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

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Message-ID: <alpine.DEB.2.20.1711100144410.2690@nanos>
Date:   Fri, 10 Nov 2017 01:46:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>
cc:     Kees Cook <keescook@...omium.org>,
        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 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.

> Also, I have a related patch series that renames getrawmonotonic64(),
> current_kernel_time64() and get_monotonic_coarse64() to
> ktime_get_raw_ts64(), ktime_get_coarse_real_ts64() and
> ktime_get_coarse_ts64(), for consistency, but then I couldn't
> come up with a good name for __getnstimeofday64(), as the
> __ktime_get_*() naming is already used for a number of other
> things and I did not want to overload that more. Completely
> removing __getnstimeofday64() would be handier here.

Oh yes, it's an abomination.

> > The normal timekeeping accessor functions cannot be called between
> > timekeeping_suspend() and timekeeping_resume() at all. They will emit a
> > warning and can indeed crash and burn in one of the ways you described
> > above. This does not happen on x86 because the TSC will just work on
> > systems with pstore.
> 
> Sure, except for __getnstimeofday64(), which will intentionally not warn but
> could crash in the clocksource driver (on non-x86). We do ignore the result
> from __getnstimeofday64() when timekeeping is suspended, but only after
> we call into the clocksource driver.

Right, let's get rid of it before it grows another user.

Thanks,

	tglx