Re: [PATCH RFC v1] wireguard: queueing: get rid of per-peer ring buffers

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hey Bjorn,

On Thu, Feb 18, 2021 at 2:50 PM Björn Töpel <bjorn@kernel.org> wrote:
> > +
> > +static void __wg_prev_queue_enqueue(struct prev_queue *queue, struct sk_buff *skb)
> > +{
> > +       WRITE_ONCE(NEXT(skb), NULL);
> > +       smp_wmb();
> > +       WRITE_ONCE(NEXT(xchg_relaxed(&queue->head, skb)), skb);
> > +}
> > +
>
> I'll chime in with Toke; This MPSC and Dmitry's links really took me
> to the "verify with pen/paper"-level! Thanks!
>
> I'd replace the smp_wmb()/_relaxed above with a xchg_release(), which
> might perform better on some platforms. Also, it'll be a nicer pair
> with the ldacq below. :-P In general, it would be nice with some
> wording how the fences pair. It would help the readers (like me!) a
> lot.

Exactly. This is what's been in my dev tree for the last week or so:

+static void __wg_prev_queue_enqueue(struct prev_queue *queue, struct
sk_buff *skb)
+{
+       WRITE_ONCE(NEXT(skb), NULL);
+       WRITE_ONCE(NEXT(xchg_release(&queue->head, skb)), skb);
+}

Look good?

Jason