linux-kernel - Re: [tip:timers/urgent] time: Fix timekeeping_freqadjust()' s incorrect use of abs() instead of abs64()

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Date:	Mon, 5 Oct 2015 16:10:28 +0100
From:	Nuno Gonçalves <nunojpg@...il.com>
To:	stable@...r.kernel.org
Cc:	linux-tip-commits@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	john.stultz@...aro.org, mlichvar@...hat.com,
	richardcochran@...il.com, peterz@...radead.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, prarit@...hat.com,
	torvalds@...ux-foundation.org
Subject: Re: [tip:timers/urgent] time: Fix timekeeping_freqadjust()' s
 incorrect use of abs() instead of abs64()

On Sun, Sep 13, 2015 at 12:07 PM, tip-bot for John Stultz
<tipbot@...or.com> wrote:
> Commit-ID:  2619d7e9c92d524cb155ec89fd72875321512e5b
> Gitweb:     http://git.kernel.org/tip/2619d7e9c92d524cb155ec89fd72875321512e5b
> Author:     John Stultz <john.stultz@...aro.org>
> AuthorDate: Wed, 9 Sep 2015 16:07:30 -0700
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Sun, 13 Sep 2015 10:30:47 +0200
>
> time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()
>
> The internal clocksteering done for fine-grained error
> correction uses a logarithmic approximation, so any time
> adjtimex() adjusts the clock steering, timekeeping_freqadjust()
> quickly approximates the correct clock frequency over a series
> of ticks.
>
> Unfortunately, the logic in timekeeping_freqadjust(), introduced
> in commit:
>
>   dc491596f639 ("timekeeping: Rework frequency adjustments to work better w/ nohz")
>
> used the abs() function with a s64 error value to calculate the
> size of the approximated adjustment to be made.
>
> Per include/linux/kernel.h:
>
>   "abs() should not be used for 64-bit types (s64, u64, long long) - use abs64()".
>
> Thus on 32-bit platforms, this resulted in the clocksteering to
> take a quite dampended random walk trying to converge on the
> proper frequency, which caused the adjustments to be made much
> slower then intended (most easily observed when large
> adjustments are made).
>
> This patch fixes the issue by using abs64() instead.
>
> Reported-by: Nuno Gonçalves <nunojpg@...il.com>
> Tested-by: Nuno Goncalves <nunojpg@...il.com>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> Cc: <stable@...r.kernel.org> # v3.17+
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Miroslav Lichvar <mlichvar@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Richard Cochran <richardcochran@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Link: http://lkml.kernel.org/r/1441840051-20244-1-git-send-email-john.stultz@linaro.org
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  kernel/time/timekeeping.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index f6ee2e6..3739ac6 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1614,7 +1614,7 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
>         negative = (tick_error < 0);
>
>         /* Sort out the magnitude of the correction */
> -       tick_error = abs(tick_error);
> +       tick_error = abs64(tick_error);
>         for (adj = 0; tick_error > interval; adj++)
>                 tick_error >>= 1;
>

I think this got lost on its way to the linux-stable queue (or I don't
understand the stable rules).

Thanks,
Nuno
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