linux-kernel - Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND

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Message-ID: <20121220112202.3ead1fe1@endymion.delvare>
Date:	Thu, 20 Dec 2012 11:22:02 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org,
	Juergen Beisert <jbe@...gutronix.de>
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned
  divisors

Hi Guenter,

On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> > The changelog didn't describe the end-user visible effects of the bug. 
> > Please always include this information.  Because...
>
> One observed effect is that the s2c_hwmon driver reports a value of 4198403
> instead of 0 if the ADC reads 0.
> 
> Other impact is unpredictable. Problem is seen if the divisor is an unsigned
> variable or constant and the dividend is less than (divisor/2).

Really? In my own testing, the problem only shows with dividend == 0,
and even then, only when dividend is signed and divisor is not.
DIV_ROUND_CLOSEST(5, 20U) returns 0 as expected, and so do
DIV_ROUND_CLOSEST(0 / 20), DIV_ROUND_CLOSEST(0U / 20) and
DIV_ROUND_CLOSEST(0U / 20U).

Are your observations different?

> > The patch is applicable to 3.7.x.  Should we backport it?
>
> Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
> unpredictable.
> 
> 3.6 needs it as well.

-- 
Jean Delvare
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