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Subject: Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc
To:     Cong Wang <xiyou.wangcong@gmail.com>
CC:     Jamal Hadi Salim <jhs@mojatatu.com>, Jiri Pirko <jiri@resnulli.us>,
        "David Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        "Linux Kernel Network Developers" <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, <linuxarm@huawei.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <ae6993be-3af2-793c-1621-9f211e52e446@huawei.com>
Date:   Thu, 5 Nov 2020 14:22:55 +0800
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On 2020/11/5 14:04, Cong Wang wrote:
> On Mon, Nov 2, 2020 at 11:24 PM Yunsheng Lin <linyunsheng@huawei.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()

There is no skb in the dqisc now.

>                         net_tx_action()
>                          -> sch_direct_xmit()

So when CPU1 calls net_tx_action(), there is no skb to send and
no skb to be requeued too.

>                            -> dev_requeue_skb()
> some_qdisc_is_busy()
> // waiting for TX action on CPU1
> // now some packets are requeued
>