Message-ID: <1398328643.2805.102.camel@ThinkPad-T5421.cn.ibm.com>
Subject: Re: [RFC PATCH v5 1/2] Use lock_device_hotplug() in
 cpu_probe_store() and cpu_release_store()
From: Li Zhong <zhong@linux.vnet.ibm.com>
To: Tejun Heo <tj@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, gregkh@linuxfoundation.org,
        rafael.j.wysocki@intel.com, toshi.kani@hp.com
Date: Thu, 24 Apr 2014 16:37:23 +0800
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On Wed, 2014-04-23 at 10:39 -0400, Tejun Heo wrote:
> On Wed, Apr 23, 2014 at 10:00:26AM +0800, Li Zhong wrote:
> > If you remove cpu0, then the cpu0 directory will be removed, together
> > with the "online" file in the directory, while some other process might
> > be writing 0 or 1 to it.
> > 
> > But here, for the probe/release, take "release" for example, if user
> > writes something that stands for cpu0 to it, the cpu0 will be removed, 
> > and the cpu0 directory and the files under it will be removed. But
> > "release" itself is not removed.
> > 
> > They are attributes of cpu_subsys, not of some specific cpus.
> 
> OIC, so the file itself which triggers removal doesn't get removed.
> Hmmm... but can't you still fall into deadlock?  If on/offline takes
> the same lock under active protection which is also taken while
> removing the cpu files, it doesn't really matter whether the release
> file itself is removed in the process or not.  You can still have ABBA
> deadlock.  What am I missing here?

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? 

Thanks, Zhong
> 
> Thanks.
> 


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