From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754664AbZHKOO7 (ORCPT ); Tue, 11 Aug 2009 10:14:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753059AbZHKOO6 (ORCPT ); Tue, 11 Aug 2009 10:14:58 -0400 Received: from mtagate2.de.ibm.com ([195.212.17.162]:46267 "EHLO mtagate2.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751462AbZHKOO5 (ORCPT ); Tue, 11 Aug 2009 10:14:57 -0400 Date: Tue, 11 Aug 2009 12:56:53 +0200 From: Martin Schwidefsky To: paulmck@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner , Gerald Schaefer , manfred@colorfullife.com, Ihno Krumreich , Greg KH Subject: Re: [BUG] race of RCU vs NOHU Message-ID: <20090811125653.12c35ee8@skybase> In-Reply-To: <20090810150807.GA6791@linux.vnet.ibm.com> References: <20090807151529.2806b8b4@skybase> <20090807142957.GB6700@linux.vnet.ibm.com> <20090810142535.3e685109@skybase> <20090810150807.GA6791@linux.vnet.ibm.com> Organization: IBM Corporation X-Mailer: Claws Mail 3.7.2 (GTK+ 2.16.5; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 10 Aug 2009 08:08:07 -0700 "Paul E. McKenney" wrote: > On Mon, Aug 10, 2009 at 02:25:35PM +0200, Martin Schwidefsky wrote: > > On Fri, 7 Aug 2009 07:29:57 -0700 > > "Paul E. McKenney" wrote: > > > > > On Fri, Aug 07, 2009 at 03:15:29PM +0200, Martin Schwidefsky wrote: > > > > Hi Paul, > > > > I analysed a dump of a hanging 2.6.30 system and found what I think is > > > > a bug of RCU vs NOHZ. There are a number of patches ontop of that > > > > kernel but they should be independent of the bug. > > > > > > > > The systems has 4 cpus and uses classic RCU. cpus #0, #2 and #3 woke up > > > > recently, cpu #1 has been sleeping for 5 minutes, but there is a pending > > > > rcu batch. The timer wheel for cpu #1 is empty, it will continue to > > > > sleep for NEXT_TIMER_MAX_DELTA ticks. > > > > > > Congratulations, Martin! You have exercised what to date has been a > > > theoretical bug identified last year by Manfred Spraul. The fix is to > > > switch from CONFIG_RCU_CLASSIC to CONFIG_RCU_TREE, which was added in > > > 2.6.29. > > > > > > Of course, if you need to work with an old kernel version, you might > > > still need a patch, perhaps for the various -stable versions. If so, > > > please let me know -- otherwise, I will focus forward on CONFIG_RCU_TREE > > > rather than backwards on CONFIG_RCU_CLASSIC. > > > > SLES11 is 2.6.27 and uses classic RCU. The not-so theoretical bug is > > present there and I think it needs to be fixed :-/ > > I was afraid of that. ;-) > > Given that there are some other theoretical bugs in Classic RCU involving > interrupts and CONFIG_NO_HZ, would backporting CONFIG_TREE_RCU make more > sense than playing whack-a-mole on Classic RCU bugs? Fine with me but I don't know if SuSE/Novell is willing to accept such a big change for an existing distribution. I've put Ihno and Greg on Cc. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.