RX_RING_MASK
head = (head + 1) & RX_RING_MASK;
passive_buf_list->head = (passive_buf_list->head + 1) & RX_RING_MASK;
tail = (tail + 1) & RX_RING_MASK;
passive_buf_list->tail = (passive_buf_list->tail + 1) & RX_RING_MASK;