From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751305Ab2LSWlS (ORCPT ); Wed, 19 Dec 2012 17:41:18 -0500 Received: from mail.active-venture.com ([67.228.131.205]:57791 "EHLO mail.active-venture.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750933Ab2LSWlN (ORCPT ); Wed, 19 Dec 2012 17:41:13 -0500 X-Originating-IP: 108.223.40.66 Date: Wed, 19 Dec 2012 14:41:22 -0800 From: Guenter Roeck To: Andrew Morton Cc: linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org, Juergen Beisert , Jean Delvare Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors Message-ID: <20121219224122.GA26863@roeck-us.net> References: <1355928015-26212-1-git-send-email-linux@roeck-us.net> <20121219134721.bbde212f.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121219134721.bbde212f.akpm@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote: > On Wed, 19 Dec 2012 06:40:15 -0800 > Guenter Roeck 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... > 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). > 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.