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

[Linus Torvalds]

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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