Re: 100% CPU load when generating traffic to destination network that nexthop is not reachable

[Julian Anastasov]

        Hello,

On Tue, 15 Aug 2017, Eric Dumazet wrote:

> It must be possible to add a fast path without locks.
> 
> (say if jiffies has not changed before last state change)

        New day - new idea. Something like this? But it
has bug: without checking neigh->dead under lock we don't
have the right to access neigh->parms, it can be destroyed
immediately by neigh_release->neigh_destroy->neigh_parms_put->
neigh_parms_destroy->kfree. Not sure, may be kfree_rcu can help
for this...

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 9816df2..f52763c 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -428,10 +428,10 @@ static inline int neigh_event_send(struct neighbour 
*neigh, struct sk_buff *skb)
 {
        unsigned long now = jiffies;
        
-       if (neigh->used != now)
-               neigh->used = now;
        if (!(neigh->nud_state&(NUD_CONNECTED|NUD_DELAY|NUD_PROBE)))
                return __neigh_event_send(neigh, skb);
+       if (neigh->used != now)
+               neigh->used = now;
        return 0;
 }
 
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 16a1a4c..52a8718 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -991,8 +991,18 @@ static void neigh_timer_handler(unsigned long arg)
 
 int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
 {
-       int rc;
        bool immediate_probe = false;
+       unsigned long now = jiffies;
+       int rc;
+
+       if (neigh->used != now) {
+               neigh->used = now;
+       } else if (neigh->nud_state == NUD_INCOMPLETE &&
+                  (!skb || neigh->arp_queue_len_bytes + skb->truesize >
+                   NEIGH_VAR(neigh->parms, QUEUE_LEN_BYTES))) {
+               kfree_skb(skb);
+               return 1;
+       }
 
        write_lock_bh(&neigh->lock);
 
@@ -1005,7 +1015,7 @@ int __neigh_event_send(struct neighbour *neigh, struct 
sk_buff *skb)
        if (!(neigh->nud_state & (NUD_STALE | NUD_INCOMPLETE))) {
                if (NEIGH_VAR(neigh->parms, MCAST_PROBES) +
                    NEIGH_VAR(neigh->parms, APP_PROBES)) {
-                       unsigned long next, now = jiffies;
+                       unsigned long next;
 
                        atomic_set(&neigh->probes,
                                   NEIGH_VAR(neigh->parms, UCAST_PROBES));

Regards

--
Julian Anastasov <j...@ssi.bg>