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

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Message-ID: <779d3346-0344-9064-15d5-4d565647a556@pengutronix.de>
Date:   Thu, 9 Jan 2020 18:39:50 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Paolo Abeni <pabeni@...hat.com>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     Sascha Hauer <kernel@...gutronix.de>
Subject: Re: [BUG] pfifo_fast may cause out-of-order CAN frame transmission

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.

Thanks
Ahmad

> 
> Thanks,
> 
> Paolo
> ---
> 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..3c46575a5af5 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3625,6 +3625,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
>  			qdisc_run_end(q);
>  		} else {
>  			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> +			if (rc != NET_XMIT_DROP && READ_ONCE(q->empty))
> +				WRITE_ONCE(q->empty, false);
>  			qdisc_run(q);
>  		}
>  
> 
> 

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