1 files changed, 13 insertions, 1 deletions

diff --git a/threads/synch.c b/threads/synch.c
index d7e5e39..98905b1 100644
--- a/threads/synch.c
+++ b/threads/synch.c
@@ -34,6 +34,8 @@
 
 static bool lock_priority_cmp_greater(const struct list_elem *a, const struct list_elem *b,
     void *AUX UNUSED);
+static bool semaphore_priority_cmp_greater(const struct list_elem *a, const struct list_elem *b,
+    void *aux UNUSED);
 
 /* Initializes semaphore SEMA to VALUE.  A semaphore is a
    nonnegative integer along with two atomic operators for
@@ -351,6 +353,16 @@ cond_init (struct condition *cond)
   list_init (&cond->waiters);
 }
 
+/* comparison function for our descending ordered condition waiters list. */
+static bool
+semaphore_priority_cmp_greater(const struct list_elem *a, const struct list_elem *b,
+    void *aux UNUSED)
+{
+  const struct semaphore_elem *s1 = list_entry (a, struct semaphore_elem, elem);
+  const struct semaphore_elem *s2 = list_entry (b, struct semaphore_elem, elem);
+  return (s1->thread->priority > s2->thread->priority);
+}
+
 /* 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
@@ -386,7 +398,7 @@ cond_wait (struct condition *cond, struct lock *lock)
   waiter.thread = thread_current ();
   /* and add condition to our waiters list. sorting isn't useful here either.
      see sema_up()/cond_signal() for more details */
-  list_push_back (&cond->waiters, &waiter->elem);
+  list_push_back (&cond->waiters, &waiter.elem);
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);