Messages in this thread | | | Date | Wed, 15 Apr 2009 17:02:57 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3) |
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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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