netdev - Re: [net-next PATCH 14/14] net: sched: pfifo_fast use skb

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Date:   Tue, 12 Dec 2017 14:06:50 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [net-next PATCH 14/14] net: sched: pfifo_fast use skb_array

On Thu, Dec 7, 2017 at 9:58 AM, John Fastabend <john.fastabend@...il.com> wrote:
> This converts the pfifo_fast qdisc to use the skb_array data structure
> and set the lockless qdisc bit. pfifo_fast is the first qdisc to support
> the lockless bit that can be a child of a qdisc requiring locking. So
> we add logic to clear the lock bit on initialization in these cases when
> the qdisc graft operation occurs.
>
> This also removes the logic used to pick the next band to dequeue from
> and instead just checks a per priority array for packets from top priority
> to lowest. This might need to be a bit more clever but seems to work
> for now.

A very dumb question:

Why do we call it lockless? With skb_array you just shift the per qdisc
spinlock down to each pfifo band, skb_array still uses a spinlock underneath.

What am I missing here?