Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update

[Oleg Nesterov <oleg@tv-sign.ru>]

On 01/09, Srivatsa Vaddagiri wrote:
>
> On Mon, Jan 08, 2007 at 08:06:35PM +0300, Oleg Nesterov wrote:
> > Ah, missed you point, thanks. Yet another old problem which was not introduced
> > by recent changes. And yet another indication we should avoid kthread_stop()
> > on CPU_DEAD event :) I believe this is easy to fix, but need to think more.
> 
> I think the problem is not just with CPU_DEAD. Anyone who calls
> cleanup_workqueue_thread (say destroy_workqueue?) will see this race. 

destroy_workqueue() first does flush_workqueue(), so it should be ok.

Anyway I agree with you, we shouldn't clear cwq->thread until it exits,

> Do you see any problems if cleanup_workqueue_thread is changed as:
> 
> cleanup_workqueue_thread()
> {
> 	kthread_stop(p);
> 	spin_lock(cwq->lock);
> 	cwq->thread = NULL;
> 	spin_unlock(cwq->lock);
> }

I think the same. In fact I suspect we even don't need spin_lock, but didn't
have a time to read the code since our discussion.

Oleg.