netdev - Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

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Date:	Wed, 22 Apr 2009 10:00:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>, paulmck@...ux.vnet.ibm.com,
	Eric Dumazet <dada1@...mosbay.com>,
	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>, kaber@...sh.net,
	jeff.chua.linux@...il.com, laijs@...fujitsu.com,
	jengelh@...ozas.de, r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org, mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

* Stephen Hemminger <shemminger@...tta.com> wrote:

> > That way there's no global cacheline bouncing (just the 
> > _reading_ of a global cacheline - which will be nicely localized 
> > - on NUMA too) - and we will hold at most 1-2 locks at once!
> > 
> > Something like:
> > 
> > 	__cacheline_aligned DEFINE_RWLOCK(global_wrlock);
> > 
> > 	DEFINE_PER_CPU(rwlock_t local_lock);
> > 
> > 
> > 	void local_read_lock(void)
> > 	{
> > 	again:
> > 		read_lock(&per_cpu(local_lock, this_cpu));
> > 
> > 		if (unlikely(!read_can_lock(&global_wrlock))) {
> > 			read_unlock(&per_cpu(local_lock, this_cpu));
> > 			/*
> > 			 * Just wait for any global write activity:
> > 			 */
> > 			read_unlock_wait(&global_wrlock);
> > 			goto again;
> > 		}
> > 	}
> 
> Quit trying to be so damn f*cking cool. [...]

You make it quite hard to give reasonable feedback to your code :-/
First you attack me personally here, then - 30 minutes later - in 
the next iteration of your patch, you do:

+       /* wait for each other cpu to see new table */
+       for_each_possible_cpu(i)
+               if (i != smp_processor_id()) {
+                       xt_info_wrlock(i);
+                       xt_info_wrunlock(i);
+               }

... which i have not seen in your previous patch and which looks 
awfully similar to the write_lock_wait() based primitive i 
suggested.

( Just open-coded in an ugly fashion and slower than a real, proper
  wait-unlock would be, because it dirties all those locks 
  needlessly. )

So you must have agreed with me to a certain degree - i just dont 
see that in any of the discussion. (you seem to totally disagree 
with me to the level of ridiculing me.) Which makes it hard to 
discuss this on a rational basis.

> Your version fails for the case of nested local rules. [...]

Yes, as Eric pointed it out, more than an hour before your reply. I 
find the nesting uninteresting (in fact i find it harmful - see my 
reply to Eric). If you were only interested in nesting then a plain 
old-fashioned rwlock would have done the job.

The detail that is interesting here is how to avoid the global 
rwlock cacheline bounce - not the recursion. (the same-CPU recursion 
is avoidable via proper design or workaround-able via a counter in 
so many ways)

Anyway, i'm back into lurker mode.

	Ingo
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