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Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
From:   Paolo Abeni <pabeni@redhat.com>
To:     Jonas Bonn <jonas.bonn@netrounds.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Josh Hunt <johunt@akamai.com>
Cc:     Michael Zhivich <mzhivich@akamai.com>,
        David Miller <davem@davemloft.net>,
        John Fastabend <john.fastabend@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>
Date:   Thu, 02 Jul 2020 11:45:52 +0200
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Hi all,

On Thu, 2020-07-02 at 08:14 +0200, Jonas Bonn wrote:
> Hi Cong,
> 
> On 01/07/2020 21:58, Cong Wang wrote:
> > On Wed, Jul 1, 2020 at 9:05 AM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> > > On Tue, Jun 30, 2020 at 2:08 PM Josh Hunt <johunt@akamai.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?

qdisc_run() returns true if it was able to acquire the seq lock. We
need to take special action in the opposite case, so Cong's patch LGTM
from a functional PoV.

> 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.

That has been reverted with
commit 379349e9bc3b42b8b2f8f7a03f64a97623fff323

---
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 90b59fc50dc9..c7e48356132a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3744,7 +3744,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> 
>  	if (q->flags & TCQ_F_NOLOCK) {
>  		rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> -		qdisc_run(q);
> +		if (!qdisc_run(q) && rc == NET_XMIT_SUCCESS)
> +			__netif_schedule(q);

I fear the __netif_schedule() call may cause performance regression to
the point of making a revert of TCQ_F_NOLOCK preferable. I'll try to
collect some data.

Thanks!

Paolo