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

[Rafael J. Wysocki]

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.

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

Thanks,
Rafael
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