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

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Date:	Mon, 23 Apr 2007 21:50:30 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, ego@...ibm.com,
	Oleg Nesterov <oleg@...sign.ru>, linux-kernel@...r.kernel.org,
	vatsa@...ibm.com, paulmck@...ibm.com
Subject: Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop

Hi,

On Monday, 23 April 2007 12:40, Pavel Machek wrote:
> Hi!
> 
> > Fix the problem with kthread_stop() that causes the freezer to fail if a
> > freezable thread is attempting to stop a frozen one and that may cause the
> > freezer to fail if the thread being stopped is freezable and
> > try_to_freeze_tasks() is running concurrently with kthread_stop().
> 
> Parse error.

OK, will fix.

> > Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> > ===================================================================
> > --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c	2007-04-09 15:23:48.000000000 +0200
> > +++ linux-2.6.21-rc6-mm1/kernel/kthread.c	2007-04-22 19:05:29.000000000 +0200
> > @@ -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);
> > +	}
> >  	wake_up_process(k);
> >  	put_task_struct(k);
> >  
> 
> Do we need to take some locks to access other process' flags? Or do
> frozen_exempt() etc take enough locks, already?

After the previous patch we only use set_bit(), clear_bit() and test_bit() to
access freezer_falgs, so no special locking is needed to protect them.

Greetings,
Rafael
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