netdev - Re: Lockdep warning in vxlan

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Message-ID: <50D6D1C9.7040702@mellanox.com>
Date:	Sun, 23 Dec 2012 11:41:29 +0200
From:	Yan Burman <yanb@...lanox.com>
To:	Eric Dumazet <erdnetdev@...il.com>
CC:	Stephen Hemminger <shemminger@...tta.com>, <netdev@...r.kernel.org>
Subject: Re: Lockdep warning in vxlan

On 20-Dec-12 20:16, Eric Dumazet wrote:
> On Thu, 2012-12-20 at 08:34 -0800, Stephen Hemminger wrote:
>> On Thu, 20 Dec 2012 16:00:32 +0200
>> Yan Burman <yanb@...lanox.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)
>
>

I am not being able to reproduce the problem now either with or without 
the patch...
I did get the warning twice when I first reported the issue

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