Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

[Jakub Kicinski]

On Tue, 1 Jun 2021 16:18:54 +0800 Yunsheng Lin wrote:
> > I see, thanks! That explains the need. Perhaps we can rephrase the
> > comment? Maybe:
> > 
> > +			/* Retest nolock_qdisc_is_empty() within the protection
> > +			 * of q->seqlock to protect from racing with requeuing.
> > +			 */  
> 
> Yes if we still decide to preserve the nolock_qdisc_is_empty() rechecking
> under q->seqlock.

Sounds good.

> >> --- a/net/sched/sch_generic.c
> >> +++ b/net/sched/sch_generic.c
> >> @@ -38,6 +38,15 @@ EXPORT_SYMBOL(default_qdisc_ops);
> >>  static void qdisc_maybe_clear_missed(struct Qdisc *q,
> >>                                      const struct netdev_queue *txq)
> >>  {
> >> +       set_bit(__QDISC_STATE_DRAINING, &q->state);
> >> +
> >> +       /* Make sure DRAINING is set before clearing MISSED
> >> +        * to make sure nolock_qdisc_is_empty() always return
> >> +        * false for aoviding transmitting a packet directly
> >> +        * bypassing the requeued packet.
> >> +        */
> >> +       smp_mb__after_atomic();
> >> +
> >>         clear_bit(__QDISC_STATE_MISSED, &q->state);
> >>
> >>         /* Make sure the below netif_xmit_frozen_or_stopped()
> >> @@ -52,8 +61,6 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q,
> >>          */
> >>         if (!netif_xmit_frozen_or_stopped(txq))
> >>                 set_bit(__QDISC_STATE_MISSED, &q->state);
> >> -       else
> >> -               set_bit(__QDISC_STATE_DRAINING, &q->state);
> >>  }  
> > 
> > But this would not be enough because we may also clear MISSING 
> > in pfifo_fast_dequeue()?  
> 
> For the MISSING clearing in pfifo_fast_dequeue(), it seems it
> looks like the data race described in RFC v3 too?
> 
>       CPU1                 CPU2               CPU3
> qdisc_run_begin(q)          .                  .
>         .              MISSED is set           .
>   MISSED is cleared         .                  .
>     q->dequeue()            .                  .
>         .              enqueue skb1     check MISSED # true
> qdisc_run_end(q)            .                  .
>         .                   .         qdisc_run_begin(q) # true
>         .            MISSED is set      send skb2 directly

Not sure what you mean.