linux-kernel - Re: [PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control

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Date:   Mon, 13 Sep 2021 08:40:33 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <llong@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Juri Lelli <juri.lelli@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup: Fix incorrect warning from
 cgroup_apply_control_disable()

Hello,

On Mon, Sep 13, 2021 at 02:35:03PM -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?

So, it's checking that cgroup_lock_and_drain_offline() which is used by
cgroup_lock_and_drain_offline() actually waited for csses which are in the
process of going offline because some operations aren't safe when racing
against them. I think the right solution here is making sure that [u]mount
path drains the offlining cgroups rather than getting rid of the warning. I
think the warning is pointing out an actual problem.

Thanks.

-- 
tejun