Re: [PATCH RESEND] PM / sleep: Fix racing timers

[SÃren Brinkmann]

Hi Rafael,

On Thu, 2014-11-06 at 01:33AM +0100, Rafael J. Wysocki wrote:
> On Thursday, October 02, 2014 09:01:15 AM SÃren Brinkmann wrote:
> > Hi Rafael,
> 
> Hi,
> 
> Sorry for the huge delay.
> 
> > On Tue, 2014-09-23 at 01:01AM +0200, Rafael J. Wysocki wrote:
> > > On Monday, September 22, 2014 10:07:03 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()
> > > >       -> timekeeping_suspend()
> > > >        -> clockevents_notify(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 move the clockevents suspend/resume notification
> > > > out of the syscore_suspend step and explictly call them at the appropriate
> > > > time in the suspend/hibernation paths. I.e. timers are suspend _before_
> > > > IRQs get disabled. And accordingly in the resume path.
> > > > 
> > > > Signed-off-by: Soren Brinkmann <soren.brinkmann@xxxxxxxxxx>
> > > > ---
> > > > Hi,
> > > > 
> > > > there was not a lot of discussion on the last submission. Just one comment from
> > > > Rafael (https://lkml.org/lkml/2014/8/26/780), which - as I outlined in my
> > > > response, does not apply, IMHO, since the platform does not re-enable
> > > > interrupts.
> > > 
> > > Well, you just don't agree with it.
> > > 
> > > The problem with your approach is that timer interrupts aren't actually as
> > > special as you think and any other IRQF_NO_SUSPEND interrupts would have caused
> > > similar issues to appear under specific conditions.
> > > 
> > > The solution I would suggest and that actually covers all IRQF_NO_SUSPEND
> > > interrupts would be to use a wait_event() loop like the one in freeze_enter()
> > > (on top of the current linux-next or the pm-genirq branch of linux-pm.git),
> > > but wait for pm_abort_suspend to become true, to implement system suspend.
> > 
> > sorry, it took me a while since I needed to get some dependencies ported
> > to the pm-genirq base. Once I had that, it reproduced my original issue.
> > So far so good. I then looked into finding a solution following your
> > guidance. I'm not sure I really found what you had in mind, but below is
> > what I came up with, which seems to do it.
> > Please let me know how far off I am.
> > 
> > 	Thanks,
> > 	SÃren
> > 
> > -------8<------------------8<----------------8<----------------8<---------------
> > diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> > index c2744b30d5d9..a4f9914571f1 100644
> > --- a/drivers/base/power/wakeup.c
> > +++ b/drivers/base/power/wakeup.c
> > @@ -25,7 +25,7 @@
> >  bool events_check_enabled __read_mostly;
> >  
> >  /* If set and the system is suspending, terminate the suspend. */
> > -static bool pm_abort_suspend __read_mostly;
> > +bool pm_abort_suspend __read_mostly;
> >  
> >  /*
> >   * Combined counters of registered wakeup events and wakeup events in progress.
> > diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> > index 6dadb25cb0d8..e6a6de8f76d0 100644
> > --- a/kernel/power/suspend.c
> > +++ b/kernel/power/suspend.c
> > @@ -33,6 +33,7 @@
> >  
> >  static const char *pm_labels[] = { "mem", "standby", "freeze", };
> >  const char *pm_states[PM_SUSPEND_MAX];
> > +extern bool pm_abort_suspend;
> >  
> >  static const struct platform_suspend_ops *suspend_ops;
> >  static const struct platform_freeze_ops *freeze_ops;
> > @@ -294,25 +295,27 @@ static int suspend_enter(suspend_state_t state, bool *wakeup)
> >  	if (error || suspend_test(TEST_CPUS))
> >  		goto Enable_cpus;
> >  
> > -	arch_suspend_disable_irqs();
> > -	BUG_ON(!irqs_disabled());
> > -
> > -	error = syscore_suspend();
> > -	if (!error) {
> > -		*wakeup = pm_wakeup_pending();
> > -		if (!(suspend_test(TEST_CORE) || *wakeup)) {
> > -			trace_suspend_resume(TPS("machine_suspend"),
> > -				state, true);
> > -			error = suspend_ops->enter(state);
> > -			trace_suspend_resume(TPS("machine_suspend"),
> > -				state, false);
> > -			events_check_enabled = false;
> > +	while (!pm_abort_suspend) {
> 
> That won't work in general, because pm_abort_suspend may not be set on some
> platforms on wakeup.  It is only set if a wakeup interrupt triggers which
> may not be the case on ACPI systems if the BIOS has woken up the system.
> 
> But that could be addressed by making those platforms simply set pm_wakeup_pending
> in their BIOS exit path.
> 
> > +		arch_suspend_disable_irqs();
> > +		BUG_ON(!irqs_disabled());
> > +
> > +		error = syscore_suspend();
> 
> Also it shouldn't be necessary to do syscore_suspend()/syscore_resume() in
> every iteration of the loop.
> 
> > +		if (!error) {
> > +			*wakeup = pm_wakeup_pending();
> 
> Plus pm_wakeup_pending() returns true if pm_abort_suspend is set
> 
> > +			if (!(suspend_test(TEST_CORE) || *wakeup)) {
> > +				trace_suspend_resume(TPS("machine_suspend"),
> > +					state, true);
> 
> Did you try to add the loop here instead of above?  Like:
> 
> 			for (;;) {
> 				*wakeup = pm_wakeup_pending();
> 				if (*wakeup)
> 					break;

I think, that doesn't work. I chose the start/end points of the loop
to include the IRQ enable/disable calls. AFAICT, pm_abort_suspend is
set in an ISR. Without enabling interrupts the abort condition of
this loop never becomes true.

	Thanks,
	SÃren
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