Re: [RFC PATCH 0/6] vhost-user: Shutdown/Flush slave channel properly

[Michael S. Tsirkin]

On Mon, Jan 25, 2021 at 01:01:09PM -0500, Vivek Goyal wrote:
> Hi,
> 
> We are working on DAX support in virtiofs and have some patches out of
> the tree hosted here.
> 
> https://gitlab.com/virtio-fs/qemu/-/commits/virtio-fs-dev

ping anyone wants to pick this up and post a non-rfc version?

> These patches have not been proposed for merge yet, becasue David
> Gilbert noticed that we can run into a deadlock during an emergency
> reboot of guest kernel. (echo b > /proc/sysrq-trigger).
> 
> I have provided details of deadlock in 4th path of the series with
> subject "qemu, vhost-user: Extend protocol to start/stop/flush slave
> channel".
> 
> Basic problem seems to be that we don't have a proper mechanism to
> shutdown slave channel when vhost-user device is stopping. This means
> there might be pending messages in slave channel and slave is blocked
> and waiting for response.
> 
> This is an RFC patch series to enhance vhost-user protocol to 
> properly shutdown/flush slave channel and avoid the deadlock. Though
> we faced the issue in the context of virtiofs, any vhost-user
> device using slave channel can potentially run into issues and
> can benefit from these patches.
> 
> Any feedback is welcome. Currently patches are based on out of
> tree code but after I get some feedback, I can only take pieces
> which are relevant to upstream and post separately.
> 
> Thanks
> Vivek
> 
> Vivek Goyal (6):
>   virtiofsd: Drop ->vu_dispatch_rwlock while waiting for thread to exit
>   libvhost-user: Use slave_mutex in all slave messages
>   vhost-user: Return error code from slave_read()
>   qemu, vhost-user: Extend protocol to start/stop/flush slave channel
>   libvhost-user: Add support to start/stop/flush slave channel
>   virtiofsd: Opt in for slave start/stop/shutdown functionality
> 
>  hw/virtio/vhost-user.c                    | 151 +++++++++++++++++++++-
>  subprojects/libvhost-user/libvhost-user.c | 147 +++++++++++++++++----
>  subprojects/libvhost-user/libvhost-user.h |   8 +-
>  tools/virtiofsd/fuse_virtio.c             |  20 +++
>  4 files changed, 294 insertions(+), 32 deletions(-)
> 
> -- 
> 2.25.4