From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756164AbYDBNlA (ORCPT ); Wed, 2 Apr 2008 09:41:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754220AbYDBNkw (ORCPT ); Wed, 2 Apr 2008 09:40:52 -0400 Received: from brick.kernel.dk ([87.55.233.238]:14854 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753835AbYDBNkv (ORCPT ); Wed, 2 Apr 2008 09:40:51 -0400 Date: Wed, 2 Apr 2008 15:40:46 +0200 From: Jens Axboe To: "Paul E. McKenney" 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: <20080402134046.GZ12774@kernel.dk> References: <19f34abd0804011408v19e13b6cje1ca89a2a471484c@mail.gmail.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080402133232.GE2813@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 :-) > Acked-by: Paul E. McKenney Thanks, added. -- Jens Axboe