netpoll: Respect NETIF_F_LLTX

Stop taking the transmit lock when a network device has specified
NETIF_F_LLTX.

If no locks needed to trasnmit a packet this is the ideal scenario for
netpoll as all packets can be trasnmitted immediately.

Even if some locks are needed in ndo_start_xmit skipping any unnecessary
serialization is desirable for netpoll as it makes it more likely a
debugging packet may be trasnmitted immediately instead of being
deferred until later.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2909,6 +2909,11 @@ static inline void netif_tx_unlock_bh(struct net_device *dev)
 	}						\
 }
 
+#define HARD_TX_TRYLOCK(dev, txq)			\
+	(((dev->features & NETIF_F_LLTX) == 0) ?	\
+		__netif_tx_trylock(txq) :		\
+		true )
+
 #define HARD_TX_UNLOCK(dev, txq) {			\
 	if ((dev->features & NETIF_F_LLTX) == 0) {	\
 		__netif_tx_unlock(txq);			\

net/core/netpoll.c

@@ -119,17 +119,17 @@ static void queue_process(struct work_struct *work)
 		txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));
 
 		local_irq_save(flags);
-		__netif_tx_lock(txq, smp_processor_id());
+		HARD_TX_LOCK(dev, txq, smp_processor_id());
 		if (netif_xmit_frozen_or_stopped(txq) ||
 		    netpoll_start_xmit(skb, dev, txq) != NETDEV_TX_OK) {
 			skb_queue_head(&npinfo->txq, skb);
-			__netif_tx_unlock(txq);
+			HARD_TX_UNLOCK(dev, txq);
 			local_irq_restore(flags);
 
 			schedule_delayed_work(&npinfo->tx_work, HZ/10);
 			return;
 		}
-		__netif_tx_unlock(txq);
+		HARD_TX_UNLOCK(dev, txq);
 		local_irq_restore(flags);
 	}
 }
@@ -345,11 +345,11 @@ void netpoll_send_skb_on_dev(struct netpoll *np, struct sk_buff *skb,
 		/* try until next clock tick */
 		for (tries = jiffies_to_usecs(1)/USEC_PER_POLL;
 		     tries > 0; --tries) {
-			if (__netif_tx_trylock(txq)) {
+			if (HARD_TX_TRYLOCK(dev, txq)) {
 				if (!netif_xmit_stopped(txq))
 					status = netpoll_start_xmit(skb, dev, txq);
 
-				__netif_tx_unlock(txq);
+				HARD_TX_UNLOCK(dev, txq);
 
 				if (status == NETDEV_TX_OK)
 					break;