Update lock_acquire to attempt donation of priorities, w/ S

src/threads/synch.c

@@ -187,6 +187,9 @@ lock_init (struct lock *lock)
   sema_init (&lock->semaphore, 1);
 }
 
+static void
+donate_priority (struct thread *donor, struct thread *donee);
+
 /* Acquires LOCK, sleeping until it becomes available if
    necessary.  The lock must not already be held by the current
    thread.
@@ -202,8 +205,18 @@ lock_acquire (struct lock *lock)
   ASSERT (!intr_context ());
   ASSERT (!lock_held_by_current_thread (lock));
 
+  struct thread *t = thread_current ();
+
+  /* TODO: If a high-priority thread holding a lock sleeps, this may cause a
+     race condition here or break the assumption that donation must occur. */
+  if (lock->holder != NULL)
+    {
+      t->waiting_lock = lock;
+    }
+
   sema_down (&lock->semaphore);
   lock->holder = thread_current ();
+  t->waiting_lock = NULL;
 }
 
 /* Tries to acquires LOCK and returns true if successful or false