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

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Date:	Thu, 20 Dec 2012 06:13:44 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jean Delvare <khali@...ux-fr.org>
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

On Thu, Dec 20, 2012 at 11:22:02AM +0100, Jean Delvare wrote:
> 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?
> 

Hmm, you are right - it only happens with 0. I thought I had also seen it with
other values.

> > > 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.
> 
Turns out 3.6 is EOL, so we'll only need it in 3.7.

Guenter
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