[PATCH 0/2] memcg: cgroup fix rmdir hang

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

previous discussion was this => http://marc.info/?t=124478543600001&r=1&w=2

This patch tries to fix problem as
  - rmdir can sleep very very long if swap entry is shared between multiple
    cgroups

Now, cgroup's rmdir path does following

==
again:
	check there are no tasks and children group.
	call pre_destroy()
	check css's refcnt
	if (refcnt > 0) {
		sleep until css's refcnt goes down to 0.
		goto again
	}
==

Unfortunately, memory cgroup does following at charge.

	css_get(&memcg->css)
	....
	charge(memcg) (increase USAGE)
	...
And this "memcg" is not necessary to include the caller, task.

pre_destroy() tries to reduce memory usage until USAGE goes down to 0.
Then, there is a race that
	- css's refcnt > 0 (and memcg's usage > 0)
	- rmdir() caller sleeps until css->refcnt goes down 0.
	- But to make css->refcnt be 0, pre_destroy() should be called again.

This patch tries to fix this in asyhcnrounos way (i.e. without big lock.)
Any comments are welcome.

Thanks,
-Kame

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