memcg: fix wake up in oom wait queue

OOM-waitqueue should be waken up when oom_disable is canceled.  This is a
fix for 3c11ecf448 ("memcg: oom kill disable and oom status").

How to test:
 Create a cgroup A...
 1. set memory.limit and memory.memsw.limit to be small value
 2. echo 1 > /cgroup/A/memory.oom_control, this disables oom-kill.
 3. run a program which must cause OOM.

A program executed in 3 will sleep by oom_waiqueue in memcg.  Then, how to
wake it up is problem.

 1. echo 0 > /cgroup/A/memory.oom_control (enable OOM-killer)
 2. echo big mem > /cgroup/A/memory.memsw.limit_in_bytes(allow more swap)

etc..

Without the patch, a task in slept can not be waken up.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1370,7 +1370,7 @@ static void memcg_wakeup_oom(struct mem_cgroup *mem)
 
 static void memcg_oom_recover(struct mem_cgroup *mem)
 {
-	if (mem->oom_kill_disable && atomic_read(&mem->oom_lock))
+	if (atomic_read(&mem->oom_lock))
 		memcg_wakeup_oom(mem);
 }
 
@@ -3781,6 +3781,8 @@ static int mem_cgroup_oom_control_write(struct cgroup *cgrp,
 		return -EINVAL;
 	}
 	mem->oom_kill_disable = val;
+	if (!val)
+		memcg_oom_recover(mem);
 	cgroup_unlock();
 	return 0;
 }