Fix donate_priority to disable interrupts for entire update of possibly-ready donatee's priority

src/threads/synch.c

@@ -210,23 +210,25 @@ donate_priority (struct thread *donee) {
       if (donor->priority <= donee->priority)
           break;
 
-      donee->priority = donor->priority;
-
       /* Also stop propagation of donation once a donee is reached with
          no donees of its own (sink node in WFG). */
       if (donee->waiting_lock == NULL)
         {
-
           /* Only the sink node of the WFG isn't waiting for a lock and
              could be on the ready list. Thus, as its priority changed,
              it must be reinserted into the list. */
           enum intr_level old_level = intr_disable ();
+          donee->priority = donor->priority;
           ready_list_reinsert (donee);
           intr_set_level (old_level);
+          
           donee = NULL;
         }
       else
-        donee = donee->waiting_lock->holder;
+        {
+          donee->priority = donor->priority;
+          donee = donee->waiting_lock->holder;
+        }
     }
 }