Re: [PATCH 4/4] cgroup freezer: --- replacement patch 4/4 (b)

[Matt Helsley <matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Wed, Jun 03, 2009 at 08:12:19PM -0400, Oren Laadan wrote:
> From 3c24531980764a71705492c2dfc2cc99366784f3 Mon Sep 17 00:00:00 2001
> From: Oren Laadan <orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org>
> Date: Wed, 3 Jun 2009 19:31:21 -0400
> Subject: [PATCH] c/r: use CHECKPOINTING state for hierarchy's cgroup freezer
> 
> Set state of freezer cgroup of checkpointed task hierarchy to
> "CHECKPOINTING" during a checkpoint, to ensure that tasks cannot
> be thawed while at it.
>
> get_container() grabs a reference to the root task's freezer cgroup.
> Then in may_checkpoint_task() each it verifies that all tasks belong
> to the same freezer group.

Ugh, I really don't like grabbing this reference. It may be necessary
though -- I need to think about it more.

> 
> In particular, the root task is also tested, such that if the root
> tasks changes its freezer cgroups before it moves to "CHECKPOINTING",
> it will be notived and an error returned.

Tasks in a CHECKPOINTING cgroup are frozen and frozen tasks cannot move.
See freezer_can_attach(). This does rather complicate self-checkpoint
though! See my reply to your follow-on patches..

Cheers,
	-Matt Helsley