[PATCH] PG_writeback: fix waitqueue_active memory barrier

The spin_unlock() in rotate_reclaimable_page() is not a sufficient memory
barrier.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -420,8 +420,8 @@ void end_page_writeback(struct page *page)
 	if (!TestClearPageReclaim(page) || rotate_reclaimable_page(page)) {
 		if (!test_clear_page_writeback(page))
 			BUG();
-		smp_mb__after_clear_bit();
 	}
+	smp_mb__after_clear_bit();
 	wake_up_page(page, PG_writeback);
 }