netdev - Re: [PATCH v2] netlink: Replace rhash

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Date:	Tue, 22 Sep 2015 11:42:33 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>,
	Cong Wang <cwang@...pensource.com>, tom@...bertland.com,
	kafai@...com, kernel-team@...com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Jiří Pírko <jiri@...nulli.us>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Thomas Graf <tgraf@...g.ch>, Scott Feldman <sfeldma@...il.com>
Subject: Re: [PATCH v2] netlink: Replace rhash_portid with bound

On Tue, Sep 22, 2015 at 9:10 AM, Tejun Heo <tj@...nel.org> wrote:
>
> That's a pentium pro era errata.  Virtually no working machine is
> affected by that anymore and nobody builds kernel with that option.
> In most cases, store_release and load_acquire are cheaper as they're
> more specific.  On x86, store_release and load_acquire boil down to
> compiler reordering barriers.  You're running in the opposite
> direction.

Well, to be fair, there are lots of machines where acquire/release is
actually quite expensive.

In general, the cheapest barrier there is (apart from the "no barrier
at all" or just "compiler barrier") is "smp_wmb()".  If an
architecture gets that one wrong, the architects were f*cking morons.
It should be a fundamentally cheap operation, since writes are
buffered and it should simply be a buffer barrier.

The acquire/release things are generally fairly cheap on modern
architectures. Not free (except on x86), but fairly low-cost. HOWEVER,
they are not at all free on some older architectures, including 32-bit
ARM.

smp_rmb() should generally be about the same cost as an acquire. It
can go either way.

So *if* the algorithm is amenable to smp_wmb()/smp_rmb() kind of
barriers, that's actually quite possibly better than acquire/release.

smp_mb() is expensive pretty much everywhere.

Looking forward, I suspect long-term acquire/release is what hardware
is going to be "reasonably good at", but as things are right now, you
can't necessarily rely on them being fast.

                   Linus
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