linux-kernel - Re: [Update][PATCH] PM / Hibernate: Fix s2disk regression related to unlock_system

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Date:	Wed, 18 Jan 2012 09:30:37 -0800
From:	Tejun Heo <tj@...nel.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, horms@...ge.net.au,
	"pavel@....cz" <pavel@....cz>, Len Brown <lenb@...nel.org>
Subject: Re: [Update][PATCH] PM / Hibernate: Fix s2disk regression related
 to unlock_system_sleep()

Hello, Srivatsa.

On Wed, Jan 18, 2012 at 10:49:09PM +0530, Srivatsa S. Bhat wrote:
> I agree, but I was trying to keep the comment from growing too long ;)

It doesn't have to be long.  It just has to give some meaning to the
decision.  AFAICS, it is correct to call try_to_freeze() on
unlock_system_sleep() regardless of 20sec window.  There's no
guarantee the unlocking task is gonna hit try_to_freeze() elsewhere
and not calling it actually makes the interface buggy.

That said, it causes a problem because unlock_system_sleep() is called
in a special context during later stage of hibernation where the usual
expectation - that a freezable task which sees a freezing condition
should freeze - doesn't hold.

The correct solution would be somehow marking that condition so that
either try_to_freeze() doesn't get invoked or gets nullified -
e.g. making the SKIP thing a counter and ensure the hibernating task
has it elevated throughout the whole process.  Alternatively, if the
code path is limited enough, using a different version of the unlock
function, unlock_system_sleep_nofreeze() or whatever, would work too -
this is a popular approach for synchronization functions which
interacts with scheduler and preemption.

For now, as a quick fix, maybe not calling try_to_freeze()
unconditionally is okay, I don't know, but it's a hack.

Thanks.

-- 
tejun
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