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SubjectRe: [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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