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

From: Andrew Morton
Date: Wed Dec 19 2012 - 16:47:14 EST

On Wed, 19 Dec 2012 06:40:15 -0800
Guenter Roeck <linux@xxxxxxxxxxxx> wrote:

> Commit 263a523 fixes a warning seen with W=1 due to change in
> DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> with unsigned divisors to unsigned, even if the dividend is signed and
> negative (for example, -10 / 5U = 858993457). The C standard says "If one
> operand has unsigned int type, the other operand is converted to unsigned
> int", so the compiler is not to blame.
> As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> to unsigned was not taken into account.
> Fix by checking for the divisor variable type when deciding which operation
> to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> for negative dividends divided by unsigned divisors. Mark the latter case as
> unsupported.

The changelog didn't describe the end-user visible effects of the bug.
Please always include this information. Because...

The patch is applicable to 3.7.x. Should we backport it?
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