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

[Waiman Long]

On 9/13/21 2:35 PM, Waiman Long wrote:
> On 9/13/21 2:05 PM, Tejun Heo wrote:
> > Hello,
> >
> > On Thu, Sep 09, 2021 at 10:42:55PM -0400, Waiman Long wrote:
> > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > > index 881ce1470beb..e31bca9fcd46 100644
> > > --- a/kernel/cgroup/cgroup.c
> > > +++ b/kernel/cgroup/cgroup.c
> > > @@ -3140,7 +3140,16 @@ static void 
> > > cgroup_apply_control_disable(struct cgroup *cgrp)
> > >               if (!css)
> > >                   continue;
> > >   - WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
> > > +            /*
> > > +             * A kill_css() might have been called previously, but
> > > +             * the css may still linger for a while before being
> > > +             * removed. Skip it in this case.
> > > +             */
> > > +            if (percpu_ref_is_dying(&css->refcnt)) {
> > > +                WARN_ON_ONCE(css->parent &&
> > > +                    cgroup_ss_mask(dsct) & (1 << ss->id));
> > > +                continue;
> > > +            }
> > This warning did help me catch some gnarly bugs. Any chance we can 
> > keep it
> > for normal cases and elide it just for remounting?
>
> 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;
+
WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));

                        if (css->parent &&

Cheers,
Longman