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

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Message-Id: <201108252301.41778.rjw@sisk.pl>
Date:	Thu, 25 Aug 2011 23:01:41 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, menage@...gle.com,
	linux-kernel@...r.kernel.org, arnd@...db.de
Subject: Re: [PATCH 06/16] freezer: make exiting tasks properly unfreezable

On Thursday, August 25, 2011, Oleg Nesterov wrote:
> On 08/25, Tejun Heo wrote:
> >
> > Yeah, what 'freeze' should do is a bit vague on the edges, I think.
> > The freezer can't really halt the whole system operation including
> > IOs.  Tasks aren't the only source which can kick those off.  There
> > are other asynchronous sources,
> 
> Of course.
> 
> 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.

Suspend needs the freezer to ensure that processes (user space mostly)
won't interact with drivers in any way while devices are being suspended,
so if a process is in a state in which it won't talk to any driver
and make changes to filesystems any more, it's irrelevant from the
suspend's point of view.

> 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.
> 
> But since we can't see the threads after exit_notify (in general),
> the current ->exit_state check looks reasonable.
> 
> But again, again, I won't argue.
> 
> > Rafael, can you please enlighten us on the subject?
> 
> Please ;)

Is the above sufficient?

Rafael
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