core: add stat to track unresolved discards in neighbor cache

in __neigh_event_send, if we have a neighbour entry which is in
NUD_INCOMPLETE state, we enqueue any outbound frames to that neighbour
to the neighbours arp_queue, which is default capped to a length of 3
skbs. If that queue exceeds its set length, it will drop an skb on
the queue to enqueue the newly arrived skb. This results in a drop
for which we have no statistics incremented. This patch adds an
unresolved_discards stat to /proc/net/stat/ndisc_cache to track these
lost frames.

Signed-off-by: Neil Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/neighbour.h

···
 	unsigned long destroys;		/* number of destroyed neighs */
 	unsigned long hash_grows;	/* number of hash resizes */
 
-	unsigned long res_failed;	/* nomber of failed resolutions */
+	unsigned long res_failed;	/* number of failed resolutions */
 
 	unsigned long lookups;		/* number of lookups */
 	unsigned long hits;		/* number of hits (among lookups) */
···
 
 	unsigned long periodic_gc_runs;	/* number of periodic GC runs */
 	unsigned long forced_gc_runs;	/* number of forced GC runs */
+
+	unsigned long unres_discards;	/* number of unresolved drops */
 };
 
 #define NEIGH_CACHE_STAT_INC(tbl, field)				\

net/core/neighbour.c

···
 				buff = neigh->arp_queue.next;
 				__skb_unlink(buff, &neigh->arp_queue);
 				kfree_skb(buff);
+				NEIGH_CACHE_STAT_INC(neigh->tbl, unres_discards);
 			}
 			__skb_queue_tail(&neigh->arp_queue, skb);
 		}
···
 	struct neigh_statistics *st = v;
 
 	if (v == SEQ_START_TOKEN) {
-		seq_printf(seq, "entries  allocs destroys hash_grows  lookups hits  res_failed  rcv_probes_mcast rcv_probes_ucast  periodic_gc_runs forced_gc_runs\n");
+		seq_printf(seq, "entries  allocs destroys hash_grows  lookups hits  res_failed  rcv_probes_mcast rcv_probes_ucast  periodic_gc_runs forced_gc_runs unresolved_discards\n");
 		return 0;
 	}
 
 	seq_printf(seq, "%08x  %08lx %08lx %08lx  %08lx %08lx  %08lx  "
-			"%08lx %08lx  %08lx %08lx\n",
+			"%08lx %08lx  %08lx %08lx %08lx\n",
 		   atomic_read(&tbl->entries),
 
 		   st->allocs,
···
 		   st->rcv_probes_ucast,
 
 		   st->periodic_gc_runs,
-		   st->forced_gc_runs
+		   st->forced_gc_runs,
+		   st->unres_discards
 		   );
 
 	return 0;