Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753017Ab0KGNjy (ORCPT ); Sun, 7 Nov 2010 08:39:54 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:43494 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752968Ab0KGNjx (ORCPT ); Sun, 7 Nov 2010 08:39:53 -0500 Date: Sun, 7 Nov 2010 05:39:50 -0800 From: "Paul E. McKenney" To: Andi Kleen Cc: Davidlohr Bueso , LKML Subject: Re: [PATCH] mce: fix RCU lockdep from mce_log() Message-ID: <20101107133950.GV15561@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1288993499.2065.4.camel@cowboy> <20101106185350.GA23824@basil.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101106185350.GA23824@basil.fritz.box> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1642 Lines: 40 On Sat, Nov 06, 2010 at 07:53:50PM +0100, Andi Kleen wrote: > On Fri, Nov 05, 2010 at 06:44:59PM -0300, Davidlohr Bueso wrote: > > Hi, > > > > Please review this patch, I am not very familiar with MCE/RCU so I'm not sure that this is the correct fix (otherwise consider it a bug report :)). > > This does "fix" the message though and I can use MCE normally. > > The patch is certainly not correct. The variable needs to be read > independently of the mutex. This code is simply checking the value of the pointer, and therefore need not protect any actual dereferences. So why not replace the rcu_dereference_check_mce() with rcu_access_pointer()? If this is OK, please see the patch below. BTW, assigning the value returned by rcu_access_pointer() into a variable often indicates a bug. ;-) Thanx, Paul Signed-off-by: Paul E. McKenney diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c index 7a35b72..4d29d50 100644 --- a/arch/x86/kernel/cpu/mcheck/mce.c +++ b/arch/x86/kernel/cpu/mcheck/mce.c @@ -1625,7 +1625,7 @@ out: static unsigned int mce_poll(struct file *file, poll_table *wait) { poll_wait(file, &mce_wait, wait); - if (rcu_dereference_check_mce(mcelog.next)) + if (rcu_access_pointer(mcelog.next)) return POLLIN | POLLRDNORM; if (!mce_apei_read_done && apei_check_mce()) return POLLIN | POLLRDNORM; -- 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/