Re: Lockdep warning in vxlan

[Eric Dumazet <erdnetdev@gmail.com>]

On Thu, 2012-12-20 at 08:34 -0800, Stephen Hemminger wrote:
> On Thu, 20 Dec 2012 16:00:32 +0200
> Yan Burman <yanb@mellanox.com> wrote:
> 
> > Hi.
> > 
> > When working with vxlan from current net-next, I got a lockdep warning 
> > (below).
> > It seems to happen when I have host B pinging host A and while the pings 
> > continue,
> > I do "ip link del" on the vxlan interface on host A. The lockdep warning 
> > is on host A.
> > Tell me if you need some more info.
> > 
> 
> Looks like the case of nested ARP requests, the initial request is coming
> from neigh_timer (ARP retransmit), but inside neigh_probe the lock
> is dropped?

Bug is from arp_solicit(), releasing the lock after arp_send()

Its used to protect neigh->ha

We could instead copy neigh->ha, without taking n->lock but ha_lock
seqlock, using neigh_ha_snapshot() helper

Yan, could you test the following patch ?

Thanks
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index ce6fbdf..1169ed4 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -321,7 +321,7 @@ static void arp_error_report(struct neighbour *neigh, struct sk_buff *skb)
 static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb)
 {
 	__be32 saddr = 0;
-	u8  *dst_ha = NULL;
+	u8 dst_ha[MAX_ADDR_LEN];
 	struct net_device *dev = neigh->dev;
 	__be32 target = *(__be32 *)neigh->primary_key;
 	int probes = atomic_read(&neigh->probes);
@@ -363,9 +363,9 @@ static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb)
 	if (probes < 0) {
 		if (!(neigh->nud_state & NUD_VALID))
 			pr_debug("trying to ucast probe in NUD_INVALID\n");
-		dst_ha = neigh->ha;
-		read_lock_bh(&neigh->lock);
+		neigh_ha_snapshot(dst_ha, neigh, dev);
 	} else {
+		memset(dst_ha, 0, dev->addr_len);
 		probes -= neigh->parms->app_probes;
 		if (probes < 0) {
 #ifdef CONFIG_ARPD
@@ -377,8 +377,6 @@ static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb)
 
 	arp_send(ARPOP_REQUEST, ETH_P_ARP, target, dev, saddr,
 		 dst_ha, dev->dev_addr, NULL);
-	if (dst_ha)
-		read_unlock_bh(&neigh->lock);
 }
 
 static int arp_ignore(struct in_device *in_dev, __be32 sip, __be32 tip)