From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755321AbYDBLHb (ORCPT ); Wed, 2 Apr 2008 07:07:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753427AbYDBLHX (ORCPT ); Wed, 2 Apr 2008 07:07:23 -0400 Received: from brick.kernel.dk ([87.55.233.238]:19520 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753370AbYDBLHX (ORCPT ); Wed, 2 Apr 2008 07:07:23 -0400 Date: Wed, 2 Apr 2008 13:07:19 +0200 From: Jens Axboe To: Pekka Enberg Cc: paulmck@linux.vnet.ibm.com, Ingo Molnar , Peter Zijlstra , Vegard Nossum , Linux Kernel Mailing List Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context) Message-ID: <20080402110718.GU12774@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> <84144f020804020401j4e5863dcofd16662baa54574@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <84144f020804020401j4e5863dcofd16662baa54574@mail.gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 02 2008, Pekka Enberg wrote: > Hi Paul, > > On Wed, Apr 2, 2008 at 1:55 PM, Paul E. McKenney > wrote: > > 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(). > > No, kmemcheck is work in progress and does not know about > SLAB_DESTROY_BY_RCU yet. The reason I asked Vegard to post the warning > was because Peter, Vegard, and myself identified this particular > warning as a real problem. But yeah, kmemcheck can cause false > positives for RCU for now. Makes sense, and to me Pauls analysis of the code looks totally correct - there's no bug there, at least related to hlist traversal and kmem_cache_free(), since we are under rcu_read_lock() and thus hold off the grace for freeing. -- Jens Axboe