memcg, kmem: fix reference count handling on the error path

mem_cgroup_css_online calls mem_cgroup_put if memcg_init_kmem fails.
This is not correct because only memcg_propagate_kmem takes an
additional reference while mem_cgroup_sockets_init is allowed to fail as
well (although no current implementation fails) but it doesn't take any
reference.  This all suggests that it should be memcg_propagate_kmem
that should clean up after itself so this patch moves mem_cgroup_put
over there.

Unfortunately this is not that easy (as pointed out by Li Zefan) because
memcg_kmem_mark_dead marks the group dead (KMEM_ACCOUNTED_DEAD) if it is
marked active (KMEM_ACCOUNTED_ACTIVE) which is the case even if
memcg_propagate_kmem fails so the additional reference is dropped in
that case in kmem_cgroup_destroy which means that the reference would be
dropped two times.

The easiest way then would be to simply remove mem_cgrroup_put from
mem_cgroup_css_online and rely on kmem_cgroup_destroy doing the right
thing.

Signed-off-by: Michal Hocko <[email protected]>
Signed-off-by: Li Zefan <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Glauber Costa <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: <[email protected]>	[3.8]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -6345,14 +6345,6 @@ mem_cgroup_css_online(struct cgroup *cont)
 
 	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
 	mutex_unlock(&memcg_create_mutex);
-	if (error) {
-		/*
-		 * We call put now because our (and parent's) refcnts
-		 * are already in place. mem_cgroup_put() will internally
-		 * call __mem_cgroup_free, so return directly
-		 */
-		mem_cgroup_put(memcg);
-	}
 	return error;
 }