diff --git a/src/threads/synch.c b/src/threads/synch.c
index 7776cb0..e1bcf9f 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -68,7 +68,8 @@ sema_down (struct semaphore *sema)
   old_level = intr_disable ();
   while (sema->value == 0) 
     {
-      list_push_back (&sema->waiters, &thread_current ()->elem);
+      list_insert_ordered (&sema->waiters, &thread_current ()->elem,
+                           &thread_priority_greater, NULL);
       thread_block ();
     }
   sema->value--;
@@ -118,6 +119,7 @@ sema_up (struct semaphore *sema)
                                 struct thread, elem));
   sema->value++;
   intr_set_level (old_level);
+  thread_yield ();
 }
 
 static void sema_test_helper (void *sema_);