linux-kernel - Re: [RFC][PATCH 4/7] Freezer: Fix vfork problem

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Date:	Sun, 25 Feb 2007 21:33:37 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>
Cc:	Aneesh Kumar <aneesh.kumar@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	ego@...ibm.com, akpm@...l.org, mingo@...e.hu, vatsa@...ibm.com,
	dipankar@...ibm.com, venkatesh.pallipadi@...el.com
Subject: Re: [RFC][PATCH 4/7] Freezer: Fix vfork problem

On Sunday, 25 February 2007 21:31, Oleg Nesterov wrote:
> On 02/25, Rafael J. Wysocki wrote:
> >
> > On Sunday, 25 February 2007 16:40, Aneesh Kumar wrote:
> > > On 2/25/07, Aneesh Kumar <aneesh.kumar@...il.com> wrote:
> > > > On 2/25/07, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > > > On Sunday, 25 February 2007 15:33, Aneesh Kumar wrote:
> > > > > > On 2/25/07, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > [--snip--]
> > > 
> > > Thinking about this  i guess we have a problem with the above approach
> > > i outlined. if we have one task that is waiting on the event and more
> > > than one that can generate the event then the above logic would not
> > > work. Also with cases other than vfork; logic of tracking the waiting
> > > task gets complex. I guess what we have right now is better.
> > 
> > I assume by "righ now" you mean the latest version of my patch. ;-)
> > 
> > Still, having pondered the Pavel's suggestion for a while I think it's doable
> > without the addtitional process flag.  Patch below.
> 
> Probably I missed something, (I didn't see this patch and I missed the
> start of discussion), but I can't understand this patch.

Please see http://lkml.org/lkml/2007/2/25/53 .

> > +__wait_for_completion(struct completion *x, int freezable)
> >  {
> >  	might_sleep();
> >  
> > @@ -3817,6 +3818,9 @@ void fastcall __sched wait_for_completio
> >  			__set_current_state(TASK_UNINTERRUPTIBLE);
> >  			spin_unlock_irq(&x->wait.lock);
> >  			schedule();
> > +			if (freezable)
> > +				try_to_freeze();
> > +
> >  			spin_lock_irq(&x->wait.lock);
> >  		} while (!x->done);
> >  		__remove_wait_queue(&x->wait, &wait);
> > @@ -3824,7 +3828,7 @@ void fastcall __sched wait_for_completio
> >  	x->done--;
> >  	spin_unlock_irq(&x->wait.lock);
> >  }
> >
> > ..........
> >
> > @@ -48,6 +48,9 @@ void refrigerator(void)
> >  		task_unlock(current);
> >  		return;
> >  	}
> > +	if (current->vfork_done)
> > +		wake_up_process(current->parent);
> > +
> 
> What if current->parent doesn't have TIF_FREEZE yet? ->parent will schedule()
> again, child goes to refrigerator. Now, how can we freeze the ->parent?

Good point.  I didn't think about it.

All in all, having tried some different approaches I think that the patch at
http://lkml.org/lkml/2007/2/25/80 is the right thing to do.

Pavel, do you agree?

Rafael
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