linux-kernel - Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

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Date:   Wed, 4 Nov 2020 22:16:23 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...wei.com,
        John Fastabend <john.fastabend@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc

On Wed, Nov 4, 2020 at 10:04 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Mon, Nov 2, 2020 at 11:24 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
> > >> From my understanding, we can do anything about the old qdisc (including
> > >> destorying the old qdisc) after some_qdisc_is_busy() return false.
> > >
> > > But the current code does the reset _before_ some_qdisc_is_busy(). ;)
> >
> > If lock is taken when doing reset, it does not matter if the reset is
> > before some_qdisc_is_busy(), right?
>
> Why not? How about the following scenario?
>
> CPU0:                   CPU1:
> dev_reset_queue()
>                         net_tx_action()
>                          -> sch_direct_xmit()
>                            -> dev_requeue_skb()
> some_qdisc_is_busy()
> // waiting for TX action on CPU1
> // now some packets are requeued

Never mind, the skb_bad_txq is also cleared by dev_reset_queue().
TX action after resetting should get NULL.

Thanks.