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

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Message-Id: <20121219134721.bbde212f.akpm@linux-foundation.org>
Date:	Wed, 19 Dec 2012 13:47:21 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org,
	Juergen Beisert <jbe@...gutronix.de>,
	Jean Delvare <khali@...ux-fr.org>
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned
 divisors

On Wed, 19 Dec 2012 06:40:15 -0800
Guenter Roeck <linux@...ck-us.net> 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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