Refactor list_remove

Refactor so we know when an element is removed by checking that
element.

All tests pass on qemu. syn-read has an assertion fail in list_next
and multi-oom hangs on bochs.

src/lib/kernel/list.c

@@ -251,7 +251,19 @@ list_remove (struct list_elem *elem)
   ASSERT (is_interior (elem));
   elem->prev->next = elem->next;
   elem->next->prev = elem->prev;
-  return elem->next;
+  struct list_elem *ret = elem->next;
+  elem->prev = NULL;
+  elem->next = NULL;
+  return ret;
+}
+
+struct list_elem *
+try_remove (struct list_elem *elem)
+{
+  if (is_interior(elem)) {
+      return list_remove(elem);
+  }
+  return NULL;
 }
 
 /* Removes the front element from LIST and returns it.

src/lib/kernel/list.h

@@ -145,6 +145,7 @@ void list_push_back (struct list *, struct list_elem *);
 
 /* List removal. */
 struct list_elem *list_remove (struct list_elem *);
+struct list_elem *try_remove (struct list_elem *);
 struct list_elem *list_pop_front (struct list *);
 struct list_elem *list_pop_back (struct list *);
 

src/threads/thread.c

@@ -385,6 +385,7 @@ void thread_exit(void) {
 
     /* Let kids know that parent is dead so that their page is freed without
        waiting for the parent. Will be freed in thread_schedule_tail().*/
+
     while (!list_empty(&cur->kids)) {
         e = list_pop_front(&cur->kids);
         struct thread *kid = list_entry(e, struct thread, kid_elem);
@@ -847,6 +848,14 @@ void thread_schedule_tail(struct thread *prev) {
     if (prev != NULL && prev->status == THREAD_DYING &&
         prev != initial_thread) {
         ASSERT(prev != cur);
+        ASSERT(try_remove(&prev->allelem) == NULL);
+        ASSERT(try_remove(&prev->sleep_elem) == NULL);
+        ASSERT(try_remove(&prev->elem) == NULL);
+        ASSERT(try_remove(&prev->lock_elem) == NULL);
+        ASSERT(try_remove(&prev->kid_elem) == NULL);
+        ASSERT(list_empty(&prev->locks_acquired));
+        ASSERT(list_empty(&prev->open_files));
+        ASSERT(list_empty(&prev->kids));
         palloc_free_page(prev);
     }
 }

src/userprog/syscall.c

@@ -177,10 +177,6 @@ void sys_exit(int status) {
     printf("%s: exit(%d)\n", cur->name, status);
     cur->exit_status = status;
 
-    if (lock_held_by_current_thread(&filesys_lock)) {
-        release_file_lock();
-    }
-
     /* Free all file buffers. */
     struct list_elem *e;
     while (!list_empty(&cur->open_files)) {
@@ -199,6 +195,8 @@ pid_t sys_exec(const char *cmd_line) {
         return ERR;
     }
     struct thread *cur = thread_current();
+
+    /* File system call */
     acquire_file_lock();
     pid_t new_process_pid = process_execute(cmd_line);