linux-kernel - [PATCH 0/3 v2] Fixes for two recently found timekeeping bugs

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Date:   Wed, 31 May 2017 20:07:55 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     lkml <linux-kernel@...r.kernel.org>
Cc:     John Stultz <john.stultz@...aro.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>,
        Daniel Mentz <danielmentz@...gle.com>
Subject: [PATCH 0/3 v2] Fixes for two recently found timekeeping bugs

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.

As always feedback would be appreciated!

thanks
-john

v2:
* Addressed style/phrasing feedback from Ingo
* Dropped the final cleanup patch, since it can wait for 4.13-rc

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 (2):
  time: Fix clock->read(clock) race around clocksource changes
  time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting

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   |  4 +--
 kernel/time/timekeeping.c             | 59 +++++++++++++++++++++++------------
 4 files changed, 44 insertions(+), 25 deletions(-)

-- 
2.7.4