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 15:27:28 +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:
>> 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 :)

> 
>> 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? :)


Yes sure, thanks again.
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