netdev - Re: [PATCH net] net: sched: do not requeue a NULL skb

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Date:	Wed, 6 Apr 2016 10:14:03 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Lars Persson <lars.persson@...s.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Lars Persson <larper@...s.com>
Subject: Re: [PATCH net] net: sched: do not requeue a NULL skb

On Wed, Apr 6, 2016 at 6:10 AM, Lars Persson <lars.persson@...s.com> wrote:
> A failure in validate_xmit_skb_list() triggered an unconditional call
> to dev_requeue_skb with skb=NULL. This slowly grows the queue
> discipline's qlen count until all traffic through the queue stops.
>

Sounds reasonable.

> Fixes: 55a93b3ea780 ("qdisc: validate skb without holding lock")
> Signed-off-by: Lars Persson <larper@...s.com>
> ---
>  net/sched/sch_generic.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index f18c350..1031536 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -165,6 +165,9 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
>                         skb = dev_hard_start_xmit(skb, dev, txq, &ret);
>
>                 HARD_TX_UNLOCK(dev, txq);
> +       } else {
> +               spin_lock(root_lock);
> +               return qdisc_qlen(q);

I think we should return 0 for this failure case so that qdisc_restart()
will stop. How about teaching dev_requeue_skb() to skip skb==NULL
case?