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

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Date:   Wed, 2 Sep 2020 16:14:48 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        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>
Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc

On 2020/9/2 15:32, Eric Dumazet wrote:
> 
> 
> On 9/1/20 11:34 PM, Yunsheng Lin wrote:
> 
>>
>> I am not familiar with TCQ_F_CAN_BYPASS.
>> From my understanding, the problem is that there is no order between
>> qdisc enqueuing and qdisc reset.
> 
> Thw qdisc_reset() should be done after rcu grace period, when there is guarantee no enqueue is in progress.
> 
> qdisc_destroy() already has a qdisc_reset() call, I am not sure why qdisc_deactivate() is also calling qdisc_reset()

That is a good point.
Do we allow skb left in qdisc when the qdisc is deactivated state?
And qdisc_destroy() is not always called after qdisc_deactivate() is called.
If we allow skb left in qdisc when the qdisc is deactivated state, then it is
huge change of semantics for qdisc_deactivate(), and I am not sure how many
cases will affected by this change.


> 
> 
>