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

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Date:	Mon, 11 May 2015 10:55:02 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] workqueue: ensure attrs-changing be sequentially

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

> 				| 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.

> 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.

-- 
tejun
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