netdev - Re: [PATCH net-next-2.6] ip

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Date:	Wed, 29 Sep 2010 12:24:28 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] ip_gre: lockless xmit

On Wed, Sep 29, 2010 at 11:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mercredi 29 septembre 2010 à 10:33 -0700, Jesse Gross a écrit :
>
>> The tx lock has another use here: to break local loops.  With this
>> change, a misconfigured tunnel can bring down the machine with a stack
>> overflow.  There are clearly other ways to fix this that don't require
>> a lock that restricts parallelism, such as a loop counter, but that's
>> the way it is now.
>
> Thats a very good point !
>
> We could use a loop counter in the skb, but this use a bit of ram,
> or percpu counters in tunnel drivers, to avoid a given level of
> recursion.

I agree, a percpu loop counter is the way to go.  I implemented
something nearly identical to deal with this problem in Open vSwitch.

There are actually a number of optimizations in the Open vSwitch
tunneling stack that you may be interested in taking a look at as well
(for example, it has had this sort of lockless transmit for a while):

http://openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=blob;f=datapath/tunnel.c;hb=HEAD

The plan is to upstream all of the kernel code (or at least offer it),
just trying to get the userspace interfaces settled down first.
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