From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753324AbXFZBmB (ORCPT ); Mon, 25 Jun 2007 21:42:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751382AbXFZBlw (ORCPT ); Mon, 25 Jun 2007 21:41:52 -0400 Received: from dsl.myri.com ([64.172.73.26]:1966 "EHLO myri.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751272AbXFZBlw (ORCPT ); Mon, 25 Jun 2007 21:41:52 -0400 Message-ID: <46806E8D.90403@myri.com> Date: Mon, 25 Jun 2007 21:40:29 -0400 From: Loic Prylli User-Agent: Thunderbird/2.0.0.4 Mnenhy/0.7.5.0 MIME-Version: 1.0 To: Andi Kleen CC: Chuck Ebbert , linux-kernel@vger.kernel.org Subject: Re: [PATCH] MTRR: Fix race causing set_mtrr to go into infinite loop References: <468035BC.9080008@myri.com> <46803C1D.2040009@redhat.com> <200706260034.41940.ak@suse.de> In-Reply-To: <200706260034.41940.ak@suse.de> X-Enigmail-Version: 0.95.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 6/25/2007 6:34 PM, Andi Kleen wrote: > On Tuesday 26 June 2007 00:05:17 Chuck Ebbert wrote: > >> On 06/25/2007 05:38 PM, Loic Prylli wrote: >> >> [cc: Andi] >> >> >>> Processors synchronization in set_mtrr requires the .gate field >>> to be set after .count field is properly initialized. Without an explicit >>> barrier, the compiler was reordering those memory stores. That was sometimes >>> causing a processor (in ipi_handler) to see the .gate change and >>> decrement .count before the latter is set by set_mtrr() (which >>> then hangs in a infinite loop with irqs disabled). >>> > > Hmm, perhaps we should just put the smp_wmb into atomic_set(). > Near all other atomic operations have memory barriers too. I think > that would be the better fix. > > -Andi > In Documentation/atomic_ops.txt atomic_set/atomic_read are described as nothing more than a type-safe assignement or reading, without any extra semantics. For other atomic operations, the rule is that any atomic operation that doesn't return a value doesn't come with a barrier (and any operation that returns the atomic value must have memory barriers). So I guess you are suggesting to change the doc and the implementation for all arches. I should admit I did not knew a number of atomic operations did not imply memory-barriers before. But maybe the extra cost might not be completely negligible, especially if, for consistency with other "barrier-implied" atomic operations, a new memory barrier is put before and after, Are you suggested changing just atomic_set(), or also other barrier-free atomic operations :"atomic_dec", "atomic_inc", "atomic_add", "atomic_sub" ? Independently of what is done to atomic, what about not making the .gate field an atomic_t, but a simple "int" in the mttr code, since the only operations done on it are atomic_read and atomic_set? Loic