freezer: fix racy usage of try_to_freeze in kswapd

Currently we can miss freeze_process()->signal_wake_up() in kswapd() if it
happens between try_to_freeze() and prepare_to_wait().  To prevent this
from happening we should check freezing(current) before calling schedule().
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1323,8 +1323,6 @@ static int kswapd(void *p)
 	for ( ; ; ) {
 		unsigned long new_order;
 
-		try_to_freeze();
-
 		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 		new_order = pgdat->kswapd_max_order;
 		pgdat->kswapd_max_order = 0;
@@ -1335,13 +1333,20 @@ static int kswapd(void *p)
 			 */
 			order = new_order;
 		} else {
+			if (!freezing(current))
 				schedule();
+
 			order = pgdat->kswapd_max_order;
 		}
 		finish_wait(&pgdat->kswapd_wait, &wait);
 
+		if (!try_to_freeze()) {
+			/* We can speed up thawing tasks if we don't call
+			 * balance_pgdat after returning from the refrigerator
+			 */
 			balance_pgdat(pgdat, order);
 		}
+	}
 	return 0;
 }