Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753322AbXL0HJX (ORCPT ); Thu, 27 Dec 2007 02:09:23 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750998AbXL0HJO (ORCPT ); Thu, 27 Dec 2007 02:09:14 -0500 Received: from mgw1.diku.dk ([130.225.96.91]:38706 "EHLO mgw1.diku.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751085AbXL0HJN (ORCPT ); Thu, 27 Dec 2007 02:09:13 -0500 Date: Thu, 27 Dec 2007 08:08:53 +0100 (CET) From: Julia Lawall To: "H. Peter Anvin" Cc: Ray Lee , autofs@linux.kernel.org, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org Subject: Re: [PATCH 1/4] fs/autofs: Use time_before, time_before_eq, etc. In-Reply-To: <4772BD6C.1010502@zytor.com> Message-ID: References: <2c0942db0712261158q34cddebeifceef6ceda683320@mail.gmail.com> <4772BD6C.1010502@zytor.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1555 Lines: 41 On Wed, 26 Dec 2007, H. Peter Anvin wrote: > Ray Lee wrote: > > On Dec 26, 2007 7:21 AM, Julia Lawall wrote: > > > - if (jiffies - ent->last_usage < timeout) > > > + if (time_before(jiffies, ent->last_usage + timeout)) > > > > I don't think this is a safe change? subtraction is always safe (if > > you think about it as 'distance'), addition isn't always safe unless > > you know the range. The time_before macro will expand that out to > > (effectively): > > > > if ( (long)(ent->last_usage + timeout) - (long)(jiffies) < 0 ) > > > > which seems to introduce an overflow condition in the first term. > > > > Dunno, I may be wrong (happens often), but at the very least what > > you've transformed it into is no longer obviously correct, and so it's > > not a great change. > > Indeed. The bottom form will have overflow issues at time > jiffies_wraparound/2, whereas the top form will have overflow issues only near > jiffies_wraparound/1. OK, so it seems like it is not such a good idea. There are, however, over 200 files that contain calls to the various time functions that follow this pattern, eg: arch/arm/kernel/ecard.c:563 if (!last || time_after(jiffies, last + 5*HZ)) { Perhaps they should be coverted to use a subtraction as well? julia -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/