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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Tue, 16 Mar 2021 11:41:28 -0700
Message-ID: <CAM_iQpXT+tS1NdpiF2M0hAocWJ90mxd5Wp8HoxkEhp4k9QM4hw@mail.gmail.com>
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb
To:     Yunsheng Lin <linyunsheng@huawei.com>
Cc:     David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linuxarm@openeuler.org
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On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> Currently qdisc_lock(q) is taken before enqueuing and dequeuing
> for lockless qdisc's skb_bad_txq/gso_skb queue, qdisc->seqlock is
> also taken, which can provide the same protection as qdisc_lock(q).
>
> This patch removes the unnecessay qdisc_lock(q) protection for
> lockless qdisc' skb_bad_txq/gso_skb queue.
>
> And dev_reset_queue() takes the qdisc->seqlock for lockless qdisc
> besides taking the qdisc_lock(q) when doing the qdisc reset,
> some_qdisc_is_busy() takes both qdisc->seqlock and qdisc_lock(q)
> when checking qdisc status. It is unnecessary to take both lock
> while the fast path only take one lock, so this patch also changes
> it to only take qdisc_lock(q) for locked qdisc, and only take
> qdisc->seqlock for lockless qdisc.
>
> Since qdisc->seqlock is taken for lockless qdisc when calling
> qdisc_is_running() in some_qdisc_is_busy(), use qdisc->running
> to decide if the lockless qdisc is running.

What's the benefit here? Since qdisc->q.lock is also per-qdisc,
so there is no actual contention to take it when we already acquire
q->seqlock, right?

Also, is ->seqlock supposed to be used for protecting skb_bad_txq
etc.? From my understanding, it was introduced merely for replacing
__QDISC_STATE_RUNNING. If you want to extend it, you probably
have to rename it too.

Thanks.