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

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Date:	Wed, 22 Apr 2009 10:18:50 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Stephen Hemminger <shemminger@...tta.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, paulmck@...ux.vnet.ibm.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)

On Wed, 22 Apr 2009, Eric Dumazet wrote:
> 
> I actually sent *one* buggy patch, and you already gave your feedback 
> and NACK.

Actually, the thing is, I don't even think your original patch was even 
buggy. The bug crept in later. I NAK'ed it not because it was buggy, but 
because of the ad-hoc'ness and the naming. Really.

And I actually even said so in my original rant:

 'The fact that code "happens to work by mistake" (and I'm not saying that
  your does - but it might just because of the per-cpu'ness of it [..]'

because your original patch still had the

	rcu_read_lock_bh();

in place before the whole

	rl = &__get_cpu_var(arp_tables_lock);
	if (likely(rl->count++ == 0))
		spin_lock(&rl->lock);

and that should have protected against both BH callers and preemption.

So I actually believe that your original patch probably worked fine (but 
as I said in my reaction to it, I thought it was almost by mistake and I 
wasn't going to review it)

So the actual _bug_ crept in later, when the RCU lock was removed, and the 
lock was cleaned up and separated into a function of its own.

And in fact, that is kind of my point: "uncommented locking with ad-hoc 
semantics is very fragile". Even _correct_ code ends up not being correct 
in the long run, because people don't realize all the subtle issues.

> I even relayed this to Stephen suggesting him not calling this a recursive lock.
> (Note how I use 'suggesting' here)
> 
> So, what do you want from me ? Should I copy 100 times :

So I consider this thread ended from a technical standpoint.

[ That said, I will not be at all shocked to hear if people decide later 
  that the RCU method was better after all, and that even the per-cpu
  rwlock or spinlock is just too expensive. ]

My problem today (apart from the relatively minor issue of also wanting to 
get the commit log fixed up) is just that I see emails from you finding my 
reaction shocking and from Jarek Poplawski that seem to still think that 
I'm a troll.

Just because I pointed out real technical problems? Is that shocking or 
trolling?

Really - please go back to my _original_ email. No, it was not polite. But 
here's another quote from it:

  "Because even if it works today, it's just a bug waiting to happen."

and dammit, I sent that out _before_ the very next version of the patch 
that actually _did_ introduce that exact bug.

So dammit - what part of my email was "shocking" or "trolling"? The part 
where I was right? Or what?

			Linus
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