Update set priority to eliminate race condition when accessing ready list

src/threads/thread.c

@@ -389,6 +389,8 @@ thread_set_priority (int new_priority)
 
   thread_current ()->priority = new_priority;
 
+  enum intr_level old_level = intr_disable ();
+
   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.
@@ -401,6 +403,8 @@ thread_set_priority (int new_priority)
       thread_yield();
     }
   }
+
+  intr_set_level (old_level);
 }
 
 /* Returns the current thread's priority. */