mm,oom: make oom_killer_disable() killable

While oom_killer_disable() is called by freeze_processes() after all
user threads except the current thread are frozen, it is possible that
kernel threads invoke the OOM killer and sends SIGKILL to the current
thread due to sharing the thawed victim's memory.  Therefore, checking
for SIGKILL is preferable than TIF_MEMDIE.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -455,15 +455,11 @@ void exit_oom_victim(void)
 bool oom_killer_disable(void)
 {
 	/*
-	 * Make sure to not race with an ongoing OOM killer
-	 * and that the current is not the victim.
+	 * Make sure to not race with an ongoing OOM killer. Check that the
+	 * current is not killed (possibly due to sharing the victim's memory).
 	 */
-	mutex_lock(&oom_lock);
-	if (test_thread_flag(TIF_MEMDIE)) {
-		mutex_unlock(&oom_lock);
+	if (mutex_lock_killable(&oom_lock))
 		return false;
-	}
-
 	oom_killer_disabled = true;
 	mutex_unlock(&oom_lock);