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

Jean Delvare
2012-12-20T10:22:17Z

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