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 14:36:14 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
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()

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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