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

[Cong Wang]

On Fri, Oct 30, 2020 at 12:38 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2020/10/30 3:05, Cong Wang wrote:
> >
> > I do not see how and why it should. synchronize_net() is merely an optimized
> > version of synchronize_rcu(), it should wait for RCU readers, softirqs are not
> > necessarily RCU readers, net_tx_action() does not take RCU read lock either.
>
> Ok, make sense.
>
> Taking RCU read lock in net_tx_action() does not seems to solve the problem,
> what about the time window between __netif_reschedule() and net_tx_action()?
>
> It seems we need to re-dereference the qdisc whenever RCU read lock is released
> and qdisc is still in sd->output_queue or wait for the sd->output_queue to drain?

Not suggesting you to take RCU read lock. We already wait for TX action with
a loop of sleep. To me, the only thing missing is just moving the
reset after that
wait.


> >>>> If we do any additional reset that is not related to qdisc in dev_reset_queue(), we
> >>>> can move it after some_qdisc_is_busy() checking.
> >>>
> >>> I am not suggesting to do an additional reset, I am suggesting to move
> >>> your reset after the busy waiting.
> >>
> >> There maybe a deadlock here if we reset the qdisc after the some_qdisc_is_busy() checking,
> >> because some_qdisc_is_busy() may require the qdisc reset to clear the skb, so that
> >
> > some_qdisc_is_busy() checks the status of qdisc, not the skb queue.
>
> Is there any reason why we do not check the skb queue in the dqisc?
> It seems there may be skb left when netdev is deactivated, maybe at least warn
> about that when there is still skb left when netdev is deactivated?
> Is that why we call qdisc_reset() to clear the leftover skb in qdisc_destroy()?
>
> >
> >
> >> some_qdisc_is_busy() can return false. I am not sure this is really a problem, but
> >> sch_direct_xmit() may requeue the skb when dev_hard_start_xmit return TX_BUSY.
> >
> > Sounds like another reason we should move the reset as late as possible?
>
> Why?

You said "sch_direct_xmit() may requeue the skb", I agree. I assume you mean
net_tx_action() calls sch_direct_xmit() which does the requeue then races with
reset. No?


>
> There current netdev down order is mainly below:
>
> netif_tx_stop_all_queues()
>
> dev_deactivate_queue()
>
> synchronize_net()
>
> dev_reset_queue()
>
> some_qdisc_is_busy()
>
>
> You suggest to change it to below order, right?
>
> netif_tx_stop_all_queues()
>
> dev_deactivate_queue()
>
> synchronize_net()
>
> some_qdisc_is_busy()
>
> dev_reset_queue()

Yes.

>
>
> What is the semantics of some_qdisc_is_busy()?

Waiting for flying TX action.

> 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(). ;)

Thanks.