Re: [BUG] pfifo_fast may cause out-of-order CAN frame transmission

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

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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