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

Linus Torvalds
2009-04-22T17:30:33Z

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