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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH 1/2] cgroup: Fix incorrect warning from
 cgroup_apply_control_disable()
To:     Tejun Heo <tj@kernel.org>, Waiman Long <llong@redhat.com>
Cc:     Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Juri Lelli <juri.lelli@redhat.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210910024256.7615-1-longman@redhat.com>
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Message-ID: <e1190877-f5b7-b052-cd80-a7e558c379a4@redhat.com>
Date:   Mon, 13 Sep 2021 14:46:05 -0400
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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