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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Tue, 16 Mar 2021 11:43:36 -0700
Message-ID: <CAM_iQpWPSouO-JP4xHFqOLM8H4Rn5ucF68sa_EK5hUWSYw8feA@mail.gmail.com>
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb
To:     David Miller <davem@redhat.com>
Cc:     Yunsheng Lin <linyunsheng@huawei.com>,
        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 4:42 PM David Miller <davem@redhat.com> wrote:
>
> From: Yunsheng Lin <linyunsheng@huawei.com>
> Date: Mon, 15 Mar 2021 17:30:10 +0800
>
> > 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.
> >
> > Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>
> What about other things protected by this lock, such as statistics and qlen?
>
> This change looks too risky to me.

They are per-cpu for pfifo_fast which sets TCQ_F_CPUSTATS too.

Thanks.