src/threads/thread.c

@@ -388,6 +388,19 @@ thread_set_priority (int new_priority)
     return;
 
   thread_current ()->priority = new_priority;
+
+  if (new_priority < old_priority && !list_empty (&ready_list)) {
+    /* If the new priority is lower than the old priority, check if the current
+       thread no longer has the highest priority. If it doesn't, yield the CPU.
+     */
+
+    struct thread *next_thread =
+      list_entry (list_front (&ready_list), struct thread, elem);
+
+    if (next_thread->priority > new_priority) {
+      thread_yield();
+    }
+  }
 }
 
 /* Returns the current thread's priority. */