[NETPOLL]: add retry timeout

Add limited retry logic to netpoll_send_skb

Each time we attempt to send, decrement our per-device retry counter.
On every successful send, we reset the counter.

We delay 50us between attempts with up to 20000 retries for a total of
1 second. After we've exhausted our retries, subsequent failed
attempts will try only once until reset by success.

Signed-off-by: Matt Mackall <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netpoll.h

···
 struct netpoll_info {
 	spinlock_t poll_lock;
 	int poll_owner;
+	int tries;
 	int rx_flags;
 	spinlock_t rx_lock;
 	struct netpoll *rx_np; /* netpoll that registered an rx_hook */

net/core/netpoll.c

···
 #define MAX_UDP_CHUNK 1460
 #define MAX_SKBS 32
 #define MAX_QUEUE_DEPTH (MAX_SKBS / 2)
+#define MAX_RETRIES 20000
 
 static DEFINE_SPINLOCK(skb_list_lock);
 static int nr_skbs;
···
 		return;
 	}
 
-	while (1) {
+	do {
+		npinfo->tries--;
 		spin_lock(&np->dev->xmit_lock);
 		np->dev->xmit_lock_owner = smp_processor_id();
 
···
 			np->dev->xmit_lock_owner = -1;
 			spin_unlock(&np->dev->xmit_lock);
 			netpoll_poll(np);
+			udelay(50);
 			continue;
 		}
 
···
 		spin_unlock(&np->dev->xmit_lock);
 
 		/* success */
-		if(!status)
+		if(!status) {
+			npinfo->tries = MAX_RETRIES; /* reset */
 			return;
+		}
 
 		/* transmit busy */
 		netpoll_poll(np);
-	}
+		udelay(50);
+	} while (npinfo->tries > 0);
 }
 
 void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
···
 		npinfo->rx_np = NULL;
 		npinfo->poll_lock = SPIN_LOCK_UNLOCKED;
 		npinfo->poll_owner = -1;
+		npinfo->tries = MAX_RETRIES;
 		npinfo->rx_lock = SPIN_LOCK_UNLOCKED;
 	} else
 		npinfo = ndev->npinfo;