net: renesas: sh_eth: Fix freeing wrong tx descriptor

The cur_tx counter must be incremented after TACT bit of
txdesc->status was set. However, a CPU is possible to reorder
instructions and/or memory accesses between cur_tx and
txdesc->status. And then, if TX interrupt happened at such a
timing, the sh_eth_tx_free() may free the descriptor wrongly.
So, add wmb() before cur_tx++.
Otherwise NETDEV WATCHDOG timeout is possible to happen.

Fixes: 86a74ff2 ("net: sh_eth: add support for Renesas SuperH Ethernet")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/sh_eth.c

@@ -2533,6 +2533,7 @@ static netdev_tx_t sh_eth_start_xmit(struct sk_buff *skb,
 	else
 		txdesc->status |= cpu_to_le32(TD_TACT);
 
+	wmb(); /* cur_tx must be incremented after TACT bit was set */
 	mdp->cur_tx++;
 
 	if (!(sh_eth_read(ndev, EDTRR) & mdp->cd->edtrr_trns))