linux-kernel - Re: [Patch] workqueue: move lockdep annotations up to destroy

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Date:	Thu, 01 Apr 2010 13:14:38 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Cong Wang <amwang@...hat.com>
CC:	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.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()

Hello,

On 04/01/2010 01:09 PM, Cong Wang wrote:
>> 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.

Yeah yeah, I'm just failing to see how the other direction is
completed.  ie. where does the kernel try to grab cpu_add_remove_lock
*after* grabbing wq lock?

>> 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. :)

Ah... great.  :-)

> After this patch, another lockdep warning appears, it is exactly what
> you expect.

Hmmm... can you please try to see whether this circular locking
warning involving wq->lockdep_map is reproducible w/ the bonding
locking fixed?  I still can't see where wq -> cpu_add_remove_lock
dependency is created.

Thanks.

-- 
tejun
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