From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755487AbYDBLUg (ORCPT ); Wed, 2 Apr 2008 07:20:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753174AbYDBLU2 (ORCPT ); Wed, 2 Apr 2008 07:20:28 -0400 Received: from viefep25-int.chello.at ([62.179.121.45]:20125 "EHLO viefep25-int.chello.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753041AbYDBLU1 (ORCPT ); Wed, 2 Apr 2008 07:20:27 -0400 X-SourceIP: 80.56.237.116 Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context) From: Peter Zijlstra To: Jens Axboe Cc: Pekka Enberg , paulmck@linux.vnet.ibm.com, Ingo Molnar , Vegard Nossum , Linux Kernel Mailing List In-Reply-To: <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> <20080402111422.GW12774@kernel.dk> Content-Type: text/plain Date: Wed, 02 Apr 2008 13:20:12 +0200 Message-Id: <1207135212.8514.782.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.21.92 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2008-04-02 at 13:14 +0200, Jens Axboe wrote: > 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. > A B C cfq_cic_lookup(cfqd_1, ioc) rcu_read_lock() cic = radix_tree_lookup(, cfqd_q); cfq_cic_free() cfq_cic_link(cfqd_2, ioc,) rcu_read_unlock() and now we have that: cic->key == cfqd_2 I'm not seeing anything stopping this from happening. Which is also why we need hlist_for_each_safe_rcu() because as soon as we kfree()d the thing, someone else might get the object and start poking at the hlist pointers, wrecking out iteration.