Re: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw_sem
From: Michal Hocko
Date: Wed Aug 30 2017 - 11:43:25 EST
The previous patch is insufficient. drain_all_stock can still race with
the memory offline callback and the underlying memcg disappear. So we
need to be more careful and pin the css on the memcg. This patch
instead...
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