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Date:	Thu, 23 Feb 2012 11:55:31 +0530
From:	"Srivatsa S. Bhat" <>
To:	"Rafael J. Wysocki" <>
CC:	John Stultz <>,
	Linux PM list <>,
	LKML <>,
	Magnus Damm <>,,
	Matthew Garrett <>,
	Greg KH <>,
	Arve Hjønnevåg <>,
	Brian Swetland <>,
	Neil Brown <>,
	Alan Stern <>,
	Dmitry Torokhov <>
Subject: Re: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2

On 02/23/2012 03:40 AM, Rafael J. Wysocki wrote:

> On Wednesday, February 22, 2012, Srivatsa S. Bhat wrote:
>> On 02/22/2012 10:19 AM, John Stultz wrote:
>>> On Wed, 2012-02-22 at 00:31 +0100, Rafael J. Wysocki wrote:
>>>> Hi all,
>>>> After the feedback so far I've decided to follow up with a refreshed patchset.
>>>> The first two patches from the previous one went to linux-pm/linux-next
>>>> and I included the recent evdev patch from Arve (with some modifications)
>>>> to this patchset for completness.
>>> Hey Rafael, 
>>> 	Thanks again for posting this! I've started playing around with it in a
>>> kvm environment, and got the following warning after echoing off >
>>> autosleep:
>>> ...
>>> PM: resume of devices complete after 185.615 msecs
>>> PM: Finishing wakeup.
>>> Restarting tasks ... done.
>>> PM: Syncing filesystems ... done.
>>> PM: Preparing system for mem sleep
>>> Freezing user space processes ... 
>>> Freezing of tasks failed after 20.01 seconds (1 tasks refusing to freeze, wq_busy=0):
>>> bash            D ffff880015714010 
>> Ah.. I think I know what is the problem here..
>> The kernel was freezing userspace processes and meanwhile, you wrote "off"
>> to autosleep. So, as a result, this userspace process (bash) just now
>> entered kernel mode. Unfortunately, the autosleep_lock is held for too long,
>> that is, something like:
>> acquire autosleep_lock
>> modify autosleep_state
>>                                <============== "A"
>>  pm_suspend or hibernate()
>> release autosleep_lock
>> At point marked "A", we should have released the autosleep lock and only then
>> entered pm_suspend or hibernate(). Since the current code holds the lock and
>> enters suspend/hibernate, the userspace process that wrote "off" to autosleep
>> (or even userspace process that writes to /sys/power/state will end up waiting
>> on autosleep_lock, thus failing the freezing operation.)
>> So the solution is to always release the autosleep lock before entering
>> suspend/hibernation.
> Well, the autosleep lock is intentionally held around suspend/hibernation in
> try_to_suspend(), because otherwise it would be possible to trigger automatic
> suspend right after user space has disabled it.

Hmm.. I was just wondering if we could avoid holding yet another lock in the
suspend/hibernate path, if possible.. 

> I think the solution is to make pm_autosleep_lock() do a _trylock() and
> return error code if already locked.

... and also do a trylock() in pm_autosleep_set_state() right?.... that is
where John hit the problem..

By the way, I am just curious.. how difficult will this make it for userspace
to disable autosleep? I mean, would a trylock mean that the user has to keep
fighting until he finally gets a chance to disable autosleep?

Srivatsa S. Bhat

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