Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

[Wei Wang]

On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
> On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:
> >   
> > +static void virtballoon_changed(struct virtio_device *vdev)
> > +{
> > +	struct virtio_balloon *vb = vdev->priv;
> > +	unsigned long flags;
> > +	__u32 cmd_id;
> > +	s64 diff = towards_target(vb);
> > +
> > +	if (diff) {
> > +		spin_lock_irqsave(&vb->stop_update_lock, flags);
> > +		if (!vb->stop_update)
> Why do you ignore stop_update for freeze?
> This means new wq entries can be added during remove
> causing use after free issues.

I think stop_update isn't needed, because the lock has already been 
handled internally by the APIs. Similar examples like 
mem_cgroup_css_free() in "mm/memcontrol.c", there is no such locks used 
for cancel_work_sync(&memcg->high_work).

Best,
Wei