netdev - Re: [patch 2/3] atomics: Provide rcuref

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Date:   Thu, 2 Mar 2023 11:36:06 -0800
From:   Linus Torvalds <torvalds@...uxfoundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Wangyang Guo <wangyang.guo@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [patch 2/3] atomics: Provide rcuref - scalable reference counting

On Wed, Mar 1, 2023 at 5:05 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> The result of staring more is:
>
> get():
>     6b57:       f0 41 83 45 40 01       lock addl $0x1,0x40(%r13)
>     6b5d:       0f 88 cd 00 00 00       js     6c30                     // -> slowpath if negative

[ rest removed ]

Yeah, so this looks like I was hoping for.

That PREEMPT=y case of 'put() makes me slightly unhappy, and I'm
wondering if it can be improved with better placement of the
preempt_disable/enable, but apart from maybe some massaging to that I
don't see a good way to avoid it.

And the ugliness is mostly about the preemption side, not about the
refcount itself. I've looked at that "preempt_enable ->
preempt_schedule" code generation before, and I've disliked it before,
and I don't have an answer to it.

> but the actual network code does some sanity checking:

Ok. Not pretty. But at least it's just an xadd on the access itself,
there's just some extra noise around it.

            Linus