bna: Add synchronization for tx ring.

We received two reports of BUG_ON in bnad_txcmpl_process() where
hw_consumer_index appeared to be ahead of producer_index. Out of order
write/read of these variables could explain these reports.

bnad_start_xmit(), as a producer of tx descriptors, has a few memory
barriers sprinkled around writes to producer_index and the device's
doorbell but they're not paired with anything in bnad_txcmpl_process(), a
consumer.

Since we are synchronizing with a device, we must use mandatory barriers,
not smp_*. Also, I didn't see the purpose of the last smp_mb() in
bnad_start_xmit().
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/brocade/bna/bnad.c

@@ -177,6 +177,7 @@ bnad_txcmpl_process(struct bnad *bnad, struct bna_tcb *tcb)
 		return 0;
 
 	hw_cons = *(tcb->hw_consumer_index);
+	rmb();
 	cons = tcb->consumer_index;
 	q_depth = tcb->q_depth;
 
@@ -3094,7 +3095,7 @@ bnad_start_xmit(struct sk_buff *skb, struct net_device *netdev)
 	BNA_QE_INDX_INC(prod, q_depth);
 	tcb->producer_index = prod;
 
-	smp_mb();
+	wmb();
 
 	if (unlikely(!test_bit(BNAD_TXQ_TX_STARTED, &tcb->flags)))
 		return NETDEV_TX_OK;
@@ -3102,7 +3103,6 @@ bnad_start_xmit(struct sk_buff *skb, struct net_device *netdev)
 	skb_tx_timestamp(skb);
 
 	bna_txq_prod_indx_doorbell(tcb);
-	smp_mb();
 
 	return NETDEV_TX_OK;
 }