linux-kernel - Re: [PATCH V2] workqueue: fix possible race condition when rescuer VS pwq-release

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Date:	Wed, 16 Apr 2014 09:25:16 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: [PATCH V2] workqueue: fix possible race condition when rescuer
 VS pwq-release

On 04/16/2014 12:47 AM, Tejun Heo wrote:
> On Fri, Mar 28, 2014 at 08:07:58PM +0800, Lai Jiangshan wrote:
>> +static inline void get_unbound_pwq(struct pool_workqueue *pwq)
>> +{
>> +	if (pwq->wq->flags & WQ_UNBOUND)
>> +		get_pwq(pwq);
>> +}
>> +
>>  /**
>>   * put_pwq - put a pool_workqueue reference
>>   * @pwq: pool_workqueue to put
>> @@ -1075,6 +1081,12 @@ static void put_pwq(struct pool_workqueue *pwq)
>>  	schedule_work(&pwq->unbound_release_work);
>>  }
>>  
>> +static inline void put_unbound_pwq(struct pool_workqueue *pwq)
>> +{
>> +	if (pwq->wq->flags & WQ_UNBOUND)
>> +		put_pwq(pwq);
>> +}
> 
> Ugh... please drop these helpers.
> 
>> +		get_unbound_pwq(pwq);
> 
> Why not just do get_pwq() here?

V1 patch just do get_pwq().

> 
> Thanks.
> 

1) Our aim is to protect unbound pwq, not percpu pwq which can't be be protected by get_pwq().
2) get_pwq() will make reviewers confused/surprised, destroy_workqueue() may destroy percpu pwqs
   with ref > 1. At least we need to add more comments explain this behavior. Origin comments:
		/*
		 * The base ref is never dropped on per-cpu pwqs.  Directly
		 * free the pwqs and wq.
		 */
3) get_unbound_pwq() self document.

Thanks,
Lai
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