On Mon, Mar 08, 2021 at 01:31:41PM +0100, Greg Kurz 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 <gr...@kaod.org> > --- > tools/virtiofsd/fuse_virtio.c | 6 ++++++ > 1 file changed, 6 insertions(+)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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