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

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Date:	Fri, 13 Feb 2015 17:03:51 +0800
From:	John Stultz <john.stultz@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Cox <alan@...ux.intel.com>,
	"Li, Aubrey" <aubrey.li@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 3/6] timekeeping: Make it safe to use the fast timekeeper
 while suspended

On Fri, Feb 13, 2015 at 10:03 AM, Rafael J. Wysocki <rjw@...ysocki.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@...ysocki.net> wrote:
>> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.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
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