Re: [PATCH net v6 3/3] net: sched: fix tx action reschedule issue with stopped queue

[Jakub Kicinski]

On Tue, 11 May 2021 20:13:56 +0800 Yunsheng Lin wrote:
> On 2021/5/11 17:04, Yunsheng Lin wrote:
> > On 2021/5/11 12:22, Jakub Kicinski wrote:  
> >> The queues are woken asynchronously without holding any locks via
> >> netif_tx_wake_queue(). Theoretically we can have a situation where:
> >>
> >> CPU 0                            CPU 1   
> >>   .                                .
> >> dequeue_skb()                      .
> >>   netif_xmit_frozen..() # true     .
> >>   .                              [IRQ]
> >>   .                              netif_tx_wake_queue()
> >>   .                              <end of IRQ>
> >>   .                              netif_tx_action()
> >>   .                              set MISSED
> >>   clear MISSED
> >>   return NULL
> >> ret from qdisc_restart()
> >> ret from __qdisc_run()
> >> qdisc_run_end()  
>  [...]  
> > 
> > Yes, the above does seems to have the above data race.
> > 
> > As my understanding, there is two ways to fix the above data race:
> > 1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
> >    case, just check the netif_xmit_frozen_or_stopped() before
> >    calling __netif_schedule() at the end of qdisc_run_end(). This seems
> >    to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
> >    we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
> >    I am not sure q->dev_queue is pointint to which netdev queue when qdisc
> >    is not set with TCQ_F_ONETXQUEUE flag.

Isn't the case where we have a NOLOCK qdisc without TCQ_F_ONETXQUEUE
rather unexpected? It'd have to be a single pfifo on multi-queue
netdev, right? Sounds not worth optimizing for. How about:

 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
	if (qdisc->flags & TCQ_F_NOLOCK) {
 		spin_unlock(&qdisc->seqlock);

		if (unlikely(test_bit(__QDISC_STATE_MISSED,
				      &qdisc->state))) {
			clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
			if (!(q->flags & TCQ_F_ONETXQUEUE) ||
			    !netif_xmit_frozen_or_stopped(q->dev_queue))
				__netif_schedule(qdisc);
		}
	}
 }

For the strange non-ONETXQUEUE case we'd have an occasional unnecessary
net_tx_action, but no infinite loop possible.

> > 2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
> >    as this patch does, and protect the __netif_schedule() with q->seqlock
> >    for netif_tx_wake_queue(), which might bring unnecessary overhead for
> >    non-stopped queue case
> > 
> > Any better idea?  
> 
> 3. Or check the netif_xmit_frozen_or_stopped() again after clearing
>    STATE_MISSED, like below:
> 
>    if (netif_xmit_frozen_or_stopped(txq)) {
> 	  clear_bit(__QDISC_STATE_MISSED, &q->state);
> 
> 	  /* Make sure the below netif_xmit_frozen_or_stopped()
> 	   * checking happens after clearing STATE_MISSED.
> 	   */
> 	  smp_mb__after_atomic();
> 
> 	  /* Checking netif_xmit_frozen_or_stopped() again to
> 	   * make sure __QDISC_STATE_MISSED is set if the
> 	   * __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
> 	   * rescheduling of net_tx_action() is cleared by the
> 	   * above clear_bit().
> 	   */
> 	  if (!netif_xmit_frozen_or_stopped(txq))
> 	  	set_bit(__QDISC_STATE_MISSED, &q->state);
>   }
> 
>   It is kind of ugly, but it does seem to fix the above data race too.
>   And it seems like a common pattern to deal with the concurrency between
>   xmit and NAPI polling, as below:
> 
> https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c#L1409

This is indeed the idiomatic way of dealing with Tx queue stopping race,
but it's a bit of code to sprinkle around. My vote would be option 1.