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

[Yunsheng Lin]

On 2020/11/5 14:04, Cong Wang wrote:
> On Mon, Nov 2, 2020 at 11:24 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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
>