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Date:	Thu, 23 Feb 2012 22:32:58 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, markgross@...gnar.org,
	Matthew Garrett <mjg@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2

On Thursday, February 23, 2012, Rafael J. Wysocki wrote:
> On Thursday, February 23, 2012, Srivatsa S. Bhat wrote:
> > 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?
> 
> That's a good point, so I think it may be a good idea to do
> mutex_lock_interruptible() in pm_autosleep_set_state() instead.

Now that I think of it, perhaps it's a good idea to just make
pm_autosleep_lock() do mutex_lock_interruptible() _and_ make
pm_autosleep_set_state() use pm_autosleep_lock().

What do you think?

Rafael
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