r8169: change wmb to smb_wmb in rtl8169_start_xmit

A barrier is needed here to ensure that rtl_tx sees the descriptor
changes (DescOwn set) before the updated tp->cur_tx value. Else it may
wrongly assume that the transfer has been finished already. For this
purpose smp_wmb() is sufficient.

No separate barrier is needed for ordering the descriptor changes
with the MMIO doorbell write. The needed barrier is included in
the non-relaxed writel() used by rtl8169_doorbell().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_main.c

@@ -4249,8 +4249,8 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
 
 	txd_first->opts1 |= cpu_to_le32(DescOwn | FirstFrag);
 
-	/* Force all memory writes to complete before notifying device */
-	wmb();
+	/* rtl_tx needs to see descriptor changes before updated tp->cur_tx */
+	smp_wmb();
 
 	tp->cur_tx += frags + 1;