From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765177AbXKPDxK (ORCPT ); Thu, 15 Nov 2007 22:53:10 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756047AbXKPDw4 (ORCPT ); Thu, 15 Nov 2007 22:52:56 -0500 Received: from smtp2.linux-foundation.org ([207.189.120.14]:41351 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755804AbXKPDwz (ORCPT ); Thu, 15 Nov 2007 22:52:55 -0500 Date: Thu, 15 Nov 2007 19:52:34 -0800 From: Andrew Morton To: Arjan van de Ven Cc: Linux Kernel Mailing List , mingo@elte.hu Subject: Re: x86: disable preemption in delay_tsc() Message-Id: <20071115195234.2dc8406d.akpm@linux-foundation.org> In-Reply-To: <20071115194116.12c7a0f6@laptopd505.fenrus.org> References: <200711150400.lAF40lIr020160@hera.kernel.org> <20071115194116.12c7a0f6@laptopd505.fenrus.org> X-Mailer: Sylpheed 2.4.1 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Nov 2007 19:41:16 -0800 Arjan van de Ven wrote: > On Thu, 15 Nov 2007 04:00:47 GMT > Linux Kernel Mailing List wrote: > > > Gitweb: > > http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=35d5d08a085c56f153458c3f5d8ce24123617faf > > Commit: 35d5d08a085c56f153458c3f5d8ce24123617faf Parent: > > 7eea436433b7b18045f272562e256976f593f7c0 Author: Andrew Morton > > AuthorDate: Wed Nov 14 17:00:41 2007 -0800 > > Committer: Linus Torvalds > > CommitDate: Wed Nov 14 18:45:44 2007 -0800 > > > > x86: disable preemption in delay_tsc() > > > > Marin Mitov points out that delay_tsc() can misbehave if it is > > preempted and rescheduled on a different CPU which has a skewed TSC. > > Fix it by disabling preemption. > > > > this worries me.. this appears to effectively disable preemption during > udelay() and mdelay() loops... which are very obvious latency inducers. > > Now you can argue that if you're preemptible you should have used > msleep() and co, and I'll totally buy that. > > > Maybe we should just check if we're still on the same cpu or something, > or have a cheap way to pin a process to a cpu.... but both are longer > term solutions. > Yes, we can do better. But this bug can cause very rare failures in probably a large number of device drivers on a minorty of machines. Ugly. So I felt it best to plug it fast while people think about more sophisticated fixes.