linux-kernel - Re: [PATCH cgroup/for-next 1/3] cgroup: remove redundancy online

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Message-ID: <0cd291b3-012d-412d-b5d3-72e96330d468@huaweicloud.com>
Date: Wed, 27 Aug 2025 08:54:02 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: tj@...nel.org, hannes@...xchg.org, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org, lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH cgroup/for-next 1/3] cgroup: remove redundancy online_cnt



On 2025/8/26 22:14, Michal Koutný wrote:
> Hello Ridong.
> 
> On Tue, Aug 26, 2025 at 03:40:20AM +0000, Chen Ridong <chenridong@...weicloud.com> wrote:
>> @@ -5949,7 +5944,7 @@ static void css_killed_work_fn(struct work_struct *work)
>>  		css_put(css);
>>  		/* @css can't go away while we're holding cgroup_mutex */
>>  		css = css->parent;
>> -	} while (css && atomic_dec_and_test(&css->online_cnt));
>> +	} while (css && css_is_dying(css) && !css->nr_descendants);
> 
> Here it's OK...
> 
>>  
>>  	cgroup_unlock();
>>  }
>> @@ -5960,7 +5955,7 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
>>  	struct cgroup_subsys_state *css =
>>  		container_of(ref, struct cgroup_subsys_state, refcnt);
>>  
>> -	if (atomic_dec_and_test(&css->online_cnt)) {
>> +	if (!css->nr_descendants) {
>>  		INIT_WORK(&css->destroy_work, css_killed_work_fn);
>>  		queue_work(cgroup_offline_wq, &css->destroy_work);
>>  	}
> 
> ... but here in percpu_ref's confirm callback you're accessing
> nr_descendants without cgroup_mutex where the atomic would have
> prevented the data race.
> 

Thank you very much, Michal, I miss this case.

> Also the semantics of online_cnt and nr_descendants is slightly
> different -- killed vs offlined. Or can you add a description why
> they're same (after workqueue split)?
> 

The nr_descendants value does not include the dying CSS; it only reflects the number of currently
living descendants. Moreover, a CSS can only be taken offline when no living CSS remains. Therefore,
I believe the online_cnt is no longer necessary. This is unrelated to workqueue splitting.

-- 
Best regards,
Ridong