Implement priority scheduling for condition variables

src/threads/synch.c

@@ -265,43 +265,25 @@ struct semaphore_elem
     priority of the first semaphore. This is useful when the thread has not been
     sema'd down yet. */
 static bool
-sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
-                   void *aux)
+sema_priority_more(const struct list_elem *a, const struct list_elem *b,
+                   void *inserting_telem)
 {
-  int a_priority, b_priority;
+  struct list_elem *te_a, *te_b;
   
-  /* If an aux is provided, then use it as the priority of the first semaphore.
-     Otherwise, get the priority of the first semaphore. */
-  if (aux != NULL)
-    a_priority = *(int *) aux;
+  te_b = list_front (
+    &list_entry (b, struct semaphore_elem, elem)->semaphore.waiters);
+
+  if (inserting_telem == NULL)
+    {
+      te_a = list_front (
+        &list_entry (a, struct semaphore_elem, elem)->semaphore.waiters);
+    }
   else
     {
-      struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
-
-      /* If waiters list is empty, return false (i.e., a has lower priority) */
-      if (list_empty(&a->semaphore.waiters))
-        return false;
-
-      /* Otherwise, get the thread with the highest priority from the waiters
-         list. By design, this is the first one in the list (See sema_down). */
-      struct thread *a_thread =
-        list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
-
-      a_priority = a_thread->priority;
+      te_a = inserting_telem;
     }
 
-  struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
-
-  /* If waiters list is empty, return true (i.e., a has higher priority) */
-  if (list_empty(&b->semaphore.waiters))
-    return true;
-
-  struct thread *b_thread =
-    list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
-
-  b_priority = b_thread->priority;
-
-  return a_priority > b_priority;
+  return priority_more (te_a, te_b, NULL);
 }
 
 /* Initializes condition variable COND.  A condition variable
@@ -346,7 +328,8 @@ cond_wait (struct condition *cond, struct lock *lock)
   ASSERT (lock_held_by_current_thread (lock));
   
   sema_init (&waiter.semaphore, 0);
-  list_push_back (&cond->waiters, &waiter.elem);
+  list_insert_ordered (&cond->waiters, &waiter.elem, sema_priority_more,
+                       &thread_current ()->elem);
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);