_join() > > Unfortunately, the queue thread ends up calling virtio_send_msg() > at some point, which itself needs to grab the lock: > > fv_queue_thread() > g_list_foreach() > fv_queue_worker() > fuse_session_process_buf_int() > do_release() > lo_release() > fuse_reply_err() > send_reply() > send_reply_iov() > fuse_send_reply_iov_nofree() > fuse_send_msg() > virtio_send_msg() > vu_dispatch_rdlock() <-- Deadlock ! > > Simply have the main thread to release the lock before going to > sleep and take it back afterwards. A very similar patch was already > sent by Vivek Goyal sometime back: > > https://listman.redhat.com/archives/virtio-fs/2021-January/msg00073.html > > The only difference here is that this done in fv_queue_set_started() > because fv_queue_cleanup_thread() can also be called from virtio_loop() > without the lock being held. > > Signed-off-by: Greg Kurz <groug@kaod.org> > Reviewed-by: Vivek Goyal <vgoyal@redhat.com> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> I've queued just this 7/7 in the virtiofsd pull I'm just making at the moment. I'll leave the rest to ride the vhost-user train. Dave > --- > tools/virtiofsd/fuse_virtio.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c > index 523ee64fb7ae..3e13997406bf 100644 > --- a/tools/virtiofsd/fuse_virtio.c > +++ b/tools/virtiofsd/fuse_virtio.c > @@ -792,7 +792,13 @@ static void fv_queue_set_started(VuDev *dev, int qidx, bool started) > assert(0); > } > } else { > + /* > + * Temporarily drop write-lock taken in virtio_loop() so that > + * the queue thread doesn't block in virtio_send_msg(). > + */ > + vu_dispatch_unlock(vud); > fv_queue_cleanup_thread(vud, qidx); > + vu_dispatch_wrlock(vud); > } > } > > -- > 2.26.2 >
diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c index 523ee64fb7ae..3e13997406bf 100644 --- a/tools/virtiofsd/fuse_virtio.c +++ b/tools/virtiofsd/fuse_virtio.c @@ -792,7 +792,13 @@ static void fv_queue_set_started(VuDev *dev, int qidx, bool started) assert(0); } } else { + /* + * Temporarily drop write-lock taken in virtio_loop() so that + * the queue thread doesn't block in virtio_send_msg(). + */ + vu_dispatch_unlock(vud); fv_queue_cleanup_thread(vud, qidx); + vu_dispatch_wrlock(vud); } }
QEMU can stop a virtqueue by sending a VHOST_USER_GET_VRING_BASE request to virtiofsd. As with all other vhost-user protocol messages, the thread that runs the main event loop in virtiofsd takes the vu_dispatch lock in write mode. This ensures that no other thread can access virtqueues or memory tables at the same time. In the case of VHOST_USER_GET_VRING_BASE, the main thread basically notifies the queue thread that it should terminate and waits for its termination: main() virtio_loop() vu_dispatch_wrlock() vu_dispatch() vu_process_message() vu_get_vring_base_exec() fv_queue_cleanup_thread() pthread_join() Unfortunately, the queue thread ends up calling virtio_send_msg() at some point, which itself needs to grab the lock: fv_queue_thread() g_list_foreach() fv_queue_worker() fuse_session_process_buf_int() do_release() lo_release() fuse_reply_err() send_reply() send_reply_iov() fuse_send_reply_iov_nofree() fuse_send_msg() virtio_send_msg() vu_dispatch_rdlock() <-- Deadlock ! Simply have the main thread to release the lock before going to sleep and take it back afterwards. A very similar patch was already sent by Vivek Goyal sometime back: https://listman.redhat.com/archives/virtio-fs/2021-January/msg00073.html The only difference here is that this done in fv_queue_set_started() because fv_queue_cleanup_thread() can also be called from virtio_loop() without the lock being held. Signed-off-by: Greg Kurz <groug@kaod.org> Reviewed-by: Vivek Goyal <vgoyal@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> --- tools/virtiofsd/fuse_virtio.c | 6 ++++++ 1 file changed, 6 insertions(+)