Hello Paolo,
On 1/10/20 5:31 PM, Paolo Abeni wrote:
> On Thu, 2020-01-09 at 18:39 +0100, Ahmad Fatoum wrote:
>> Hello Paolo,
>>
>> On 1/9/20 1:51 PM, Paolo Abeni wrote:
>>> On Wed, 2020-01-08 at 15:55 +0100, Ahmad Fatoum wrote:
>>>> I've run into an issue of CAN frames being sent out-of-order on an i.MX6 Dual
>>>> with Linux v5.5-rc5. Bisecting has lead me down to this commit:
>>>
>>> Thank you for the report.
>>
>> Thanks for the prompt patch. :-)
>>
>>> The code is only build-tested, could you please try it in your setup?
>>
>> Issue still persists, albeit appears to have become much less frequent. Took 2 million
>> frames till first two were swapped. What I usually saw was a swap every few thousand
>> frames at least and quite often more frequent than that. Might just be noise though.
>
> Thank you for testing. Even with the proposed patch there is still a
> possible race condition: the CPU holding the seqlock can clear the
> 'empty' flag after that the CPU xmitting the packet enqueue it and set
> the 'empty' flag.
>
> The only option I can think of - beyond plain revert - is updating the
> 'empty' flag in a even a more coarse way, as in the following patch.
>
> Again, the code only build tested and very rough, but it would be
> helpful if you could give it a spin.
Issue still reproducible despite the new patch.
Thanks
Ahmad
>
> Thank you!
>
> Paolo
>
> ---
> diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
> index 6a70845bd9ab..fb365fbf65f8 100644
> --- a/include/net/pkt_sched.h
> +++ b/include/net/pkt_sched.h
> @@ -113,7 +113,7 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
> struct net_device *dev, struct netdev_queue *txq,
> spinlock_t *root_lock, bool validate);
>
> -void __qdisc_run(struct Qdisc *q);
> +int __qdisc_run(struct Qdisc *q);
>
> static inline void qdisc_run(struct Qdisc *q)
> {
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index fceddf89592a..df460fe0773a 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -158,7 +158,6 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
> if (qdisc->flags & TCQ_F_NOLOCK) {
> if (!spin_trylock(&qdisc->seqlock))
> return false;
> - WRITE_ONCE(qdisc->empty, false);
> } else if (qdisc_is_running(qdisc)) {
> return false;
> }
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0ad39c87b7fd..b6378bb7b64a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3624,10 +3624,22 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> end_run:
> qdisc_run_end(q);
> } else {
> + int quota = 0;
> +
> rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> - qdisc_run(q);
> + if (!qdisc_run_begin(q))
> + goto out;
> +
> + WRITE_ONCE(q->empty, false);
> + if (likely(!test_bit(__QDISC_STATE_DEACTIVATED,
> + &q->state)))
> + quota = __qdisc_run(q);
> + if (quota > 0)
> + WRITE_ONCE(q->empty, true);
> + qdisc_run_end(q);
> }
>
> +out:
> if (unlikely(to_free))
> kfree_skb_list(to_free);
> return rc;
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 5ab696efca95..1bd2c4e9c4c2 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -376,7 +376,7 @@ static inline bool qdisc_restart(struct Qdisc *q, int *packets)
> return sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
> }
>
> -void __qdisc_run(struct Qdisc *q)
> +int __qdisc_run(struct Qdisc *q)
> {
> int quota = dev_tx_weight;
> int packets;
> @@ -388,6 +388,7 @@ void __qdisc_run(struct Qdisc *q)
> break;
> }
> }
> + return quota;
> }
>
> unsigned long dev_trans_start(struct net_device *dev)
> @@ -649,12 +650,9 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
>
> skb = __skb_array_consume(q);
> }
> - if (likely(skb)) {
> - qdisc_update_stats_at_dequeue(qdisc, skb);
> - } else {
> - WRITE_ONCE(qdisc->empty, true);
> - }
>
> + if (likely(skb))
> + qdisc_update_stats_at_dequeue(qdisc, skb);
> return skb;
> }
>
>
>
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