linux-kernel - Re: [PATCH v4] PM / Memory-hotplug: Avoid task freezing failures

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Date:	Mon, 21 Nov 2011 10:23:19 -0800
From:	Tejun Heo <tj@...nel.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, pavel@....cz, lenb@...nel.org,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-mm@...ck.org,
	Chen Gong <gong.chen@...ux.intel.com>
Subject: Re: [PATCH v4] PM / Memory-hotplug: Avoid task freezing failures

On Mon, Nov 21, 2011 at 11:42:54PM +0530, Srivatsa S. Bhat wrote:
> The lock_system_sleep() function is used in the memory hotplug code at
> several places in order to implement mutual exclusion with hibernation.
> However, this function tries to acquire the 'pm_mutex' lock using
> mutex_lock() and hence blocks in TASK_UNINTERRUPTIBLE state if it doesn't
> get the lock. This would lead to task freezing failures and hence
> hibernation failure as a consequence, even though the hibernation call path
> successfully acquired the lock.
> 
> But it is to be noted that, since this task tries to acquire pm_mutex, if it
> blocks due to this, we are *100% sure* that this task is not going to run
> as long as hibernation sequence is in progress, since hibernation releases
> 'pm_mutex' only at the very end, when everything is done.
> And this means, this task is going to be anyway blocked for much more longer
> than what the freezer intends to achieve; which means, freezing and thawing
> doesn't really make any difference to this task!
> 
> So, to fix freezing failures, we just ask the freezer to skip freezing this
> task, since it is already "frozen enough".
> 
> But instead of calling freezer_do_not_count() and freezer_count() as it is,
> we use only the relevant parts of those functions, because restrictions
> such as 'the task should be a userspace one' etc., might not be relevant in
> this scenario.
> 
> v4: Redesigned the whole fix, to ask the freezer to skip freezing the task
>     which is blocked trying to acquire 'pm_mutex' lock.
> 
> v3: Tejun suggested avoiding busy-looping by adding an msleep() since
>     it is not guaranteed that we will get frozen immediately.
> 
> v2: Tejun pointed problems with using mutex_lock_interruptible() in a
>     while loop, when signals not related to freezing are involved.
>     So, replaced it with mutex_trylock().
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks a lot. :)

-- 
tejun
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