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Date:	Fri, 02 Apr 2010 13:00:15 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [Patch] workqueue: move lockdep annotations up to	destroy_workqueue()

Oleg Nesterov wrote:
> On 04/01, Cong Wang wrote:
>>> 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.
> 
> Oh, I can never understand the output from lockdep, it is much more
> clever than me ;)
> 
> But at first glance,
> 
>> Mar 31 16:15:02 dhcp-66-70-5 kernel: -> #2 (rtnl_mutex){+.+.+.}:
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff810a6bc1>] validate_chain+0x1019/0x1540
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff815523f8>] mutex_lock_nested+0x64/0x4e9
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff8147af16>] rtnl_lock+0x1e/0x27
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffffa0836779>] bond_mii_monitor+0x39f/0x74b [bonding]
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff8108654f>] worker_thread+0x2da/0x46c
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff8108b1ea>] kthread+0xdd/0xec
>> Mar 31 16:15:02 dhcp-66-70-5 kernel:        [<ffffffff81004894>] kernel_thread_helper+0x4/0x10
> 
> OK, so work->func() takes rtnl_mutex.
> 
> This means it is not safe to do flush_workqueue() or destroy_workqueue()
> under rtnl_lock(). This is known fact.
> 
>> Mar 31 16:15:03 dhcp-66-70-5 kernel: -> #0 ((bond_dev->name)){+.+...}:
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff810a6696>] validate_chain+0xaee/0x1540
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff810a7e75>] __lock_acquire+0xd8d/0xe55
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff810aa3a4>] lock_acquire+0x160/0x1af
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff81085278>] cleanup_workqueue_thread+0x59/0x10b
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff81085428>] destroy_workqueue+0x9c/0x107
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffffa0839d32>] bond_uninit+0x524/0x58a [bonding]
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff8146967b>] rollback_registered_many+0x205/0x2e3
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff81469783>] unregister_netdevice_many+0x2a/0x75
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff8147ada3>] __rtnl_kill_links+0x8b/0x9d
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff8147adea>] __rtnl_link_unregister+0x35/0x72
>> Mar 31 16:15:03 dhcp-66-70-5 kernel:        [<ffffffff8147b293>] rtnl_link_unregister+0x2c/0x43
> 
> However, rtnl_link_unregister() takes rtnl_mutex and then bond_uninit()
> does cleanup_workqueue_thread().
> 
> So, looks like this warning is valid, this path can deadlock if
> destroy_workqueue() is called when bond->mii_work is queued.


Yeah, this is right.

> 
> 
> Lockdep decided to blaim cpu_add_remove_lock in this chain.
> 

Yes, this is what makes me confused. ;)

Thanks!

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