From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756899AbYDBLOs (ORCPT ); Wed, 2 Apr 2008 07:14:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756569AbYDBLO2 (ORCPT ); Wed, 2 Apr 2008 07:14:28 -0400 Received: from brick.kernel.dk ([87.55.233.238]:11393 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756566AbYDBLO1 (ORCPT ); Wed, 2 Apr 2008 07:14:27 -0400 Date: Wed, 2 Apr 2008 13:14:23 +0200 From: Jens Axboe To: Peter Zijlstra Cc: Pekka Enberg , paulmck@linux.vnet.ibm.com, Ingo Molnar , Vegard Nossum , Linux Kernel Mailing List Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context) Message-ID: <20080402111422.GW12774@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> <20080402110718.GU12774@kernel.dk> <1207134536.8514.773.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1207134536.8514.773.camel@twins> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 02 2008, Peter Zijlstra wrote: > On Wed, 2008-04-02 at 13:07 +0200, Jens Axboe wrote: > > 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. > > but what holds off the slab allocator re-issueing that same object and > someone else writing other stuff into it? Nothing, that's how rcu destry works here. But for the validation to be WRONG radix_tree_lookup(..., old_key) must return cic for new_key, not NULL. -- Jens Axboe