linux-kernel - Re: [RFC PATCH v5 1/2] Use lock_device_hotplug() in cpu_probe_store() and cpu_release

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Date:	Thu, 24 Apr 2014 10:32:08 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	rafael.j.wysocki@...el.com, toshi.kani@...com
Subject: Re: [RFC PATCH v5 1/2] Use lock_device_hotplug() in
 cpu_probe_store() and cpu_release_store()

Hello,

On Thu, Apr 24, 2014 at 04:37:23PM +0800, Li Zhong wrote:
> On Wed, 2014-04-23 at 10:39 -0400, Tejun Heo wrote:
> After thinking it harder, I still couldn't see ABBA here ... 
> 
> the active protection taken here is for "probe/release" which will not
> be waited for removing something like "online" under cpu#? Or my
> assumption that s_active for different files are different locks are
> completely wrong? Or I missed something else? 

I'm probably confused about the locking.  I was thinking a scenario
like the following.

A. CPU on/offline

   grabs s_active protection of online node
   grabs cpu subsys mutex
   perform on/offline
   releases cpu subsys mutex
   releases s_active protection of online node

B. CPU release

   grabs s_active protection of release node
   grabs cpu subsys mutex
   performs removal of the CPU
   removes the online node
   releases cpu subsys mutex
   releases s_active protection of release node

A nests cpu subsys mutex under s_active of the online node.  B nests
s_active of the online node under the cpu subsys mutex.  What am I
missing?

Thanks.

-- 
tejun
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