Add priority comparator function for thread list sorting

src/threads/thread.c

@@ -70,6 +70,9 @@ static void *alloc_frame (struct thread *, size_t size);
 static void schedule (void);
 void thread_schedule_tail (struct thread *prev);
 static tid_t allocate_tid (void);
+static bool priority_more (const struct list_elem *a_,
+                           const struct list_elem *b_,
+                           void *aux UNUSED)
 
 /* Initializes the threading system by transforming the code
    that's currently running into a thread.  This can't work in
@@ -350,6 +353,19 @@ thread_foreach (thread_action_func *func, void *aux)
     }
 }
 
+/* Function that compares the two threads associated with the provided
+   list_elem structures. Returns true if the thread associated with a_ has
+   a higher priority than that of b_. */
+static bool
+priority_more (const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux UNUSED)
+{
+  struct thread *a = list_entry (a_, struct thread, elem);
+  struct thread *b = list_entry (b_, struct thread, elem);
+
+  return a->priority > b->priority;
+}
+
 /* Sets the current thread's priority to NEW_PRIORITY. */
 void
 thread_set_priority (int new_priority)