Re: [PATCH 3/6] timekeeping: Make it safe to use the fast timekeeper while suspended

John Stultz
2015-02-13T09:03:51Z

On Fri, Feb 13, 2015 at 10:03 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Friday, February 13, 2015 08:53:38 AM John Stultz wrote:
>> On Wed, Feb 11, 2015 at 12:03 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> >
>> > Theoretically, ktime_get_mono_fast_ns() may be executed after
>> > timekeeping has been suspended (or before it is resumed) which
>> > in turn may lead to undefined behavior, for example, when the
>> > clocksource read from timekeeping_get_ns() called by it is
>> > not accessible at that time.
>>
>> And the callers of the ktime_get_mono_fast_ns() have to get back a
>> value?
>
> Yes, they do.
>
>> Or can we return an error on timekeeping_suspended like we do
>> w/ __getnstimeofday64()?
>
> No, we can't.
>
>> Also, what exactly is the case when the clocksource being read isn't
>> accessible? I see this is conditionalized on
>> CLOCK_SOURCE_SUSPEND_NONSTOP, so is the concern on resume we read the
>> clocksource and its been reset causing a crazy time value?
>
> The clocksource's ->suspend method may have been called (during suspend)
> and depending on what that did we may even crash things theoretically.
>
> During resume, before the clocksource's ->resume callback, it may just
> be undefined behavior (random data etc).
>
> For system suspend as we have today the window is quite narrow, but after
> patch [4/6] from this series suspend-to-idle may suspend timekeeping and
> just sit there in idle for extended time (hours even) which broadens the
> potential exposure quite a bit.
>
> Of course, it does that with interrupts disabled, but ktime_get_mono_fast_ns()
> is for NMI, so theoretically, if an NMI happens while we're in suspend-to-idle
> with timekeeping suspended and the clocksource is not CLOCK_SOURCE_SUSPEND_NONSTOP
> and the NMI calls ktime_get_mono_fast_ns(), strange and undesirable things may
> happen.

Ok.. No objection to the approach then. But maybe could you wrap the
new logic in a halt_fast_timekeeper() function? Also is there much
value in not halting it for SUSPEND_NONSTOP clocksources? If not,
might as well halt it in all cases just to simplify the conditions we
have to keep track of in our heads. :)

thanks
-john