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

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Date:   Fri, 14 May 2021 17:17:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
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, 14 May 2021 16:57:29 -0700 Cong Wang wrote:
> 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:  
>  [...]  
> > >
> > > 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.