Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932444AbZICWsW (ORCPT ); Thu, 3 Sep 2009 18:48:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932370AbZICWsV (ORCPT ); Thu, 3 Sep 2009 18:48:21 -0400 Received: from smtp2.ultrahosting.com ([74.213.174.253]:33947 "EHLO smtp.ultrahosting.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932070AbZICWsV (ORCPT ); Thu, 3 Sep 2009 18:48:21 -0400 Date: Thu, 3 Sep 2009 17:43:12 -0500 (CDT) From: Christoph Lameter X-X-Sender: cl@V090114053VZO-1 To: "Paul E. McKenney" cc: Eric Dumazet , Pekka Enberg , Zdenek Kabelac , Patrick McHardy , Robin Holt , Linux Kernel Mailing List , Jesper Dangaard Brouer , Linux Netdev List , Netfilter Developers Subject: Re: [PATCH] slub: fix slab_pad_check() In-Reply-To: <20090903174435.GF6761@linux.vnet.ibm.com> Message-ID: References: <4A9EEF07.5070800@gmail.com> <4A9F1620.2080105@gmail.com> <84144f020909022331x2b275aa5n428f88670e0ae8bc@mail.gmail.com> <4A9F7283.1090306@gmail.com> <4A9FCDC6.3060003@gmail.com> <4A9FDA72.8060001@gmail.com> <20090903174435.GF6761@linux.vnet.ibm.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1956 Lines: 46 On Thu, 3 Sep 2009, Paul E. McKenney wrote: > 2. CPU 0 discovers that the slab cache can now be destroyed. > > It determines that there are no users, and has guaranteed > that there will be no future users. So it knows that it > can safely do kmem_cache_destroy(). > > 3. In absence of rcu_barrier(), kmem_cache_destroy() would > immediately tear down the slab data structures. Of course. This has been discussed before. You need to ensure that no objects are in use before destroying a slab. In case of DESTROY_BY_RCU you must ensure that there are no potential readers. So use a suitable rcu barrier or something else like a synchronize_rcu... > > But going through the RCU period is pointless since no user of the cache > > remains. > > Which is irrelevant. The outstanding RCU callback was posted by the > slab cache itself, -not- by the user of the slab cache. There will be no rcu callbacks generated at kmem_cache_destroy with the patch I posted. > > The dismantling does not need RCU since there are no operations on the > > objects in progress. So simply switch DESTROY_BY_RCU off for close. > > Unless I am missing something, this patch re-introduces the bug that > the rcu_barrier() was added to prevent. So, in absence of a better > explanation of what I am missing: The "fix" was ill advised. Slab users must ensure that no objects are in use before destroying a slab. Only the slab users know how the objects are being used. The slab allocator itself cannot know how to ensure that there are no pending references. Putting a rcu_barrier in there creates an inconsistency in the operation of kmem_cache_destroy() and an expectation of functionality that the function cannot provide. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/