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