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

[Waiman Long <llong@redhat.com>]

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