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

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Date:	Tue, 6 Oct 2015 09:05:51 +0100
From:	Greg KH <greg@...ah.com>
To:	Nuno Gonçalves <nunojpg@...il.com>
Cc:	stable@...r.kernel.org, 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 Mon, Oct 05, 2015 at 04:10:28PM +0100, Nuno Gonçalves wrote:
> 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).

It's still in the "todo" queue, along with 159 other patches that I have
yet to get to :(

thanks,

greg k-h
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