linux-kernel - Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for lockless qdisc

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Date:   Fri, 14 May 2021 16:57:29 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        David Miller <davem@...emloft.net>,
        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>,
        Hillf Danton <hdanton@...a.com>, jgross@...e.com,
        JKosina@...e.com, Michal Kubecek <mkubecek@...e.cz>,
        Björn Töpel <bjorn@...nel.org>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for
 lockless qdisc

On Fri, May 14, 2021 at 4:39 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 14 May 2021 16:36:16 -0700 Cong Wang wrote:
> > > @@ -176,8 +202,15 @@ 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);
> > > +
> > > +               if (unlikely(test_bit(__QDISC_STATE_MISSED,
> > > +                                     &qdisc->state))) {
> > > +                       clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
> >
> > We have test_and_clear_bit() which is atomic, test_bit()+clear_bit()
> > is not.
>
> It doesn't have to be atomic, right? I asked to split the test because
> test_and_clear is a locked op on x86, test by itself is not.

It depends on whether you expect the code under the true condition
to run once or multiple times, something like:

if (test_bit()) {
  clear_bit();
  // this code may run multiple times
}

With the atomic test_and_clear_bit(), it only runs once:

if (test_and_clear_bit()) {
  // this code runs once
}

This is why __netif_schedule() uses test_and_set_bit() instead of
test_bit()+set_bit().

Thanks.