net: ll_temac: Fix race condition causing TX hang

It is possible that the interrupt handler fires and frees up space in
the TX ring in between checking for sufficient TX ring space and
stopping the TX queue in temac_start_xmit. If this happens, the
queue wake from the interrupt handler will occur before the queue is
stopped, causing a lost wakeup and the adapter's transmit hanging.

To avoid this, after stopping the queue, check again whether there is
sufficient space in the TX ring. If so, wake up the queue again.

This is a port of the similar fix in axienet driver,
commit 7de44285 ("net: axienet: Fix race condition causing TX hang").

Fixes: 23ecc4bd ("net: ll_temac: fix checksum offload logic")
Signed-off-by: NEsben Haabendal <esben@geanix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/xilinx/ll_temac_main.c

@@ -788,6 +788,9 @@ static void temac_start_xmit_done(struct net_device *ndev)
 		stat = be32_to_cpu(cur_p->app0);
 	}
 
+	/* Matches barrier in temac_start_xmit */
+	smp_mb();
+
 	netif_wake_queue(ndev);
 }
 
@@ -830,9 +833,19 @@ temac_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
 
 	if (temac_check_tx_bd_space(lp, num_frag + 1)) {
-		if (!netif_queue_stopped(ndev))
-			netif_stop_queue(ndev);
-		return NETDEV_TX_BUSY;
+		if (netif_queue_stopped(ndev))
+			return NETDEV_TX_BUSY;
+
+		netif_stop_queue(ndev);
+
+		/* Matches barrier in temac_start_xmit_done */
+		smp_mb();
+
+		/* Space might have just been freed - check again */
+		if (temac_check_tx_bd_space(lp, num_frag))
+			return NETDEV_TX_BUSY;
+
+		netif_wake_queue(ndev);
 	}
 
 	cur_p->app0 = 0;