oom: fix tasklist_lock leak

Commit 0aad4b31 ("oom: fold __out_of_memory into out_of_memory")
introduced a tasklist_lock leak.  Then it caused following obvious
danger warnings and panic.

    ================================================
    [ BUG: lock held when returning to user space! ]
    ------------------------------------------------
    rsyslogd/1422 is leaving the kernel with locks still held!
    1 lock held by rsyslogd/1422:
     #0:  (tasklist_lock){.+.+.+}, at: [<ffffffff810faf64>] out_of_memory+0x164/0x3f0
    BUG: scheduling while atomic: rsyslogd/1422/0x00000002
    INFO: lockdep is turned off.

This patch fixes it.
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -646,6 +646,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	unsigned long freed = 0;
 	unsigned int points;
 	enum oom_constraint constraint = CONSTRAINT_NONE;
+	int killed = 0;
 
 	blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
 	if (freed > 0)
@@ -683,7 +684,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 		if (!oom_kill_process(current, gfp_mask, order, 0, totalpages,
 				NULL, nodemask,
 				"Out of memory (oom_kill_allocating_task)"))
-			return;
+			goto out;
 	}
 
 retry:
@@ -691,7 +692,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 			constraint == CONSTRAINT_MEMORY_POLICY ? nodemask :
 								 NULL);
 	if (PTR_ERR(p) == -1UL)
-		return;
+		goto out;
 
 	/* Found nothing?!?! Either we hang forever, or we panic. */
 	if (!p) {
@@ -703,13 +704,15 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	if (oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
 				nodemask, "Out of memory"))
 		goto retry;
+	killed = 1;
+out:
 	read_unlock(&tasklist_lock);
 
 	/*
 	 * Give "p" a good chance of killing itself before we
 	 * retry to allocate memory unless "p" is current
 	 */
-	if (!test_thread_flag(TIF_MEMDIE))
+	if (killed && !test_thread_flag(TIF_MEMDIE))
 		schedule_timeout_uninterruptible(1);
 }