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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>
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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 <125c4202-68d1-1a4e-03d6-2b18f0794ba4@redhat.com>
Message-ID: <dbb1a221-b3d2-5086-e47b-8a2c764d60ad@redhat.com>
Date:   Mon, 13 Sep 2021 14:43:44 -0400
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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