From: Greg Kurz <gr...@kaod.org>
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>
Reviewed-by: Vivek Goyal <vgo...@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
Message-Id: <20210312092212.782255-8-gr...@kaod.org>
Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
---
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 523ee64fb7..3e13997406 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);
}
}
--
2.30.2
