diff --git a/src/threads/synch.c b/src/threads/synch.c
index 5ac1e51..65c5bde 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -212,6 +212,7 @@ donate_priority (struct thread *donee) {
   ASSERT (intr_get_level () == INTR_OFF);
 
   struct thread *donor = thread_current ();
+  list_remove (&donor->donor_elem);
   list_push_back (&donee->donors_list, &donor->donor_elem);
   
   while (donee != NULL)
@@ -342,7 +343,6 @@ lock_release (struct lock *lock)
      released, transfer the remaining orphaned donors to its donor list. */
   if (max_donor != NULL)
     {
-      list_remove (&max_donor->donor_elem);
       while (!list_empty (&orphan_list))
         list_push_back (&max_donor->donors_list, list_pop_front (&orphan_list));
     }
diff --git a/src/threads/thread.c b/src/threads/thread.c
index fd239ee..74ac41e 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -373,7 +373,9 @@ thread_exit (void)
      and schedule another process.  That process will destroy us
      when it calls thread_schedule_tail(). */
   intr_disable ();
-  list_remove (&thread_current()->allelem);
+  struct thread *t = thread_current ();
+  list_remove (&t->allelem);
+  list_remove (&t->donor_elem);
   thread_current ()->status = THREAD_DYING;
   schedule ();
   NOT_REACHED ();
@@ -679,6 +681,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->base_priority
       = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
   list_init (&t->donors_list);
+  list_push_back (&t->donors_list, &t->donor_elem);
   t->waiting_lock = NULL;
 
   t->nice = nice;
diff --git a/src/userprog/process.c b/src/userprog/process.c
index ac0cc39..02adb34 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -133,11 +133,10 @@ start_process (void *proc_start_data)
   /* Prevent writing to the file being executed. */
   struct file *exec_file = filesys_open (data->file_name);
   thread_current ()->exec_file = exec_file;
-  file_deny_write(exec_file);
+  file_deny_write (exec_file);
+  lock_release (&filesys_lock);
 
   success = load (data->file_name, &if_.eip, &if_.esp);
-  
-  lock_release (&filesys_lock);
 
   /* If load failed, quit. */
   if (!success) 
@@ -326,7 +325,13 @@ process_exit (void)
   uint32_t *pd;
 
   printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
-  file_close (cur->exec_file);
+  if (cur->exec_file != NULL)
+    {
+      lock_acquire (&filesys_lock);
+      file_allow_write (cur->exec_file);
+      file_close (cur->exec_file);
+      lock_release (&filesys_lock);
+    }
 
   /* Update process result. */
   if (cur->result != NULL)
@@ -486,6 +491,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
   off_t file_ofs;
   bool success = false;
   int i;
+  lock_acquire (&filesys_lock);
 
   /* Allocate and activate page directory. */
   t->pagedir = pagedir_create ();
@@ -585,6 +591,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
  done:
   /* We arrive here whether the load is successful or not. */
   file_close (file);
+  lock_release (&filesys_lock);
   return success;
 }