From: Andrew Morton <akpm@linux-foundation.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
Juergen Beisert <jbe@pengutronix.de>,
Jean Delvare <khali@linux-fr.org>
Subject: Re: [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Wed, 19 Dec 2012 21:47:21 +0000 [thread overview]
Message-ID: <20121219134721.bbde212f.akpm@linux-foundation.org> (raw)
In-Reply-To: <1355928015-26212-1-git-send-email-linux@roeck-us.net>
On Wed, 19 Dec 2012 06:40:15 -0800
Guenter Roeck <linux@roeck-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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WARNING: multiple messages have this Message-ID (diff)
From: Andrew Morton <akpm@linux-foundation.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
Juergen Beisert <jbe@pengutronix.de>,
Jean Delvare <khali@linux-fr.org>
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Wed, 19 Dec 2012 13:47:21 -0800 [thread overview]
Message-ID: <20121219134721.bbde212f.akpm@linux-foundation.org> (raw)
In-Reply-To: <1355928015-26212-1-git-send-email-linux@roeck-us.net>
On Wed, 19 Dec 2012 06:40:15 -0800
Guenter Roeck <linux@roeck-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?
next prev parent reply other threads:[~2012-12-19 21:47 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 ` Andrew Morton [this message]
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 ` [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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