Always release filesys_lock when checking if file is valid in process_execute

src/userprog/process.c

@@ -82,14 +82,15 @@ process_execute (const char *cmd)
      of the process. */
   char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
 
-/* NOTE: Currently, the file being executed is closed in load () and then 
+  /* NOTE: Currently, the file being executed is closed in load () and then
           reopened here. Because load is an exported public function, this
           might be necessary. */
   lock_acquire (&filesys_lock);
   /* Validates that the current file to be executed is a valid file */
-  if (filesys_open (file_name) == NULL)
-      return TID_ERROR;
+  bool valid_file = filesys_open (file_name) != NULL;
   lock_release (&filesys_lock);
+  if (!valid_file)
+    return TID_ERROR;
 
   /* Create a new thread to execute the command, by initializing
      it running the function 'start_process' with the appropriate