linux-kernel - Re: [PATCH 4/9 V3] workqueue: add non_manager_role_manager_mutex

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Message-ID: <503F2F51.8000301@cn.fujitsu.com>
Date:	Thu, 30 Aug 2012 17:16:01 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/9 V3] workqueue: add non_manager_role_manager_mutex_unlock()

On 08/30/2012 02:25 AM, Tejun Heo wrote:
> On Thu, Aug 30, 2012 at 12:51:55AM +0800, Lai Jiangshan wrote:
>> If hotplug code grabbed the manager_mutex and worker_thread try to create
>> a worker, the manage_worker() will return false and worker_thread go to
>> process work items. Now, on the CPU, all workers are processing work items,
>> no idle_worker left/ready for managing. It breaks the concept of workqueue
>> and it is bug.
>>
>> So when this case happens, the last idle should not go to process work,
>> it should go to sleep as usual and wait normal events. but it should
>> also be notified by the event that hotplug code release the manager_mutex.
>>
>> So we add non_manager_role_manager_mutex_unlock() to do this notify.
> 
> Hmmm... how about just running rebind_workers() from a work item?
> That way, it would be guaranteed that there alwyas will be an extra
> worker available on rebind completion.
> 
> Thanks.
> 

gcwq_unbind_fn() is unsafe even it is called from a work item.
so we need non_manager_role_manager_mutex_unlock().

If rebind_workers() is called from a work item, it is safe when there is
no CPU_INTENSIVE items. but we can't disable CPU_INTENSIVE items,
so it is still unsafe, we need non_manager_role_manager_mutex_unlock() too.

non_manager_role_manager_mutex_unlock() approach is good to fix it.
I'm writing V4 patch/approach to fix it too, it is a little more complicated,
but it has some benefit over non_manager_role_manager_mutex_unlock() approach.

Thanks.
Lai
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