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From: Jean Delvare <khali@linux-fr.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
	Juergen Beisert <jbe@pengutronix.de>
Subject: Re: [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Thu, 20 Dec 2012 10:22:02 +0000	[thread overview]
Message-ID: <20121220112202.3ead1fe1@endymion.delvare> (raw)
In-Reply-To: <20121219224122.GA26863@roeck-us.net>

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

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WARNING: multiple messages have this Message-ID (diff)
From: Jean Delvare <khali@linux-fr.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
	Juergen Beisert <jbe@pengutronix.de>
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Thu, 20 Dec 2012 11:22:02 +0100	[thread overview]
Message-ID: <20121220112202.3ead1fe1@endymion.delvare> (raw)
In-Reply-To: <20121219224122.GA26863@roeck-us.net>

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

  reply	other threads:[~2012-12-20 10:22 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-12-19 14:40 [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors Guenter Roeck
2012-12-19 14:40 ` Guenter Roeck
2012-12-19 21:47 ` [lm-sensors] " Andrew Morton
2012-12-19 21:47   ` Andrew Morton
2012-12-19 22:41   ` [lm-sensors] " Guenter Roeck
2012-12-19 22:41     ` Guenter Roeck
2012-12-20 10:22     ` Jean Delvare [this message]
2012-12-20 10:22       ` Jean Delvare
2012-12-20 10:30       ` [lm-sensors] " Juergen Beisert
2012-12-20 10:30         ` Juergen Beisert
2012-12-20 11:00         ` [lm-sensors] " Jean Delvare
2012-12-20 11:00           ` Jean Delvare
2012-12-20 14:13       ` [lm-sensors] " Guenter Roeck
2012-12-20 14:13         ` Guenter Roeck
2012-12-19 22:21 ` [lm-sensors] " Jean Delvare
2012-12-19 22:21   ` Jean Delvare
2012-12-19 23:01   ` [lm-sensors] " Guenter Roeck
2012-12-19 23:01     ` Guenter Roeck
2012-12-20 11:48     ` [lm-sensors] " Jean Delvare
2012-12-20 11:48       ` Jean Delvare

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