linux-kernel - Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread

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Message-Id: <200704232205.16208.rjw@sisk.pl>
Date:	Mon, 23 Apr 2007 22:05:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	vatsa@...ibm.com, paulmck@...ibm.com, pavel@....cz
Subject: Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop

On Monday, 23 April 2007 21:03, Oleg Nesterov wrote:
> 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.

Yes, I think you're right.

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

OK, I will, thanks.

Greetings,
Rafael
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