linux-kernel - Re: [bug] __nf_ct_refresh_acct(): WARNING: at lib/list_debug.c:30 __list

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Date:	Wed, 17 Jun 2009 13:55:42 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
CC:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [bug] __nf_ct_refresh_acct(): WARNING: at lib/list_debug.c:30
 __list_add+0x7d/0xad()

Patrick McHardy a écrit :
> Eric Dumazet wrote:
>> IPS_CONFIRMED_BIT is set under nf_conntrack_lock (in
>> __nf_conntrack_confirm()),
>> we probably want to add a synchronisation under ct->lock as well,
>> or __nf_ct_refresh_acct() could set ct->timeout.expires to extra_jiffies,
>> while a different cpu could confirm the conntrack.
> 
> 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.

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