Kamezawa Hiroyuki [off-list ref] writes:

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We test RES_USAGE before taking hugetlb_lock.  What prevents some other
thread from increasing RES_USAGE after that test?

After walking the list we test RES_USAGE after dropping hugetlb_lock.
What prevents another thread from incrementing RES_USAGE before that
test, triggering the BUG?

IIUC core cgroup will prevent a new task getting added to the cgroup
when we are in pre_destroy. Since we already check that the cgroup doesn't
have any task, the RES_USAGE cannot increase in pre_destroy.

You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve
the lock after that, so a task can be attached to the cgroup in this interval.

But that means rmdir can be racy right ? What happens if the task got
added, allocated few pages and then moved out ? We still would have task
count 0 but few pages, which we missed to to move to parent cgroup.

That's a problem even if it's verrrry unlikely.
I'd like to look into it and fix the race in cgroup layer.
But I'm sorry I'm a bit busy in these days...

How about moving that mutex_unlock(&cgroup_mutex) to memcg callback ? That
can be a patch for 3.5 ? 

-aneesh
 

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