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

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Date:	Wed, 29 Sep 2010 20:43:14 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesse Gross <jesse@...ira.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] ip_gre: lockless xmit

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.

/* this should be shared by all tunnels */
DEFINE_PER_CPU(tunnel_xmit_count);



tunnel_xmit()
{
if (__this_cpu_read(tunnel_xmit_count) >= LIMIT)
	goto tx_error;
__this_cpu_inc(tunnel_xmit_count);

....

__IPTUNNEL_XMIT(tstats, &dev->stats);

__this_cpu_dec(tunnel_xmit_count),
return NETDEV_TX_OK;

}



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