From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751901Ab0DAEGR (ORCPT ); Thu, 1 Apr 2010 00:06:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48014 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750706Ab0DAEGK (ORCPT ); Thu, 1 Apr 2010 00:06:10 -0400 Message-ID: <4BB41C72.3090909@redhat.com> Date: Thu, 01 Apr 2010 12:09:22 +0800 From: Cong Wang User-Agent: Thunderbird 2.0.0.23 (X11/20091001) MIME-Version: 1.0 To: Tejun Heo CC: Oleg Nesterov , linux-kernel@vger.kernel.org, Rusty Russell , akpm@linux-foundation.org, Ingo Molnar Subject: Re: [Patch] workqueue: move lockdep annotations up to destroy_workqueue() References: <20100331105534.5601.50813.sendpatchset@localhost.localdomain> <20100331112559.GA17747@redhat.com> <4BB408AF.4080908@redhat.com> <4BB41988.1030400@kernel.org> In-Reply-To: <4BB41988.1030400@kernel.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Tejun Heo wrote: > Hello, guys. > > On 04/01/2010 11:45 AM, Cong Wang wrote: >>> OK, but nobody should take cpu_maps_update_begin() under wq->lockdep_map, >>> in particular work->func() must not. >>> >>> I must have missed something, but it seems to me this patch tries to >>> supress the valid warning. >>> >>> Could you please clarify? >> Sure, below is the whole warning. Please teach me how this is valid. > > I still have some trouble interpreting lockdep warnings. Please > correct me if I get something wrong. > >> modprobe/5264 is trying to acquire lock: >> ((bond_dev->name)){+.+...}, at: [] cleanup_workqueue_thread+0x2b/0x10b >> >> but task is already holding lock: >> (cpu_add_remove_lock){+.+.+.}, at: [] cpu_maps_update_begin+0x1e/0x27 > > This (cpu hotplug -> wq) is the expected sequence. Plug cpu > hotplugging and then flush cpu workqueues. > >> which lock already depends on the new lock. > > But lockdep says the other way around has already happened. > >> the existing dependency chain (in reverse order) is: >> >> -> #3 (cpu_add_remove_lock){+.+.+.}: >> [] validate_chain+0x1019/0x1540 >> [] __lock_acquire+0xd8d/0xe55 >> [] lock_acquire+0x160/0x1af >> [] mutex_lock_nested+0x64/0x4e9 >> [] cpu_maps_update_begin+0x1e/0x27 >> [] destroy_workqueue+0x41/0x107 >> [] bond_uninit+0x524/0x58a [bonding] >> [] rollback_registered_many+0x205/0x2e3 >> [] unregister_netdevice_many+0x2a/0x75 >> [] __rtnl_kill_links+0x8b/0x9d >> [] __rtnl_link_unregister+0x35/0x72 >> [] rtnl_link_unregister+0x2c/0x43 >> [] bonding_exit+0x5a/0x76 [bonding] >> [] sys_delete_module+0x306/0x3b1 >> [] system_call_fastpath+0x16/0x1b > > This is bond_uninit() calling destroy_workqueue() but I don't get how > this thread would be already holding wq lock. destroy_workqueue() does hold wq lock and then releases it. > >> -> #2 (rtnl_mutex){+.+.+.}: >> [] validate_chain+0x1019/0x1540 >> [] __lock_acquire+0xd8d/0xe55 >> [] lock_acquire+0x160/0x1af >> [] mutex_lock_nested+0x64/0x4e9 >> [] rtnl_lock+0x1e/0x27 >> [] bond_mii_monitor+0x39f/0x74b [bonding] >> [] worker_thread+0x2da/0x46c >> [] kthread+0xdd/0xec >> [] kernel_thread_helper+0x4/0x10 >> >> -> #1 ((&(&bond->mii_work)->work)){+.+...}: >> [] validate_chain+0x1019/0x1540 >> [] __lock_acquire+0xd8d/0xe55 >> [] lock_acquire+0x160/0x1af >> [] worker_thread+0x2cd/0x46c >> [] kthread+0xdd/0xec >> [] kernel_thread_helper+0x4/0x10 > > These two are form a workqueue worker thread and I don't quite > understand why they are here. > >> -> #0 ((bond_dev->name)){+.+...}: >> [] validate_chain+0xaee/0x1540 >> [] __lock_acquire+0xd8d/0xe55 >> [] lock_acquire+0x160/0x1af >> [] cleanup_workqueue_thread+0x59/0x10b >> [] destroy_workqueue+0x9c/0x107 >> [] bond_uninit+0x524/0x58a [bonding] >> [] rollback_registered_many+0x205/0x2e3 >> [] unregister_netdevice_many+0x2a/0x75 >> [] __rtnl_kill_links+0x8b/0x9d >> [] __rtnl_link_unregister+0x35/0x72 >> [] rtnl_link_unregister+0x2c/0x43 >> [] bonding_exit+0x5a/0x76 [bonding] >> [] sys_delete_module+0x306/0x3b1 >> [] system_call_fastpath+0x16/0x1b > > This seems to be from the original thread of frame#3. It's grabbing > wq lock here but the problem is that the lock will be released > immediately, so bond_dev->name (the wq) can't be held by the time it > reaches frame#3. How is this dependency chain completed? Is it > somehow transitive through rtnl_mutex? wq lock is held *after* cpu_add_remove_lock, lockdep also said this, the process is trying to hold wq lock while having cpu_add_remove_lock. > >> other info that might help us debug this: >> >> 2 locks held by modprobe/5264: >> #0: (rtnl_mutex){+.+.+.}, at: [] rtnl_lock+0x1e/0x27 >> #1: (cpu_add_remove_lock){+.+.+.}, at: [] cpu_maps_update_begin+0x1e/0x27 > > Isn't there a circular dependency here? bonding_exit() calls > destroy_workqueue() under rtnl_mutex but destroy_workqueue() should > flush works which could be trying to grab rtnl_lock. Or am I > completely misunderstanding locking here? Sure, that is why I sent another patch for bonding. :) After this patch, another lockdep warning appears, it is exactly what you expect. Thanks.