net: r8169: a question of memory barrier in the r8169 driver

In the rt8169 driver, the function "rtl_tx" uses "smp_mb" to sync the 
writing operation with rtl8169_start_xmit:
     if (tp->dirty_tx != dirty_tx) {
         tp->dirty_tx = dirty_tx;
         smp_mb();
         ...
     }
The function rtl8169_start_xmit reads tp->dirty_tx in TX_FRAGS_READY_FOR:
     if (unlikely(!TX_FRAGS_READY_FOR(tp, skb_shinfo(skb)->nr_frags))) {
         netif_err(tp, drv, dev, "BUG! Tx Ring full when queue awake!\n");
         goto err_stop_0;
     }
But there is no memory barrier around this code.

Is there a possible data race here?
If not, how this data race is avoided?


Thanks,
Jia-Ju Bai