linux-kernel - Re: [PATCH 06/16] freezer: make exiting tasks properly unfreezable

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Message-ID: <20110825215203.GS2803@mtj.dyndns.org>
Date:	Thu, 25 Aug 2011 23:52:03 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	rjw@...k.pl, menage@...gle.com, linux-kernel@...r.kernel.org,
	arnd@...db.de
Subject: Re: [PATCH 06/16] freezer: make exiting tasks properly unfreezable

Hello, Oleg.

On Thu, Aug 25, 2011 at 06:56:50PM +0200, Oleg Nesterov wrote:
> But still I can't understand why it is better to consider the exiting
> task as "frozen" from the very beginning, right after PTRACE_EVENT_EXIT.
> do_exit() does a lot of misc things, and this patch simply makes it
> "invisible" to the freezer. This looks "unsafe" even if this is fine
> for suspend/etc.
> 
> To me, try_to_freeze_tasks() should succed when all threads either
> sleep in refrigerator(), or ->state = TASK_DEAD (the final schedule()
> was called). Until then try_to_freeze_tasks() should retry.

Hmmm... yeah, maybe moving it right after setting ->state would be
safer.  Care to send a patch?

> But since we can't see the threads after exit_notify (in general),
> the current ->exit_state check looks reasonable.

I prefer setting NOFREEZE after point of no return from exit path.
The intention is clearer that way, I think.

> > Another freezer user is the cgroup,
> 
> Yes. And I don't understand this case too. I mean, the fact we ignore
> the exiting tasks.

Because it's basically a mass (or meta) job control mechanism and it's
nicer to be able to kill processes after freezing berserk cgroups.

Thanks.

-- 
tejun
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