Subject: Re: [bug] __nf_ct_refresh_acct(): WARNING: at lib/list_debug.c:30 __list_add+0x7d/0xad()

From: Patrick McHardy <[email-masked]>
Date: Wed, 17 Jun 2009 14:00:04 +0200

Eric Dumazet wrote:
> Patrick McHardy a écrit :
>> Before the conntrack is confirmed, it is exclusively handled by a
>> single CPU. I agree that we need to make sure the IPS_CONFIRMED_BIT
>> is visible before we add the conntrack to the hash table since the
>> lookup is lockless, but simply moving the set_bit before the hash
>> insertion should be fine I think.
>>
> 
> Hmm...  now we could have the reverse case :
> 
> __nf_conntrack_confirm() could be "interrupted" by __nf_ct_refresh_acct()
> 
> index 5f72b94..22755fa 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -425,6 +425,7 @@ __nf_conntrack_confirm(struct sk_buff *skb)
>  	/* Remove from unconfirmed list */
>  	hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
>  
> +	set_bit(IPS_CONFIRMED_BIT, &ct->status);
>  	__nf_conntrack_hash_insert(ct, hash, repl_hash);
>  	/* Timer relative to confirmation time, not original
>  	   setting time, otherwise we'd get timer wrap in
> @@ -432,7 +433,6 @@ __nf_conntrack_confirm(struct sk_buff *skb)
>  	ct->timeout.expires += jiffies;
> 
> << What happens if another packet is handled by __nf_ct_refresh_acct here >>
> (seeing or not the IPS_CONFIRMED_BIT) >>
> 
>  	add_timer(&ct->timeout);
> 
> << or here ? >>
> 
> 
>  	atomic_inc(&ct->ct_general.use);
> -	set_bit(IPS_CONFIRMED_BIT, &ct->status);
>  	NF_CT_STAT_INC(net, insert);
>  	spin_unlock_bh(&nf_conntrack_lock);
>  	help = nfct_help(ct);
> 
> Problem is timeout.expires is either a relative or absolute timeout, and changes happen
> in __nf_conntrack_confirm() or __nf_ct_refresh_acct().
> 
> We must have a synchronization (an barriers), a single bit wont be enough.

Please have a look at the second patch I just sent. It relies
on the RCU barriers to make sure all stores are visible before
other CPUs can find the conntrack.