linux-kernel - Re: freezer problems

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Message-ID: <20070222104747.GA505@tv-sign.ru>
Date:	Thu, 22 Feb 2007 13:47:47 +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, 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?

> @@ -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.

Oleg.

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