Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754638Ab2JCHnv (ORCPT ); Wed, 3 Oct 2012 03:43:51 -0400 Received: from cantor2.suse.de ([195.135.220.15]:33681 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752335Ab2JCHnu (ORCPT ); Wed, 3 Oct 2012 03:43:50 -0400 Date: Wed, 3 Oct 2012 09:43:41 +0200 (CEST) From: Jiri Kosina To: "Srivatsa S. Bhat" Cc: paulmck@linux.vnet.ibm.com, "Paul E. McKenney" , Josh Triplett , linux-kernel@vger.kernel.org Subject: Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()") In-Reply-To: <506BB950.3000102@linux.vnet.ibm.com> Message-ID: References: <506B50F1.8070907@linux.vnet.ibm.com> <506BB283.4010800@linux.vnet.ibm.com> <20121003034405.GB13192@linux.vnet.ibm.com> <506BB950.3000102@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) 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: 1692 Lines: 45 On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote: > >>> CPU 0 CPU 1 > >>> kmem_cache_destroy() > >> > >> What about the get_online_cpus() right here at CPU0 before > >> calling mutex_lock(slab_mutex)? How can the cpu_up() proceed > >> on CPU1?? I still don't get it... :( > >> > >> (kmem_cache_destroy() uses get/put_online_cpus() around acquiring > >> and releasing slab_mutex). > > > > The problem is that there is a CPU-hotplug notifier for slab, which > > establishes hotplug->slab. > > Agreed. > > > Then having kmem_cache_destroy() call > > rcu_barrier() under the lock > > Ah, that's where I disagree. kmem_cache_destroy() *cannot* proceed at > this point in time, because it has invoked get_online_cpus()! It simply > cannot be running past that point in the presence of a running hotplug > notifier! So, kmem_cache_destroy() should have been sleeping on the > hotplug lock, waiting for the notifier to release it, no? Please look carefully at the scenario again. kmem_cache_destroy() calls get_online_cpus() before the hotplug notifier even starts. Hence it has no reason to block there (noone is holding hotplug lock). *Then* hotplug notifier fires up, succeeds obtaining hotplug lock, kmem_cache_destroy() calls rcu_barrier in the meantime, and blocks itself on the hotplug lock there. Please note that the get_online_cpus() call in kmem_cache_destroy() doesn't play *any* role in this scenario. -- Jiri Kosina SUSE Labs -- 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/