linux-kernel - Re: [PATCH 1/2] time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()

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Message-ID: <20150913083244.GA29934@gmail.com>
Date:	Sun, 13 Sep 2015 10:32:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Nuno Gonçalves <nunojpg@...il.com>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] time: Fix timekeeping_freqadjust()'s incorrect use
 of abs() instead of abs64()


* John Stultz <john.stultz@...aro.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
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