Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753442Ab2JCDoM (ORCPT ); Tue, 2 Oct 2012 23:44:12 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:43412 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752664Ab2JCDoK (ORCPT ); Tue, 2 Oct 2012 23:44:10 -0400 Date: Tue, 2 Oct 2012 20:44:05 -0700 From: "Paul E. McKenney" To: "Srivatsa S. Bhat" Cc: Jiri Kosina , "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()") Message-ID: <20121003034405.GB13192@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <506B50F1.8070907@linux.vnet.ibm.com> <506BB283.4010800@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <506BB283.4010800@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12100303-2398-0000-0000-00000BCB65ED X-IBM-ISS-SpamDetectors: X-IBM-ISS-DetailInfo: BY=3.00000294; HX=3.00000196; KW=3.00000007; PH=3.00000001; SC=3.00000008; SDB=6.00179343; UDB=6.00040616; UTC=2012-10-03 03:44:09 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1947 Lines: 60 On Wed, Oct 03, 2012 at 09:05:31AM +0530, Srivatsa S. Bhat wrote: > On 10/03/2012 03:47 AM, Jiri Kosina wrote: > > On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote: > > > >> I don't see how this circular locking dependency can occur.. If you are using SLUB, > >> kmem_cache_destroy() releases slab_mutex before it calls rcu_barrier(). If you are > >> using SLAB, kmem_cache_destroy() wraps its whole operation inside get/put_online_cpus(), > >> which means, it cannot run concurrently with a hotplug operation such as cpu_up(). So, I'm > >> rather puzzled at this lockdep splat.. > > > > I am using SLAB here. > > > > The scenario I think is very well possible: > > > > > > 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. Then having kmem_cache_destroy() call rcu_barrier() under the lock establishes slab->hotplug, which results in deadlock. Jiri really did explain this in an earlier email message, but both of us managed to miss it. ;-) Thanx, Paul > Regards, > Srivatsa S. Bhat > > > mutex_lock(slab_mutex) > > _cpu_up() > > cpu_hotplug_begin() > > mutex_lock(cpu_hotplug.lock) > > rcu_barrier() > > _rcu_barrier() > > get_online_cpus() > > mutex_lock(cpu_hotplug.lock) > > (blocks, CPU 1 has the mutex) > > __cpu_notify() > > mutex_lock(slab_mutex) > > > > Deadlock. > > > > Right? > > > > -- 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/