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Re: [PATCH v2 7/7] virtiofsd: Release vu_dispatch_lock when stopping que
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH v2 7/7] virtiofsd: Release vu_dispatch_lock when stopping queue |
Date: |
Mon, 15 Mar 2021 14:57:17 +0000 |
User-agent: |
Mutt/2.0.5 (2021-01-21) |
* 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
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
- Re: [PATCH v2 2/7] vhost-user: Fix double-close on slave_read() error path, (continued)
- [PATCH v2 4/7] vhost-user: Convert slave channel to QIOChannelSocket, Greg Kurz, 2021/03/12
- [PATCH v2 3/7] vhost-user: Factor out duplicated slave_fd teardown code, Greg Kurz, 2021/03/12
- [PATCH v2 6/7] vhost-user: Monitor slave channel in vhost_user_read(), Greg Kurz, 2021/03/12
- [PATCH v2 5/7] vhost-user: Introduce nested event loop in vhost_user_read(), Greg Kurz, 2021/03/12
- [PATCH v2 7/7] virtiofsd: Release vu_dispatch_lock when stopping queue, Greg Kurz, 2021/03/12
- Re: [PATCH v2 7/7] virtiofsd: Release vu_dispatch_lock when stopping queue,
Dr. David Alan Gilbert <=