diff --git a/src/threads/synch.c b/src/threads/synch.c
index 39575ca..ffdb588 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -114,11 +114,22 @@ sema_up (struct semaphore *sema)
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
-  if (!list_empty (&sema->waiters)) 
-    thread_unblock (list_entry (list_pop_front (&sema->waiters),
-                                struct thread, elem));
+
+  /* Wake up (unblock) the highest priority thread from the waiters list */
+  struct thread *t = NULL;
+  if (!list_empty (&sema->waiters))
+    {
+      t = list_entry (list_pop_front (&sema->waiters), struct thread, elem);
+      thread_unblock (t);
+    }
+
   sema->value++;
   intr_set_level (old_level);
+
+  /* If the unblocked thread has higher priority than the current running thread
+     then yield the CPU to the unblocked thread */
+  if (!intr_context() && t != NULL && t->priority > thread_get_priority ())
+    thread_yield ();
 }
 
 static void sema_test_helper (void *sema_);