Implement priority scheduling for condition variables

src/threads/synch.c

@@ -266,6 +266,35 @@ cond_init (struct condition *cond)
   list_init (&cond->waiters);
 }
 
+/* Returns true iff the priority of the only thread in the first singleton
+   semaphore is greater than the priority of the only thread in the second
+   singleton semaphore.
+   
+   If used for insertion, the third argument must be a pointer to struct
+   list_elem for the thread being inserted, otherwise must be NULL. */
+static bool
+singleton_sema_priority_greater (const struct list_elem *a, 
+                                 const struct list_elem *b,
+                                 void *insertingThread)
+{
+  struct list_elem *te_a, *te_b;
+  
+  te_b = list_front (
+    &list_entry (b, struct semaphore_elem, elem)->semaphore.waiters);
+
+  if (insertingThread == NULL)
+    {
+      te_a = list_front (
+        &list_entry (a, struct semaphore_elem, elem)->semaphore.waiters);
+    }
+  else
+    {
+      te_a = insertingThread;
+    }
+
+  return thread_priority_greater (te_a, te_b, NULL);
+}
+
 /* Atomically releases LOCK and waits for COND to be signaled by
    some other piece of code.  After COND is signaled, LOCK is
    reacquired before returning.  LOCK must be held before calling
@@ -297,7 +326,9 @@ 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, 
+                       singleton_sema_priority_greater,
+                       &thread_current ()->elem);
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);