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Date:   Fri, 14 May 2021 17:17:59 -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>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andriin@fb.com>,
        Eric Dumazet <edumazet@google.com>,
        Wei Wang <weiwan@google.com>,
        "Cong Wang ." <cong.wang@bytedance.com>,
        Taehee Yoo <ap420073@gmail.com>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
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        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
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Subject: Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for
 lockless qdisc
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On Fri, 14 May 2021 16:57:29 -0700 Cong Wang wrote:
> On Fri, May 14, 2021 at 4:39 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Fri, 14 May 2021 16:36:16 -0700 Cong Wang wrote:  
>  [...]  
> > >
> > > 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, makes sense, so hopefully the MISSED-was-set case is not common
and we can depend on __netif_schedule() to DTRT, avoiding the atomic op
in the common case.