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

Oleg Nesterov
2007-04-23T19:04:01Z

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.