linux-kernel - Re: [RFC PATCH] freezer: allow killing of frozen tasks

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Date:	Tue, 20 Aug 2013 08:27:27 -0400
From:	Tejun Heo <tj@...nel.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Pavel Machek <pavel@....cz>, linux-pm@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>
Subject: Re: [RFC PATCH] freezer: allow killing of frozen tasks

On Tue, Aug 20, 2013 at 02:34:14PM +0200, Rafael J. Wysocki wrote:
> On Tuesday, August 20, 2013 08:22:00 AM Tejun Heo wrote:
> > On Tue, Aug 20, 2013 at 02:30:18PM +0200, Rafael J. Wysocki wrote:
> > > > So, I don't think we can simply turn TASK_UNITERRUPTIBLE to
> > > > TASK_KILLABLE at this point.  We really need to strictly define where
> > > > a task can freeze before being able to do anything like this.
> > > 
> > > But we could do that for user space tasks I suppose?
> > 
> > Even for userland tasks, we don't know where the task is stuck at.  I
> > think there are enough freeze points in the kernel which are in the
> > middle of something which can be used by userland tasks excuting some
> > syscall.  We need to collect all those sites into well defined trap
> > points before doing this.
> 
> OK, thanks!

I scanned through try_to_freeze() users and it seems like we don't
have that many which can be hit by userland tasks.  I think it should
be doable to audit all the users, remove the ones which can be invoked
by userland and make try_to_freeze() whine loudly if it's running off
a userland task except from well-defined spots.  Anyways, we need to
ensure that userland task doesn't get stuck deep in the kernel before
allowing this.

Thanks.

-- 
tejun
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