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>

4 files changed, 51 insertions, 26 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
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 2044906ecd1a..b165b96355bf 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -294,6 +294,7 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl)
 
         skb_queue_head_init(&n->arp_queue);
         rwlock_init(&n->lock);
+        seqlock_init(&n->ha_lock);
         n->updated        = n->used = now;
         n->nud_state      = NUD_NONE;
         n->output         = neigh_blackhole;
@@ -1015,7 +1016,7 @@ out_unlock_bh:
 }
 EXPORT_SYMBOL(__neigh_event_send);
 
-static void neigh_update_hhs(struct neighbour *neigh)
+static void neigh_update_hhs(const struct neighbour *neigh)
 {
         struct hh_cache *hh;
         void (*update)(struct hh_cache*, const struct net_device*, const unsigned char *)
@@ -1151,7 +1152,9 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
         }
 
         if (lladdr != neigh->ha) {
+                write_seqlock(&neigh->ha_lock);
                 memcpy(&neigh->ha, lladdr, dev->addr_len);
+                write_sequnlock(&neigh->ha_lock);
                 neigh_update_hhs(neigh);
                 if (!(new & NUD_CONNECTED))
                         neigh->confirmed = jiffies -
@@ -1214,6 +1217,7 @@ static inline bool neigh_hh_lookup(struct neighbour *n, struct dst_entry *dst,
 {
         struct hh_cache *hh;
 
+        smp_rmb(); /* paired with smp_wmb() in neigh_hh_init() */
         for (hh = n->hh; hh; hh = hh->hh_next) {
                 if (hh->hh_type == protocol) {
                         atomic_inc(&hh->hh_refcnt);
@@ -1248,8 +1252,8 @@ static void neigh_hh_init(struct neighbour *n, struct dst_entry *dst,
                 kfree(hh);
                 return;
         }
-        read_unlock(&n->lock);
-        write_lock(&n->lock);
+
+        write_lock_bh(&n->lock);
 
         /* must check if another thread already did the insert */
         if (neigh_hh_lookup(n, dst, protocol)) {
@@ -1263,13 +1267,13 @@ static void neigh_hh_init(struct neighbour *n, struct dst_entry *dst,
                 hh->hh_output = n->ops->output;
 
         hh->hh_next = n->hh;
+        smp_wmb(); /* paired with smp_rmb() in neigh_hh_lookup() */
         n->hh       = hh;
 
         if (unlikely(cmpxchg(&dst->hh, NULL, hh) != NULL))
                 hh_cache_put(hh);
 end:
-        write_unlock(&n->lock);
-        read_lock(&n->lock);
+        write_unlock_bh(&n->lock);
 }
 
 /* This function can be used in contexts, where only old dev_queue_xmit
@@ -1308,16 +1312,18 @@ int neigh_resolve_output(struct sk_buff *skb)
         if (!neigh_event_send(neigh, skb)) {
                 int err;
                 struct net_device *dev = neigh->dev;
+                unsigned int seq;
 
-                read_lock_bh(&neigh->lock);
                 if (dev->header_ops->cache &&
                     !dst->hh &&
                     !(dst->flags & DST_NOCACHE))
                         neigh_hh_init(neigh, dst, dst->ops->protocol);
 
-                err = dev_hard_header(skb, dev, ntohs(skb->protocol),
-                                      neigh->ha, NULL, skb->len);
-                read_unlock_bh(&neigh->lock);
+                do {
+                        seq = read_seqbegin(&neigh->ha_lock);
+                        err = dev_hard_header(skb, dev, ntohs(skb->protocol),
+                                              neigh->ha, NULL, skb->len);
+                } while (read_seqretry(&neigh->ha_lock, seq));
 
                 if (err >= 0)
                         rc = neigh->ops->queue_xmit(skb);
@@ -1344,13 +1350,16 @@ int neigh_connected_output(struct sk_buff *skb)
         struct dst_entry *dst = skb_dst(skb);
         struct neighbour *neigh = dst->neighbour;
         struct net_device *dev = neigh->dev;
+        unsigned int seq;
 
         __skb_pull(skb, skb_network_offset(skb));
 
-        read_lock_bh(&neigh->lock);
-        err = dev_hard_header(skb, dev, ntohs(skb->protocol),
-                              neigh->ha, NULL, skb->len);
-        read_unlock_bh(&neigh->lock);
+        do {
+                seq = read_seqbegin(&neigh->ha_lock);
+                err = dev_hard_header(skb, dev, ntohs(skb->protocol),
+                                      neigh->ha, NULL, skb->len);
+        } while (read_seqretry(&neigh->ha_lock, seq));
+
         if (err >= 0)
                 err = neigh->ops->queue_xmit(skb);
         else {
@@ -2148,10 +2157,14 @@ static int neigh_fill_info(struct sk_buff *skb, struct neighbour *neigh,
 
         read_lock_bh(&neigh->lock);
         ndm->ndm_state   = neigh->nud_state;
-        if ((neigh->nud_state & NUD_VALID) &&
-            nla_put(skb, NDA_LLADDR, neigh->dev->addr_len, neigh->ha) < 0) {
-                read_unlock_bh(&neigh->lock);
-                goto nla_put_failure;
+        if (neigh->nud_state & NUD_VALID) {
+                char haddr[MAX_ADDR_LEN];
+
+                neigh_ha_snapshot(haddr, neigh, neigh->dev);
+                if (nla_put(skb, NDA_LLADDR, neigh->dev->addr_len, haddr) < 0) {
+                        read_unlock_bh(&neigh->lock);
+                        goto nla_put_failure;
+                }
         }
 
         ci.ndm_used      = jiffies_to_clock_t(now - neigh->used);
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index f35309578170..d8e540c5b071 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -502,10 +502,8 @@ int arp_find(unsigned char *haddr, struct sk_buff *skb)
 
         if (n) {
                 n->used = jiffies;
-                if (n->nud_state&NUD_VALID || neigh_event_send(n, skb) == 0) {
-                        read_lock_bh(&n->lock);
-                        memcpy(haddr, n->ha, dev->addr_len);
-                        read_unlock_bh(&n->lock);
+                if (n->nud_state & NUD_VALID || neigh_event_send(n, skb) == 0) {
+                        neigh_ha_snapshot(haddr, n, dev);
                         neigh_release(n);
                         return 0;
                 }
diff --git a/net/sched/sch_teql.c b/net/sched/sch_teql.c
index feaabc103ce6..401af9596709 100644
--- a/net/sched/sch_teql.c
+++ b/net/sched/sch_teql.c
@@ -241,11 +241,11 @@ __teql_resolve(struct sk_buff *skb, struct sk_buff *skb_res, struct net_device *
         }
         if (neigh_event_send(n, skb_res) == 0) {
                 int err;
+                char haddr[MAX_ADDR_LEN];
 
-                read_lock(&n->lock);
-                err = dev_hard_header(skb, dev, ntohs(skb->protocol),
-                                      n->ha, NULL, skb->len);
-                read_unlock(&n->lock);
+                neigh_ha_snapshot(haddr, n, dev);
+                err = dev_hard_header(skb, dev, ntohs(skb->protocol), haddr,
+                                      NULL, skb->len);
 
                 if (err < 0) {
                         neigh_release(n);