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Re: [PATCH 4/4] virtiofsd: Release vu_dispatch_lock when stopping queue
From: |
Vivek Goyal |
Subject: |
Re: [PATCH 4/4] virtiofsd: Release vu_dispatch_lock when stopping queue |
Date: |
Tue, 9 Mar 2021 09:00:46 -0500 |
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 <groug@kaod.org>
Looks good to me.
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Vivek
> ---
> 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
>