Re: [PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control_disable()

[Waiman Long]

On 9/13/21 2:45 PM, Tejun Heo wrote:
> On Mon, Sep 13, 2021 at 02:43:44PM -0400, Waiman Long wrote:
> > > The problem with percpu_ref_is_dying() is the fact that it becomes true
> > > after percpu_ref_exit() is called in css_free_rwork_fn() which has an
> > > RCU delay. If you want to catch the fact that kill_css() has been
> > > called, we can check the CSS_DYING flag which is set in kill_css() by
> > > commit 33c35aa481786 ("cgroup: Prevent kill_css() from being called more
> > > than once"). Will that be an acceptable alternative?
> > Something like
> >
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 881ce1470beb..851e54800ad8 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -3140,6 +3140,9 @@ static void cgroup_apply_control_disable(struct cgroup
> > *cg
> >                          if (!css)
> >                                  continue;
> >
> > +                       if (css->flags & CSS_DYING)
> > +                               continue;
> > +
> So, I don't think this would be correct. It is assumed that there are no
> dying csses when control reaches this point. The right fix is making sure
> that remount path clears up dying csses before calling into this path.

Thanks for the suggestion. I will look into that.

Cheers,
Longman