Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop

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

On 04/23, Gautham R Shenoy wrote:
>
> On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> >  /*
> > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> > 
> >  	/* Now set kthread_should_stop() to true, and wake it up. */
> >  	kthread_stop_info.k = k;
> > +	if (!freezer_should_exempt(current)) {
> > +		/* We are freezable, so we must make sure that the thread being
> > +		 * stopped is not frozen and will not be frozen until it dies
> > +		 */
> > +		freezer_exempt(k);
> > +		if (frozen(k))
> > +			clear_frozen_flag(k);
> > +	}
> 
> I'm trying hard to convince myself that this will work. May be I am
> missing something here, but I find a potential race window (very small though) 
> when k is entering the refrigerator.
> 
> [... snip ... ]
> 
> IMO, we need the to take the task_lock for k here. Something like
> 
> > +	if (!freezer_should_exempt(current)) {
> 		task_lock(k);
> > +		/* We are freezable, so we must make sure that the thread being
> > +		 * stopped is not frozen and will not be frozen until it dies
> > +		 */
> > +		freezer_exempt(k);
> > +		if (frozen(k))
> > +			clear_frozen_flag(k);
> 		task_unlock(k);
> > +	}

Well, probably I missed something, but why can't we do

	if (!freezer_should_exempt(current)) {
		freezer_exempt(k);
		thaw_process(k);
	}
?

thaw_process(k) is properly serialized with refrigerator(), and it checks
frozen(k). We can make an extra wake_up, but this should not matter.

Rafael, please check the recent changes in kthread.c, kthread_stop() was
reworked, we don't have kthread_stop_info any longer.

Oleg.

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