netdev - Re: DDoS attack causing bad effect on conntrack searches

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Message-ID: <4BD17F45.3030802@trash.net>
Date:	Fri, 23 Apr 2010 13:06:45 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Jesper Dangaard Brouer <hawk@...u.dk>, paulmck@...ux.vnet.ibm.com,
	Changli Gao <xiaosuo@...il.com>, hawk@...x.dk,
	Linux Kernel Network Hackers <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>
Subject: Re: DDoS attack causing bad effect on conntrack searches

Eric Dumazet wrote:
> Le vendredi 23 avril 2010 à 12:55 +0200, Patrick McHardy a écrit :
>> Eric Dumazet wrote:
>>> OK but a lookup last a fraction of a micro second, unless interrupted by
>>> hard irq.
>>>
>>> Probability of a change during a lookup should be very very small.
>>>
>>> Note that the scenario for a restart is :
>>>
>>> The lookup go through the chain.
>>> While it is examining one object, this object is deleted.
>>> The object is re-allocated by another cpu and inserted to a new chain.
>> I think another scenario that seems a bit more likely would be
>> that a new entry is added to the chain after it was fully searched.
>> Perhaps we could continue searching at the last position if the
>> last entry is not a nulls entry to improve this.
> 
> But the last entry is always a nulls entry, what do you mean exactly ?
> 
> When an unsert (of a fresh object, not a reused one) is done, this
> doesnt affect lookups in any way, since its done at the head of list.

Right, I missed that :)
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