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Date: Sun, 13 Sep 2015 10:32:44 +0200
From: Ingo Molnar <mingo@kernel.org>
To: John Stultz <john.stultz@linaro.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
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        Miroslav Lichvar <mlichvar@redhat.com>,
        Prarit Bhargava <prarit@redhat.com>,
        Richard Cochran <richardcochran@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, stable@vger.kernel.org
Subject: Re: [PATCH 1/2] time: Fix timekeeping_freqadjust()'s incorrect use
 of abs() instead of abs64()
Message-ID: <20150913083244.GA29934@gmail.com>
References: <1441840051-20244-1-git-send-email-john.stultz@linaro.org>
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* John Stultz <john.stultz@linaro.org> wrote:

> 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 dc491596f639438 (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.

>  	/* Sort out the magnitude of the correction */
> -	tick_error = abs(tick_error);
> +	tick_error = abs64(tick_error);

Ugh, and we had this bug for almost two years!

Would it be possible to make abs() warn about 64-bit types during build time,
to prevent such mishaps?

Thanks,

	Ingo