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

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Message-Id: <201111212341.39359.rjw@sisk.pl>
Date:	Mon, 21 Nov 2011 23:41:39 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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 Monday, November 21, 2011, Tejun Heo wrote:
> 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. :)

Applied to linux-pm/linux-next.

Thanks,
Rafael
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