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

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Date:   Tue, 30 May 2017 17:11:25 -0700
From:   Daniel Mentz <danielmentz@...gle.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Stephen Boyd <stephen.boyd@...aro.org>
Subject: Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs

I successfully tested these four patches on a HiKey (ARMv8) board
using a 4.9 kernel. I cherry picked various commits from upstream that
touched kernel/time/timekeeping.c to ensure that John's patches apply
cleanly. The inconsistency-check from kselftest that previously failed
because of CLOCK_MONOTONIC_RAW jumping backwards by 1 ns is now
passing.

Tested-by: Daniel Mentz <danielmentz@...gle.com>

On Tue, May 30, 2017 at 5:10 PM, Daniel Mentz <danielmentz@...gle.com> wrote:
> I successfully tested these four patches on a HiKey (ARMv8) board using a
> 4.9 kernel. I cherry picked various commits from upstream that touched
> kernel/time/timekeeping.c to ensure that John's patches apply cleanly. The
> inconsistency-check from kselftest that previously failed because of
> CLOCK_MONOTONIC_RAW jumping backwards by 1 ns is now passing.
>
> Tested-by: Daniel Mentz <danielmentz@...gle.com>
>
> On Fri, May 26, 2017 at 8:33 PM, 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.
>>
>> Thanks so much to help from Will Deacon in getting the needed
>> adjustments to the arm64 vDSO in place. Also thanks to Daniel
>> Mentz who also properly diagnosed the MONOTONIC_RAW issue in
>> parallel while I was woking on this patchset.
>>
>> I wanted to send these out for some initial review. I'm
>> unfortunately still chasing a third issue (related to
>> inconsistencies triggered during extreme freq adjustments) I've
>> uncovered on HiKey, which doesn't seem to be related to the
>> previous two.
>>
>> As always feedback would be appreciated!
>>
>> thanks
>> -john
>>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Miroslav Lichvar <mlichvar@...hat.com>
>> Cc: Richard Cochran <richardcochran@...il.com>
>> Cc: Prarit Bhargava <prarit@...hat.com>
>> Cc: Stephen Boyd <stephen.boyd@...aro.org>
>> Cc: Daniel Mentz <danielmentz@...gle.com>
>>
>>
>> John Stultz (3):
>>   time: Fix clock->read(clock) race around clocksource changes
>>   time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting
>>   time: Clean up CLOCK_MONOTONIC_RAW time handling
>>
>> Will Deacon (1):
>>   arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW
>>
>>  arch/arm64/kernel/vdso.c              |  5 +-
>>  arch/arm64/kernel/vdso/gettimeofday.S |  1 -
>>  include/linux/timekeeper_internal.h   |  8 +--
>>  kernel/time/timekeeping.c             | 96
>> ++++++++++++++++++++++-------------
>>  4 files changed, 66 insertions(+), 44 deletions(-)
>>
>> --
>> 2.7.4
>>
>