Add comparison function for thread list elements based on thread priority

src/threads/thread.c

@@ -599,6 +599,17 @@ allocate_tid (void)
   return tid;
 }
 
+/* Returns true iff the priority of the first list element's thread is greater
+   than that of the second list element's thread. */
+bool 
+thread_priority_greater (const struct list_elem *a, const struct list_elem *b,
+                      void *aux)
+{
+  struct thread *ta = list_entry (a, struct thread, elem);
+  struct thread *tb = list_entry (b, struct thread, elem);
+  return ta->priority > tb->priority;
+}
+
 /* Offset of `stack' member within `struct thread'.
    Used by switch.S, which can't figure it out on its own. */
 uint32_t thread_stack_ofs = offsetof (struct thread, stack);