Re: [PATCH wq/for-3.6-fixes 3/3] workqueue: fix possible idle worker depletion during CPU_ONLINE

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 09/07/2012 04:08 AM, Tejun Heo wrote:
>>From 985aafbf530834a9ab16348300adc7cbf35aab76 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Thu, 6 Sep 2012 12:50:41 -0700
> 
> To simplify both normal and CPU hotplug paths, while CPU hotplug is in
> progress, manager_mutex is held to prevent one of the workers from
> becoming a manager and creating or destroying workers; unfortunately,
> it currently may lead to idle worker depletion which in turn can lead
> to deadlock under extreme circumstances.
> 
> Idle workers aren't allowed to become busy if there's no other idle
> worker left to create more idle workers, but during CPU_ONLINE
> gcwq_associate() is holding all managerships and all the idle workers
> can proceed to become busy before gcwq_associate() is finished.
> 
> This patch fixes the bug by releasing manager_mutexes before letting
> the rebound idle workers go.  This ensures that by the time idle
> workers check whether management is necessary, CPU_ONLINE already has
> released the positions.
> 

Could you review manage_workers_slowpath() in V4 patchset.
It has enough changelog and comments.

After the discussion,

We don't move the hotplug code outside hotplug code. it matches this requirement.

Since we introduce manage_mutex(), any palace should be allowed to grab it
when its context allows. So it is not hotplug code's responsibility of this bug.

manage_workers() just use mutex_trylock() to grab the lock, it does not make
hard to do it jobs when need, and it does not try to find out the reason of fail.
so I think it is manage_workers()'s responsibility to handle this bug.
a manage_workers_slowpath() is enough to fix the bug.

=====
manage_workers_slowpath() just adds a little overhead over manage_workers(),
so we can use manage_workers_slowpath() to replace manage_workers(), thus we
can reduce the code of manage_workers() and we can do more cleanup for manage.

Thanks,
Lai