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

[Patrick McHardy]

Eric Dumazet wrote:
> Patrick McHardy a écrit :
> > 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.
> > >
> > > 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.
>
> Sorry, I dont understand how your second patch corrects the problem.
>
> This (unconfirmed) conntrack is visible by another cpu.

No, before it is confirmed, its only visible to the CPU handling
the initial packet of a connection. Confirmation is the step that
makes it visible to other CPUs.

> This other
> cpu can call __nf_ct_refresh_acct() while this cpu runs
> in __nf_conntrack_confirm()

Not for the same conntrack, that would be a seperate bug.

Does that explain what I'm trying to do? :)

> @@ -425,7 +425,6 @@ __nf_conntrack_confirm(struct sk_buff *skb)
>  	/* Remove from unconfirmed list */
>  	hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
>  
> -	__nf_conntrack_hash_insert(ct, hash, repl_hash);
>  	/* Timer relative to confirmation time, not original
>  	   setting time, otherwise we'd get timer wrap in
>  	   weird delay cases. */
> @@ -433,8 +432,15 @@ __nf_conntrack_confirm(struct sk_buff *skb)
>  	add_timer(&ct->timeout);
>
> <<<< another cpu could here change timeout.expires (thinking its still relative) >>>>
>
>  	atomic_inc(&ct->ct_general.use);
>  	set_bit(IPS_CONFIRMED_BIT, &ct->status);
> +
> +	/* Since the lookup is lockless, hash insertion must be after starting the
> +	 * timer and setting the CONFIRMED bit. The RCU barriers guarantee that no
> +	 * other CPU can find the conntrack before the above stores are visible.
> +	 */
> +	__nf_conntrack_hash_insert(ct, hash, repl_hash);
>  	NF_CT_STAT_INC(net, insert);
>  	spin_unlock_bh(&nf_conntrack_lock);
>  	help = nfct_help(ct);
>  	if (help && help->helper)
>  		nf_conntrack_event_cache(IPCT_HELPER, ct);

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