Update donate_priority to only attempt to sort position of the donee that isn't waiting for a lock

src/threads/synch.c

@@ -214,12 +214,17 @@ donate_priority (struct thread *donor, struct thread *donee) {
       /* 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)
-        donee = 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. */
+          ready_list_reinsert (donee);
+          donee = NULL;
+        }
       else
         donee = donee->waiting_lock->holder;
     }
-
-  ready_list_reorder ();
 }
 
 /* Acquires LOCK, sleeping until it becomes available if

src/threads/thread.c

@@ -465,10 +465,22 @@ thread_get_recent_cpu (void)
   return 0;
 }
 
+/* Reinsert thread t into the ready list at its correct position
+   in descending order of priority. Used when this thread's priority
+   may have changed. */
 void
-ready_list_reorder (void)
+ready_list_reinsert (struct thread *t)
 {
-  list_sort (&ready_list, priority_more, NULL);
+  enum intr_level old_level = intr_disable ();
+
+  /* If the thread isn't ready to run, do nothing. */
+  if (t->status == THREAD_READY)
+    {
+      list_remove (&t->elem);
+      list_insert_ordered (&ready_list, &t->elem, priority_more, NULL);
+    }
+
+  intr_set_level (old_level);
 }
 
 /* Idle thread.  Executes when no other thread is ready to run.

src/threads/thread.h

@@ -151,6 +151,6 @@ void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
 
-void ready_list_reorder (void);
+void ready_list_reinsert (struct thread *t);
 
 #endif /* threads/thread.h */