linux-kernel - Re: freezer problems

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Date:	Thu, 22 Feb 2007 18:03:37 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
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 Thursday, 22 February 2007 11:47, Oleg Nesterov wrote:
> On 02/22, Rafael J. Wysocki wrote:
> >
> > Okay, below is what I have right now (compilation tested on x86_64):
> > 
> > This patch fixes the vfork problem by adding the PF_FREEZER_SKIP flag that
> > can be used by tasks to tell the freezer not to count them as freezeable and
> > making the vfork parents set this flag before they call wait_for_completion().
> > 
> > Secondly, it fixes the race which happens it a task with TIF_FREEZE set is
> > preempted right before calling frozen_process() in refrigerator() and stays
> > unforzen until after thaw_tasks() runs and checks its status.  For this purpose
> > task_lock() is used.
> 
> Great! But please be kind to those of us who read the source control history
> trying to understand the code. Could you make 2 separate patches?

Okay, attached.  The first one closes the race between thaw_tasks() and the
refrigerator that can occurs if the freezing fails.  The second one fixes the
vfork problem (should go on top of the first one).

> > @@ -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.

Why don't we just drop the warning?  try_to_freeze_tasks() should give us a
warning if there's anything wrong anyway.

Greetings,
Rafael

View attachment "freezer-fix-theoretical-race.patch" of type "text/x-diff" (1825 bytes)

View attachment "freezer-fix-vfork-problem.patch" of type "text/x-diff" (4349 bytes)