[NET]: Convert hh_lock to seqlock.

The hard header cache is in the main output path, so using
seqlock instead of reader/writer lock should reduce overhead.
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -199,7 +199,7 @@ struct hh_cache
                                          */
 	u16		hh_len;		/* length of header */
 	int		(*hh_output)(struct sk_buff *skb);
-	rwlock_t	hh_lock;
+	seqlock_t	hh_lock;
 
 	/* cached hardware header; allow for machine alignment needs.        */
 #define HH_DATA_MOD	16

include/net/neighbour.h

@@ -309,6 +309,24 @@ static inline int neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
 	return 0;
 }
 
+static inline int neigh_hh_output(struct hh_cache *hh, struct sk_buff *skb)
+{
+	unsigned seq;
+	int hh_len;
+
+	do {
+		int hh_alen;
+
+		seq = read_seqbegin(&hh->hh_lock);
+		hh_len = hh->hh_len;
+		hh_alen = HH_DATA_ALIGN(hh_len);
+		memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
+	} while (read_seqretry(&hh->hh_lock, seq));
+
+	skb_push(skb, hh_len);
+	return hh->hh_output(skb);
+}
+
 static inline struct neighbour *
 __neigh_lookup(struct neigh_table *tbl, const void *pkey, struct net_device *dev, int creat)
 {

net/core/neighbour.c

@@ -577,9 +577,10 @@ void neigh_destroy(struct neighbour *neigh)
 	while ((hh = neigh->hh) != NULL) {
 		neigh->hh = hh->hh_next;
 		hh->hh_next = NULL;
-		write_lock_bh(&hh->hh_lock);
+
+		write_seqlock_bh(&hh->hh_lock);
 		hh->hh_output = neigh_blackhole;
-		write_unlock_bh(&hh->hh_lock);
+		write_sequnlock_bh(&hh->hh_lock);
 		if (atomic_dec_and_test(&hh->hh_refcnt))
 			kfree(hh);
 	}
@@ -897,9 +898,9 @@ static void neigh_update_hhs(struct neighbour *neigh)
 
 	if (update) {
 		for (hh = neigh->hh; hh; hh = hh->hh_next) {
-			write_lock_bh(&hh->hh_lock);
+			write_seqlock_bh(&hh->hh_lock);
 			update(hh, neigh->dev, neigh->ha);
-			write_unlock_bh(&hh->hh_lock);
+			write_sequnlock_bh(&hh->hh_lock);
 		}
 	}
 }
@@ -1089,7 +1090,7 @@ static void neigh_hh_init(struct neighbour *n, struct dst_entry *dst,
 			break;
 
 	if (!hh && (hh = kzalloc(sizeof(*hh), GFP_ATOMIC)) != NULL) {
-		rwlock_init(&hh->hh_lock);
+		seqlock_init(&hh->hh_lock);
 		hh->hh_type = protocol;
 		atomic_set(&hh->hh_refcnt, 0);
 		hh->hh_next = NULL;

net/ipv4/ip_output.c

@@ -164,7 +164,6 @@ EXPORT_SYMBOL_GPL(ip_build_and_send_pkt);
 static inline int ip_finish_output2(struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb->dst;
-	struct hh_cache *hh = dst->hh;
 	struct net_device *dev = dst->dev;
 	int hh_len = LL_RESERVED_SPACE(dev);
 
@@ -183,16 +182,9 @@ static inline int ip_finish_output2(struct sk_buff *skb)
 		skb = skb2;
 	}
 
-	if (hh) {
-		int hh_alen;
-
-		read_lock_bh(&hh->hh_lock);
-		hh_alen = HH_DATA_ALIGN(hh->hh_len);
-  		memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
-		read_unlock_bh(&hh->hh_lock);
-	        skb_push(skb, hh->hh_len);
-		return hh->hh_output(skb);
-	} else if (dst->neighbour)
+	if (dst->hh)
+		return neigh_hh_output(dst->hh, skb);
+	else if (dst->neighbour)
 		return dst->neighbour->output(skb);
 
 	if (net_ratelimit())

net/ipv6/ip6_output.c

@@ -72,20 +72,11 @@ static __inline__ void ipv6_select_ident(struct sk_buff *skb, struct frag_hdr *f
 
 static inline int ip6_output_finish(struct sk_buff *skb)
 {
-
 	struct dst_entry *dst = skb->dst;
-	struct hh_cache *hh = dst->hh;
-
-	if (hh) {
-		int hh_alen;
-
-		read_lock_bh(&hh->hh_lock);
-		hh_alen = HH_DATA_ALIGN(hh->hh_len);
-		memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
-		read_unlock_bh(&hh->hh_lock);
-	        skb_push(skb, hh->hh_len);
-		return hh->hh_output(skb);
-	} else if (dst->neighbour)
+
+	if (dst->hh)
+		return neigh_hh_output(dst->hh, skb);
+	else if (dst->neighbour)
 		return dst->neighbour->output(skb);
 
 	IP6_INC_STATS_BH(ip6_dst_idev(dst), IPSTATS_MIB_OUTNOROUTES);