linux-kernel - Re: freezer problems

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Date:	Thu, 22 Feb 2007 14:33:56 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	paulmck@...ux.vnet.ibm.com, 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: freezer problems

On 02/22, Oleg Nesterov wrote:
>
> On 02/22, Rafael J. Wysocki wrote:
> >
> > @@ -207,7 +209,7 @@ static void thaw_tasks(int thaw_user_spa
> >  		if (is_user_space(p) == !thaw_user_space)
> >  			continue;
> >  
> > -		if (!thaw_process(p))
> > +		if (!thaw_process(p) && !freezer_should_skip(p))
> >  			printk(KERN_WARNING " Strange, %s not stopped\n",
> 
> This is racy, the warning could be false. We wake up the task, testing
> its ->flags is not reliable.
> 
> Damn. PF_FREEZER_SKIP task could be woken before, clear PF_FREEZER_SKIP,
> but not frozen.
> 
> We can change freezer_count() to clear PF_FREEZER_SKIP after try_to_freeze(),
> not before. Now thaw_process() can take PF_FREEZER_SKIP into account and
> return "true".
> 
> But this means the task may be PF_FREEZER_SKIP | PF_FROZEN. What if we we
> call try_to_freeze_tasks() soon after thaw_tasks()? We may hit the task which
> leaves the refrigerator, but didn't clear PF_FREEZER_SKIP yet. This means
> that thaw_process() should clear PF_FREEZER_SKIP as well. This is messy :(
> 
> Any other ideas? In any case we should imho avoid a separate loop for
> PF_FREEZER_SKIP tasks to just fix debug messages. In fact it can't help
> anyway.

Probably: current clears PF_FREEZER_SKIP along with TIF_FREEZE "atomically"
under task_lock in refrigerator(). thaw_process() takes PF_FREEZER_SKIP into
account.

Oleg.

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