On 2021/4/13 10:21, Hillf Danton wrote:
> On Mon, 12 Apr 2021 20:00:43 Yunsheng Lin wrote:
>>
>> Yes, the below patch seems to fix the data race described in
>> the commit log.
>> Then what is the difference between my patch and your patch below:)
>
> Hehe, this is one of the tough questions over a bounch of weeks.
>
> If a seqcount can detect the race between skb enqueue and dequeue then we
> cant see any excuse for not rolling back to the point without NOLOCK.
I am not sure I understood what you meant above.
As my understanding, the below patch is essentially the same as
your previous patch, the only difference I see is it uses qdisc->pad
instead of __QDISC_STATE_NEED_RESCHEDULE.
So instead of proposing another patch, it would be better if you
comment on my patch, and make improvement upon that.
>
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -632,6 +632,7 @@ static int pfifo_fast_enqueue(struct sk_
> return qdisc_drop(skb, qdisc, to_free);
> }
>
> + qdisc->pad++;
As has been mentioned:
Doing updating in pfifo_fast_enqueue() unconditionally does not
seems to be performance friendly, which is something my patch
tries to avoid as much as possible.
> qdisc_update_stats_at_enqueue(qdisc, pkt_len);
> return NET_XMIT_SUCCESS;
> }
> @@ -642,6 +643,7 @@ static struct sk_buff *pfifo_fast_dequeu
> struct sk_buff *skb = NULL;
> int band;
>
> + qdisc->pad = 0;
> for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
> struct skb_array *q = band2list(priv, band);
>
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -176,8 +176,12 @@ static inline bool qdisc_run_begin(struc
> static inline void qdisc_run_end(struct Qdisc *qdisc)
> {
> write_seqcount_end(&qdisc->running);
> - if (qdisc->flags & TCQ_F_NOLOCK)
> + if (qdisc->flags & TCQ_F_NOLOCK) {
> spin_unlock(&qdisc->seqlock);
> +
> + if (qdisc->pad != 0)
> + __netif_schedule(qdisc);
> + }
> }
>
> static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
>
> .
>
