linux-kernel - Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of _

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Date:   Fri, 10 Nov 2017 08:04:24 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
cc:     Arnd Bergmann <arnd@...db.de>, Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of
 __getnstimeofday()

On Thu, 9 Nov 2017, Kees Cook wrote:
> On Thu, Nov 9, 2017 at 4:46 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Fri, 10 Nov 2017, Arnd Bergmann wrote:
> >> On Fri, Nov 10, 2017 at 12:00 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> > Hmm, no. None of the regular accessor functions can be called from NMI
> >> > context safely.
> >>
> >> Right, that's what I mean: it must not get called from NMI context, but it
> >> currently is, at least for this case:
> >>
> >> NMI handler:
> >>   something bad
> >>     panic()
> >>       kmsg_dump()
> >>         pstore_dump()
> >>            pstore_record_init()
> >>              __getnstimeofday()
> >>
> >> I should probably add that to the changelog text ;-)
> >
> > Indeed.
> 
> Er, so, is this safe to call there? I've had to fix this a few times
> now, so if using ktime_get_real_fast_ns() can be used here (and
> doesn't return 0) then this is easily an improvement over the existing
> "maybe read 0" case pstore has now.

ktime_get_real_fast_ns() is NMI safe and returns

before timekeeping_suspend():	correct time

after timekeeping_suspend():	timestamp which was frozen in
      				timekeeping_suspend()

after timekeeping_resume():	correct time

Thanks,

	tglx