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

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Date:	Mon, 1 Mar 2010 16:50:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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

On 03/02, Tejun Heo wrote:
>
> >> @@ -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_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.

Hmm. not sure I understand... but before this patch the code was OK.
cpu_maps_update_begin() in create/destroy protected us from the races
with hotplug.

> I don't
> think it's worth fixing them at this point with all the pending
> changes.

OK, agreed.

Oleg.

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