linux-kernel - Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs

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Message-ID: <CALAqxLWZpwS-L+jViRprXUAHzAw9TytDXDUY9y2_6DYK0k=YQg@mail.gmail.com>
Date:   Sat, 27 May 2017 09:16:26 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Daniel Mentz <danielmentz@...gle.com>
Subject: Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs

On Sat, May 27, 2017 at 12:38 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * John Stultz <john.stultz@...aro.org> wrote:
>
>> As part of the Linaro Linux Kernel Functional Test (LKFT)
>> effort, test failures from kselftest/timer's
>> inconsistency-check were reported connected to
>> CLOCK_MONOTONIC_RAW, on the HiKey platform.
>>
>> Digging in I found that an old issue with how sub-ns accounting
>> is handled with the RAW time which was fixed long ago with the
>> CLOCK_MONOTONIC/REALTIME ids, but missed with RAW time, was
>> present.
>>
>> Additionally, running further tests, I uncovered an issue with
>> how the clocksource read function is handled when clocksources
>> are changed, which can cause crashes.
>>
>> Both of these issues have not been uncovered in x86 based
>> testing due to x86 not using vDSO to accelerate
>> CLOCK_MONOTONIC_RAW, combined with the HiKey's arch_timer
>> clocksource being fast to access but incrementing slowly enough
>> to get multiple reads using the same counter value (which helps
>> uncover time handing issues), along with the fact that none of
>> the x86 clocksources making use of the clocksource argument
>> passed to the read function.
>>
>> This patchset addresses these two issues.
>
> AFAICS only the first two patches are fixes, the other two patches are
> cleanups/simplifications that resulted out of the debugging effort, right?

Actually the first three are fixes (ARM64 still sees discontinuities
until the vDSO is fixed), the last one is a cleanup.

thanks
-john