From: Guenter Roeck <linux@roeck-us.net>
To: Jean Delvare <khali@linux-fr.org>
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 14:13:44 +0000 [thread overview]
Message-ID: <20121220141344.GA32230@roeck-us.net> (raw)
In-Reply-To: <20121220112202.3ead1fe1@endymion.delvare>
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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WARNING: multiple messages have this Message-ID (diff)
From: Guenter Roeck <linux@roeck-us.net>
To: Jean Delvare <khali@linux-fr.org>
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 06:13:44 -0800 [thread overview]
Message-ID: <20121220141344.GA32230@roeck-us.net> (raw)
In-Reply-To: <20121220112202.3ead1fe1@endymion.delvare>
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
next prev parent reply other threads:[~2012-12-20 14:13 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 ` [lm-sensors] " Jean Delvare
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 ` Guenter Roeck [this message]
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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