linux-kernel - Re: [PATCH RFC v2] PM / sleep: Fix racing timers

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Date:	Mon, 28 Jul 2014 12:26:07 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Soren Brinkmann <soren.brinkmann@...inx.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH RFC v2] PM / sleep: Fix racing timers

On 07/27/2014 04:18 PM, Rafael J. Wysocki wrote:
> On Friday, July 25, 2014 02:06:48 PM 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@...inx.com>
>> ---
>> Hi,
>>
>> This is my second shot at this. I followed John's suggestion to keep the
>> timekeeping suspend where it is and just move the shutdown of the clockevent
>> devices around.
> John, what do you think?

I've not had the chance to take a closer look and do any testing. I
suspect we'll need tgxl's input here as well.

The change makes sense, but ordering modifications in this area tend to
be fragile, as there are lots of implicit dependencies.

thanks
-john

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