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

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Date:	Thu, 01 Apr 2010 14:05:44 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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()

Cong Wang wrote:
> Tejun Heo wrote:
>> Hello,
>>
>> On 04/01/2010 01:28 PM, Cong Wang wrote:
>>>> 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.
>>>>
>>> I thought this is obvious.
>>>
>>> Here it is:
>>>
>>> void destroy_workqueue(struct workqueue_struct *wq)
>>> {
>>>         const struct cpumask *cpu_map = wq_cpu_map(wq);
>>>         int cpu;
>>>
>>>         cpu_maps_update_begin();        <----------------- Hold
>>> cpu_add_remove_lock here
>>>         spin_lock(&workqueue_lock);
>>>         list_del(&wq->list);
>>>         spin_unlock(&workqueue_lock);
>>>
>>>         for_each_cpu(cpu, cpu_map)
>>>                 cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, 
>>> cpu)); <------ See below
>>>         cpu_maps_update_done();        <----------------- Release
>>> cpu_add_remove_lock here
>>>
>>> ...
>>> static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
>>> {
>>>         /*
>>>          * Our caller is either destroy_workqueue() or CPU_POST_DEAD,
>>>          * cpu_add_remove_lock protects cwq->thread.
>>>          */
>>>         if (cwq->thread == NULL)
>>>                 return;
>>>
>>>         lock_map_acquire(&cwq->wq->lockdep_map); <-------------- Lockdep
>>> complains here.
>>>         lock_map_release(&cwq->wq->lockdep_map);
>>> ...
>>
>> Yeap, the above is cpu_add_remove_lock -> wq->lockdep_map dependency.
>> I can see that but I'm failing to see where the dependency the other
>> direction is created.
>>
> 
> Hmm, it looks like I misunderstand lock_map_acquire()? From the changelog,
> I thought it was added to complain its caller is holding a lock when 
> invoking
> it, thus cpu_add_remove_lock is not an exception.
> 

Oh, I see, wq->lockdep_map is acquired again in run_workqueue(), so I was wrong. :)
I think you and Oleg are right, the lockdep warning is not irrelevant.

Sorry for the noise, ignore this patch please.

Thanks.
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