fil_node_open_file() releases fil_system.mutex allowing other thread to open its file node

static bool fil_node_open_file(fil_node_t *node)                                                                                  

{                                                                               

  ...                                                                           

  for (ulint count= 0; fil_system.n_open >= srv_max_n_open_files; count++)      

  {                                                                             

    if (fil_space_t::try_to_close(count > 1))                                   

      count= 0;                                                                 

    else if (count >= 2)                                                        

    {                                                                           

    ...                                                                         

    }                                                                           

    else                                                                        

    {                                                                           

      mutex_exit(&fil_system.mutex);                                            

      os_thread_sleep(20000);                                                   

      /* Flush tablespaces so that we can close modified files. */              

      fil_flush_file_spaces();                                                  

      mutex_enter(&fil_system.mutex);                                           

    }                                                                           

  }                                                                             

  return fil_node_open_file_low(node);                                          

}