Date: Wed, 15 Apr 2009 17:02:57 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Miller <davem@davemloft.net>
cc: dada1@cosmosbay.com, shemminger@vyatta.com, kaber@trash.net,
       jeff.chua.linux@gmail.com, paulmck@linux.vnet.ibm.com, paulus@samba.org,
       mingo@elte.hu, laijs@cn.fujitsu.com, jengelh@medozas.de,
       r000n@r000n.net, linux-kernel@vger.kernel.org,
       netfilter-devel@vger.kernel.org, netdev@vger.kernel.org,
       benh@kernel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)
In-Reply-To: <20090415.164811.19905145.davem@davemloft.net>
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References: <49E5BDF7.8090502@trash.net> <20090415135526.2afc4d18@nehalam> <49E64C91.5020708@cosmosbay.com> <20090415.164811.19905145.davem@davemloft.net>
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On Wed, 15 Apr 2009, David Miller wrote:
> 
> I really think we should entertain the idea where we don't RCU quiesce
> when adding rules.  That was dismissed as not workable because the new
> rule must be "visible" as soon as we return to userspace but let's get
> real, effectively it will be.

I never understood that dismissal.

The new rule _will_ be visible as we return to user space. It's just that 
old packets may still be in flight in other queues.

But that is true even _without_ the "synchronize_net()". The old packets 
just had to make it slightly further in the queueing - but as far as user 
space is concerned, there is absolutely _zero_ difference between the two. 
In both cases it may see packets queued with the old rules. 

> I almost cringed when the per-spinlock idea was proposed, but per-cpu
> rwlocks just takes things too far for my tastes.

I really personally would prefer the RCU approach too. I don't think 
rwlocks are any more cringe-worthy than spinlocks, although it is true 
that they tend to be slightly more expensive.

The pure RCU "just get rid of the unnecessary 'serialze_net()'" approach 
seems to be clearly superior to either.

		Linus
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