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

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Date:	Thu, 01 Apr 2010 13:59:14 +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 01:28 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.

Thanks.

-- 
tejun
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