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

[Cong Wang]

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