Re: [PATCH RFC] suspend/hibernation: Fix racing timers

[SÃren Brinkmann]

Hi John,

On Wed, 2014-07-23 at 08:55PM -0700, John Stultz wrote:
> On 07/21/2014 10:35 AM, Soren Brinkmann wrote:
> > On platforms that do not power off during suspend, successfully entering
> > suspend races with timers.
> >
> > The race happening in a couple of location is:
> >
> >   1. disable IRQs 	(e.g. arch_suspend_disable_irqs())
> >      ...
> >   2. syscore_suspend()
> >         -> tick_suspend() 	(timers are turned off here)
> >      ...
> >   3. wfi		(wait for wake-IRQ here)
> >
> > Between steps 1 and 2 the timers can still generate interrupts that are
> > not handled and stay pending until step 3. That pending IRQ causes an
> > immediate - spurious - wake.
> >
> > The solution is to remove the timekeeping suspend/resume functions from
> > the syscore functions and explictly call them at the appropriate time in
> > the suspend/hibernation patchs. I.e. timers are suspend _before_ IRQs
> > get disabled. And accordingly in the resume path.
> 
> So.. I sort of follow this, though from the description disabling
> timekeeping to turn off timers seems a little indirect (I do see that
> suspending timekeeping calls clockevents_suspend() which is the key
> part). Maybe this could be clarified in a future version of the patch
> description?
> 
> I worry that moving timekeeping_suspend earlier in the suspend process
> might cause problems where things access time in the suspend path. I
> recall these orderings have been problematic in the past, and slightly
> tweaking them can often destabilize things badly.

You're right. Just when I received this I started seeing some warning
from the kernel due to ktime_get() called with timekeeping being
suspended.
Though, stability-wise it seems to work.

> 
> I wonder if it would be better just to move the clockevent_suspend()
> call to the earlier site, that way timers are halted but timekeeping
> continues until its normal suspend point.

I'll look into this and send out a patch once I have something working.

	Thanks,
	SÃren
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