Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

From: Patrick McHardy <[email-masked]>
Date: Thu, 16 Apr 2009 14:31:48 +0200

Jan Engelhardt wrote:
> On Thursday 2009-04-16 14:12, Patrick McHardy wrote:
>> Jan Engelhardt wrote:
>>>> So change per-cpu spinlocks to per-cpu rwlocks 
>>>> and use read_lock() in ipt_do_table() to allow recursion...
>>>>
>>> iptables cannot quite recurse into itself due to the comefrom stuff.
>> Actually it can by using the REJECT target:
> 
> Yes, but it has to return an absolute verdict (which REJECT does),
> so it's not really a recursion, it's more like a goto without return.

Its recursion in the sense that we reenter the same code path,
while holding a lock. The verdict is issued *after* recursing.

A (quite ugly) workaround would be to have ipt_REJECT queue
the packets to a temporary queue and have ipt_do_table call
dst_output() after dropping the lock.