Re: [RFC][PATCH 0/7] PM: Implement autosleep and "wake locks", take2

[Rafael J. Wysocki]

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.

Thanks,
Rafael
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