Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

From: Jean Delvare
Date: Thu Dec 20 2012 - 05:22:16 EST

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

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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