Re: alarm timer/timerfd expiration does not abort suspend operation

From: Gabriel Beddingfield
Date: Fri Feb 10 2017 - 14:25:01 EST

Hi John,

Re-sending because VGER rejected my hipster HTML mail... sorry!

On Fri, Feb 10, 2017 at 10:49 AM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> I see a few ways to fix it:
>> 1. Create a wakeup_source for each timerfd, and if it's an alarm timer
>> call
>> __pm_stay_awake() in timerfd_triggered() and __pm_relax() in
>> timerfd_read().
>> 2. call pm_system_wakeup() in alarmtimer_fired()
>> 3. call `if (isalarm(ctx)) pm_system_wakeup();' in timerfd_triggered()
>> 4. call __pm_wakeup_event(ws, 2 * MSECS_PER_SEC) in alarmtimer_fired()
>> 5. call `if (isalarm(ctc)) __pm_wakeup_event(ws, 2 * MSECS_PER_SEC);'
>> in
>> timerfd_triggered() (using a static struct wakeup_source).
>> * #4 Matches the current behavior of the "happy case" if and only if
>> userspace is using the 'wakeup' system, otherwise doesn't change any
>> behavior. But, I wonder how many people think the current behavior is a
>> bug.
> The approach you took in your patch looks basically ok to me, though I
> think the __pm_wakeup_event() method in #4 sounds safer, just to avoid
> the problematic issue if no one is waiting on the fd.
> Though I worry I'm not quite understanding the con for that case
> properly, so maybe you can clarify what concerns you there?

The concern is born of my personal experience: I was ignorant of the
"wakeup_count" protocol, and so I wasn't using it. Because of this
__pm_wakeup_event() would not block a suspend because I never wrote to
wakeup_count. On the other hand, method #2 will work unconditionally.