memcg: wait for kfree's to finish before destroying cache

memcg: wait for kfree's to finish before destroying cache

kmem_cache_free doesn't expect that the cache can be destroyed as soon as
the object is freed, e.g.  SLUB's implementation may want to update cache
stats after putting the object to the free list.

Therefore we should wait for all kmem_cache_free's to finish before
proceeding to cache destruction.  Since both SLAB and SLUB versions of
kmem_cache_free are non-preemptable, we wait for rcu-sched grace period to
elapse.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>