linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Date:	Sun, 18 Feb 2007 19:51:16 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On Sunday, 18 February 2007 17:11, Oleg Nesterov wrote:
> On 02/18, Rafael J. Wysocki wrote:
> >
> > > On Sunday, 18 February 2007 12:31, Oleg Nesterov wrote:
> > > > 
> > > > A very vague idea: what if parent will do
> > > > 
> > > > 	current->flags |= PF_PLEASE_CONSIDER_ME_AS_FROZEN_BUT_SET_TIF_FREEZE
> > > > 	wait_for_completion(&vfork);
> > > > 	try_to_freeze();
> > > > 
> > > > ?
> > 
> > Hm, what about the following patch instead?
> > 
> > The problem is that if the child enters the refrigeratior, we can't freeze the
> > parent, because it's uninterruptible, but the child knows the parent will be
> > uninterruptible until it exits, so the child can mark the parent as frozen.
> > 
> > --- linux-2.6.20-mm2.orig/kernel/power/process.c	2007-02-18 15:43:30.000000000 +0100
> > +++ linux-2.6.20-mm2/kernel/power/process.c	2007-02-18 16:09:53.000000000 +0100
> > @@ -39,6 +39,13 @@ void refrigerator(void)
> >  	/* Hmm, should we be allowed to suspend when there are realtime
> >  	   processes around? */
> >  	long save;
> > +
> > +	/* The parent is uninterruptible and will stay so until this task exits,
> > +	 * so we can mark it as frozen.
> > +	 */
> > +	if (current->vfork_done)
> > +		frozen_process(current->parent);
> 
> This is not safe. task->flags is not atomic, we can change ->flags only
> if we know the task won't touch it itself (ptrace, thaw_process).
> The parent could be interrupted, irq may play with current->flags (slab,
> for example).
> 
> Please note that ->parent may do things like ptrace_notify() before
> it actually sleeps on ->vfork_done. This means that even if we could
> set PF_FROZEN in a safe manner, this doesn't look like a good idea.
> 
> > +
> > +	if (current->vfork_done && frozen(current->parent))
> > +		current->parent->flags &= ~PF_FROZEN;
> >  }
> 
> Why? If the code above works, we shouldn't take care about frozen
> ->parent?

I've added this for symmetry.  thaw_tasks() should reset PF_FROZEN for it
anyway. 

Okay, so I'll post the patch that implements your idea in the other thread.

Greetings,
Rafael
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