From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758025AbYDBQPd (ORCPT ); Wed, 2 Apr 2008 12:15:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756000AbYDBQPW (ORCPT ); Wed, 2 Apr 2008 12:15:22 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:46901 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755847AbYDBQPV (ORCPT ); Wed, 2 Apr 2008 12:15:21 -0400 Date: Wed, 2 Apr 2008 09:15:19 -0700 From: "Paul E. McKenney" To: Jens Axboe Cc: Peter Zijlstra , Ingo Molnar , Pekka J Enberg , Vegard Nossum , Linux Kernel Mailing List Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context) Message-ID: <20080402161519.GC9333@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1207085788.29991.6.camel@lappy> <20080402071709.GC12774@kernel.dk> <20080402072456.GI12774@kernel.dk> <20080402072846.GA16454@elte.hu> <20080402105539.GA5610@linux.vnet.ibm.com> <1207133961.8514.768.camel@twins> <20080402113327.GC41073@gandalf.sssup.it> <20080402133232.GE2813@linux.vnet.ibm.com> <20080402134046.GZ12774@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080402134046.GZ12774@kernel.dk> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 02, 2008 at 03:40:46PM +0200, Jens Axboe wrote: > On Wed, Apr 02 2008, Paul E. McKenney wrote: > > On Wed, Apr 02, 2008 at 01:33:27PM +0200, Fabio Checconi wrote: > > > > From: Peter Zijlstra > > > > Date: Wed, Apr 02, 2008 12:59:21PM +0200 > > > > > > > > On Wed, 2008-04-02 at 03:55 -0700, Paul E. McKenney wrote: > > > > > On Wed, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote: > > > > > > > > > > > > * Jens Axboe wrote: > > > > > > > > > > > > > On Wed, Apr 02 2008, Pekka J Enberg wrote: > > > > > > > > On Wed, 2 Apr 2008, Jens Axboe wrote: > > > > > > > > > Good catch, I wonder why it didn't complain in my testing. I've added a > > > > > > > > > patch to fix that, please see it here: > > > > > > > > > > > > > > > > You probably don't have kmemcheck in your kernel ;-) > > > > > > > > > > > > > > Ehm no, you are right :) > > > > > > > > > > > > ... and you can get kmemcheck by testing on x86.git/latest: > > > > > > > > > > > > http://people.redhat.com/mingo/x86.git/README > > > > > > > > > > > > ;-) > > > > > > > > > > I will check this when I get back to some bandwidth -- but in the meantime, > > > > > does kmemcheck special-case SLAB_DESTROY_BY_RCU? It is legal to access > > > > > newly-freed items in that case, as long as you did rcu_read_lock() > > > > > before gaining a reference to them and don't hold the reference past > > > > > the matching rcu_read_unlock(). > > > > > > > > I don't think it does. > > > > > > > > It would have to register an call_rcu callback itself in order to mark > > > > it freed - and handle the race with the object being handed out again. > > > > > > I had the same problem while debugging a cfq-derived i/o scheduler, > > > and I found nothing preventing the reuse of the freed memory. > > > The patch below seemed to fix the logic. > > > > Looks good to me from a strictly RCU viewpoint -- I must confess great > > ignorance of the CFQ code. :-/ > > That can always be rectified, given enough spare time :-) Hey, I am not complaining about 2007 being gone already, because I am still wondering what the heck happened to 2005 and 2006!!! ;-) Thanx, Paul > > Acked-by: Paul E. McKenney > > Thanks, added. > > -- > Jens Axboe >