Re: [virtio-dev] Re: [PATCH v1 1/2] virtio-balloon: tweak config_changed implementation

[Wei Wang]

On 01/04/2019 12:42 AM, Michael S. Tsirkin wrote:
> On Thu, Jan 03, 2019 at 01:31:01PM +0800, Wei Wang wrote:
> > virtio-ccw has deadlock issues with reading config registers inside the
> > interrupt context, so we tweak the virtballoon_changed implementation
> > by moving the config read operations into the related workqueue contexts.
> >
> > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> > ---
> >   drivers/virtio/virtio_balloon.c | 54 ++++++++++++++++++++---------------------
> >   1 file changed, 26 insertions(+), 28 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 728ecd1..9a82a11 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -394,33 +394,15 @@ static void virtballoon_changed(struct virtio_device *vdev)
> >   {
> >   	struct virtio_balloon *vb = vdev->priv;
> >   	unsigned long flags;
> > -	s64 diff = towards_target(vb);
> > -
> > -	if (diff) {
> > -		spin_lock_irqsave(&vb->stop_update_lock, flags);
> > -		if (!vb->stop_update)
> > -			queue_work(system_freezable_wq,
> > -				   &vb->update_balloon_size_work);
> > -		spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> > -	}
> >   
> > -	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> > -		virtio_cread(vdev, struct virtio_balloon_config,
> > -			     free_page_report_cmd_id, &vb->cmd_id_received);
> There's one problem with this approach:
>
> previously updating the cmd_id_received here would immediately
> stop the report in send_free_pages.
>
> With this approach we are waiting for the wq to schedule,
> which might be blocked waiting for report to complete.
> So host can no longer quickly stop the report in progress.
>
> A simple work-around would be to set some kind of flag whenever there
> is a change interrupt, then have send_free_pages test it
> and re-read cmd_id_received.
>
> Needs to be an atomic I guess ...

Yes, sounds better..will update the patch.

Best,
Wei