linux-kernel - Re: [PATCH 3/5] workqueue: ensure attrs-changing be sequentially

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Date:	Tue, 12 May 2015 13:09:50 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] workqueue: ensure attrs-changing be sequentially

On 05/11/2015 10:55 PM, Tejun Heo wrote:
> Hey,
> 
> Prolly a better subject is "ensure attrs changes are properly
> synchronized"
> 
> On Mon, May 11, 2015 at 05:35:50PM +0800, Lai Jiangshan wrote:
>> Current modification to attrs via sysfs is not atomically.
> 
> 						 atomic.
> 
>>
>> Process A (change cpumask)	| Process B (change numa affinity)
>> wq_cpumask_store()		|
>>   wq_sysfs_prep_attrs()		|
> 				^
> 				misaligned

It is aligned in email, misaligned in quoted email, and misaligned
in `git log` and `git show`, aligned in `git commit` when I wrote
the changelog. 

I will just remove all the |.

> 
>> 				| apply_workqueue_attrs()
>>   apply_workqueue_attrs()	|
>>
>> It results that the Process B's operation is totally reverted
>> without any notification.
> 
> Yeah, right.
> 
>> This behavior is acceptable but it is sometimes unexpected.
> 
> I don't think this is an acceptable behavior.
> 
>> Sequential model on non-performance-sensitive operations is more popular
>> and preferred. So this patch moves wq_sysfs_prep_attrs() into the protection
> 
> You can just say the previous behavior is buggy.

It depends on definitions. To me, it is just a nuisance.

> 
>> under wq_pool_mutex to ensure attrs-changing be sequentially.
>>
>> This patch is also a preparation patch for next patch which change
>> the API of apply_workqueue_attrs().
> ...
>> +static void apply_wqattrs_lock(void)
>> +{
>> +	/*
>> +	 * CPUs should stay stable across pwq creations and installations.
>> +	 * Pin CPUs, determine the target cpumask for each node and create
>> +	 * pwqs accordingly.
>> +	 */
>> +	get_online_cpus();
>> +	mutex_lock(&wq_pool_mutex);
>> +}
>> +
>> +static void apply_wqattrs_unlock(void)
>> +{
>> +	mutex_unlock(&wq_pool_mutex);
>> +	put_online_cpus();
>> +}
> 
> Separate out refactoring and extending locking coverage?
> 
> Thanks.
> 

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