linux-kernel - Re: [PATCH net v2] net: sched: fix packet stuck problem for lockless qdisc

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Date:   Wed, 24 Mar 2021 12:20:27 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Vladimir Oltean <olteanv@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        "Cong Wang ." <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...neuler.org,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-can@...r.kernel.org, Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>, kpsingh@...nel.org,
        bpf <bpf@...r.kernel.org>, Jonas Bonn <jonas.bonn@...rounds.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        Josh Hunt <johunt@...mai.com>, Jike Song <albcamus@...il.com>,
        Kehuan Feng <kehuan.feng@...il.com>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>, atenart@...nel.org,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net v2] net: sched: fix packet stuck problem for lockless qdisc

On Tue, Mar 23, 2021 at 7:24 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
> @@ -176,8 +207,23 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>  static inline void qdisc_run_end(struct Qdisc *qdisc)
>  {
>         write_seqcount_end(&qdisc->running);
> -       if (qdisc->flags & TCQ_F_NOLOCK)
> +       if (qdisc->flags & TCQ_F_NOLOCK) {
>                 spin_unlock(&qdisc->seqlock);
> +
> +               /* qdisc_run_end() is protected by RCU lock, and
> +                * qdisc reset will do a synchronize_net() after
> +                * setting __QDISC_STATE_DEACTIVATED, so testing
> +                * the below two bits separately should be fine.

Hmm, why synchronize_net() after setting this bit is fine? It could
still be flipped right after you test RESCHEDULE bit.


> +                * For qdisc_run() in net_tx_action() case, we
> +                * really should provide rcu protection explicitly
> +                * for document purposes or PREEMPT_RCU.
> +                */
> +               if (unlikely(test_bit(__QDISC_STATE_NEED_RESCHEDULE,
> +                                     &qdisc->state) &&
> +                            !test_bit(__QDISC_STATE_DEACTIVATED,
> +                                      &qdisc->state)))

Why do you want to test __QDISC_STATE_DEACTIVATED bit at all?
dev_deactivate_many() will wait for those scheduled but being
deactivated, so what's the problem of scheduling it even with this bit?

Thanks.