[PATCH RFC] memcg: fix drain_all_stock crash

[Michal Hocko]

What do you think about the half backed patch bellow? I didn't manage to
test it yet but I guess it should help. I hate asymmetry of drain_lock
locking (it is acquired somewhere else than it is released which is
not). I will think about a nicer way how to do it.
Maybe I should also split the rcu part in a separate patch.

What do you think?
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