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SubjectRe: [PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control_disable()
Date
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?

Cheers,
Longman

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