neigh: Protect neigh->ha[] with a seqlock

Add a seqlock in struct neighbour to protect neigh->ha[], and avoid
dirtying neighbour in stress situation (many different flows / dsts)

Dirtying takes place because of read_lock(&n->lock) and n->used writes.

Switching to a seqlock, and writing n->used only on jiffies changes
permits less dirtying.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 1 deletions

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index a4538d553704..f04e7a2522c5 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -105,6 +105,7 @@ struct neighbour {
         atomic_t                refcnt;
         atomic_t                probes;
         rwlock_t                lock;
+        seqlock_t               ha_lock;
         unsigned char           ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))];
         struct hh_cache         *hh;
         int                     (*output)(struct sk_buff *skb);
@@ -302,7 +303,10 @@ static inline void neigh_confirm(struct neighbour *neigh)
 
 static inline int neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
 {
-        neigh->used = jiffies;
+        unsigned long now = ACCESS_ONCE(jiffies);
+        
+        if (neigh->used != now)
+                neigh->used = now;
         if (!(neigh->nud_state&(NUD_CONNECTED|NUD_DELAY|NUD_PROBE)))
                 return __neigh_event_send(neigh, skb);
         return 0;
@@ -373,4 +377,14 @@ struct neighbour_cb {
 
 #define NEIGH_CB(skb)   ((struct neighbour_cb *)(skb)->cb)
 
+static inline void neigh_ha_snapshot(char *dst, const struct neighbour *n,
+                                     const struct net_device *dev)
+{
+        unsigned int seq;
+
+        do {
+                seq = read_seqbegin(&n->ha_lock);
+                memcpy(dst, n->ha, dev->addr_len);
+        } while (read_seqretry(&n->ha_lock, seq));
+}
 #endif