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

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Message-ID: <4BB42D05.4060207@redhat.com>
Date:	Thu, 01 Apr 2010 13:20:05 +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()

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.

Thanks!
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