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

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Date:	Fri, 31 Aug 2012 09:08:22 +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 05:17 PM, Tejun Heo wrote:
> Hello, Lai.
> 
> On Thu, Aug 30, 2012 at 05:16:01PM +0800, Lai Jiangshan wrote:
>> 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.
> 
> Can you please elaborate?  Why is it not safe if there are
> CPU_INTENSIVE items?
> 
> Thanks.
> 

Imaging there only two workers, they all have UNBOUND bit because the rebind_workers()
has not been called. The First one is processing work items, the second one is idle,
when the first one encounter the work item of rebind_workers() and handle it, at the same
the second one try to create workers and failed and go to process work items too.
but unlikely the second one encounters a CPU_INTENSIVE items, the nr_running is still
<=1 after the first one finish rebind_workers().

								nr_running.
first one:	process work item endless			+0 or +1
second one:	process the CPU_INTENSIVE item endless		+0

No one can service for manager role.

Thanks.
Lai
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