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

From: Eric Dumazet
Date: Wed Jun 17 2009 - 07:58:16 EST

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 */

+ 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) >>


<< or here ? >>

- set_bit(IPS_CONFIRMED_BIT, &ct->status);
NF_CT_STAT_INC(net, insert);
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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