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Date:   Fri, 14 May 2021 16:39:23 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Cong Wang <xiyou.wangcong@gmail.com>
Cc:     Yunsheng Lin <linyunsheng@huawei.com>,
        David Miller <davem@davemloft.net>,
        Vladimir Oltean <olteanv@gmail.com>,
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        Wei Wang <weiwan@google.com>,
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Subject: Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for
 lockless qdisc
Message-ID: <20210514163923.53f39888@kicinski-fedora-PC1C0HJN>
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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.