From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751396AbWHVO1T (ORCPT ); Tue, 22 Aug 2006 10:27:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751397AbWHVO1T (ORCPT ); Tue, 22 Aug 2006 10:27:19 -0400 Received: from smtp.osdl.org ([65.172.181.4]:9104 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1751396AbWHVO1S (ORCPT ); Tue, 22 Aug 2006 10:27:18 -0400 Date: Tue, 22 Aug 2006 07:26:56 -0700 From: Andrew Morton To: Arjan van de Ven Cc: Heiko Carstens , Greg KH , Dave Jones , linux-kernel@vger.kernel.org Subject: Re: Lockdep message on workqueue_mutex Message-Id: <20060822072656.b1a28a85.akpm@osdl.org> In-Reply-To: <1156250192.2976.47.camel@laptopd505.fenrus.org> References: <20060822121042.GA29577@osiris.boeblingen.de.ibm.com> <1156250192.2976.47.camel@laptopd505.fenrus.org> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 22 Aug 2006 14:36:32 +0200 Arjan van de Ven wrote: > On Tue, 2006-08-22 at 14:10 +0200, Heiko Carstens wrote: > > git commit 9b41ea7289a589993d3daabc61f999b4147872c4 causes the lockdep > > message below on cpu hotplug (git kernel of today). > > > > We have: > > > > cpu_down (takes cpu_add_remove_lock) > > [CPU_DOWN_PREPARE] > > blocking_notifier_call_chain (takes (cpu_chain).rwsem) > > workqueue_cpu_callback (takes workqueue_mutex) > > blocking_notifier_call_chain (releases (cpu_chain).rwsem) > > [CPU_DEAD] > > blocking_notifier_call_chain (takes (cpu_chain).rwsem) > > ^^^^^^^^^^^^^^^^^^^^^^^ > > -> reverse locking order, since we still hold workqueue_mutex. > > > > But since all of this is protected by the cpu_add_remove_lock this looks > > legal. Well, at least it's safe as long as no other cpu callback function > > does anything that will take the workqueue_mutex as well. > > so you're saying this locking is entirely redundant ? ;-) Nope, not all code paths which access the data which is protected by workqueue_mutex take cpu_add_remove_lock. Simplifying it, we have: blocking_notifier_call_chain() ->down_read(cpu_chain.rwsem) ->workqueue_cpu_callback() ->mutex_lock(workqueue_mutex) ->up_read(cpu_chain.rwsem) blocking_notifier_call_chain() ->down_read(cpu_chain.rwsem) ->workqueue_cpu_callback() ->mutex_unlock(workqueue_mutex) ->up_read(cpu_chain.rwsem) Which is OK as long as nobody runs cpu_up() or cpu_down() while holding workqueue_mutex.