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

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Date:	Thu, 16 Apr 2009 14:31:48 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Jan Engelhardt <jengelh@...ozas.de>
CC:	Eric Dumazet <dada1@...mosbay.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	paulmck@...ux.vnet.ibm.com, David Miller <davem@...emloft.net>,
	paulus@...ba.org, mingo@...e.hu, torvalds@...ux-foundation.org,
	laijs@...fujitsu.com, r000n@...0n.net,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, benh@...nel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

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.



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