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

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Date:   Thu, 5 Nov 2020 14:32:52 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Cong Wang <xiyou.wangcong@...il.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 2020/11/5 14:16, Cong Wang wrote:
> 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.

Yes, maybe it is safe to remove qdisc_reset() now? or issue a warning
if there are still skb in the qdisc.

> 
> Thanks.
> .
>