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

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Date:	Wed, 17 Jun 2009 15:29:40 +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 :
>>>> 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.
> 
> Thanks Patrick, I missed this, and your patch seems fine now :)

Thanks for your help, I'll send it to Dave later today.
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