virtiofsd: Release vu_dispatch_lock when stopping queue

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>
Message-Id: <20210312092212.782255-8-groug@kaod.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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);
     }
 }