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

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Date:	Sun, 18 Feb 2007 19:11:51 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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?

Oleg.

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