netdev - Re: Packet gets stuck in NOLOCK pfifo

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Date:   Thu, 2 Jul 2020 08:14:34 +0200
From:   Jonas Bonn <jonas.bonn@...rounds.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, Josh Hunt <johunt@...mai.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

Hi Cong,

On 01/07/2020 21:58, Cong Wang wrote:
> On Wed, Jul 1, 2020 at 9:05 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
>>
>> On Tue, Jun 30, 2020 at 2:08 PM Josh Hunt <johunt@...mai.com> wrote:
>>> Do either of you know if there's been any development on a fix for this
>>> issue? If not we can propose something.
>>
>> If you have a reproducer, I can look into this.
> 
> Does the attached patch fix this bug completely?

It's easier to comment if you inline the patch, but after taking a quick 
look it seems too simplistic.

i)  Are you sure you haven't got the return values on qdisc_run reversed?
ii) There's a "bypass" path that skips the enqueue/dequeue operation if 
the queue is empty; that needs a similar treatment:  after releasing 
seqlock it needs to ensure that another packet hasn't been enqueued 
since it last checked.

/Jonas

> 
> Thanks.
>