linux-kernel - Re: [PATCH 16/43] workqueue: kill cpu_populated

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Message-ID: <4B8BDE02.1050208@kernel.org>
Date:	Tue, 02 Mar 2010 00:32:18 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Subject: Re: [PATCH 16/43] workqueue: kill cpu_populated_map

Hello,

On 03/01/2010 01:00 AM, Oleg Nesterov wrote:
> On 02/26, Tejun Heo wrote:
>>
>> @@ -1023,41 +991,40 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
>> ...
>> +	cpu_maps_update_done();
>> ...
>> +
>> +	spin_lock(&workqueue_lock);
>> +	list_add(&wq->list, &workqueues);
>> +	spin_unlock(&workqueue_lock);
> 
> OK, but if cpu_up() happens right after we drop cpu_maps_update_done(),
> cwq->thread on the new CPU will run unbound?

Indeed, looks like I was too impatient with cpu_maps_update_done().

>> @@ -1127,47 +1091,30 @@ static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
>> ...
>>  	list_for_each_entry(wq, &workqueues, list) {
> 
> this becomes unsafe. create/destroy can modify workqueues list
> in parallel.

Yeap, has always been like that.  Will be fixed by later changes.

>>  		case CPU_ONLINE:
>> -			start_workqueue_thread(cwq, cpu);
>> +			__set_cpus_allowed(cwq->thread, get_cpu_mask(cpu),
>> +					   true);
> 
> if the thread doesn't have PF_THREAD_BOUND, who will set it?

Indeed will update the worker function to set PF_THREAD_BOUND itself
but again this problem is gone with later patches.

>>  		case CPU_POST_DEAD:
>> -			cleanup_workqueue_thread(cwq);
>> +			lock_map_acquire(&cwq->wq->lockdep_map);
>> +			lock_map_release(&cwq->wq->lockdep_map);
>> +			flush_cpu_workqueue(cwq);
> 
> This can race with destroy_workqueue(), no?

Yes, it can and, again, has been always like that and will be fixed by
later patches.

> I guess this patch is preparation, probably these problems should
> go away later...

I'll fix the new problems but leave the existing ones alone.  I don't
think it's worth fixing them at this point with all the pending
changes.

Thanks.

-- 
tejun
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