netdev - Re: [PATCH net-next 2/2] pfifo_fast: drop unneeded additional lock on dequeue

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Date:   Tue, 15 May 2018 23:17:32 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net-next 2/2] pfifo_fast: drop unneeded additional lock
 on dequeue

On Tue, May 15, 2018 at 04:24:37PM +0200, Paolo Abeni wrote:
> After the previous patch, for NOLOCK qdiscs, q->seqlock is
> always held when the dequeue() is invoked, we can drop
> any additional locking to protect such operation.
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  include/linux/skb_array.h | 5 +++++
>  net/sched/sch_generic.c   | 4 ++--
>  2 files changed, 7 insertions(+), 2 deletions(-)

Is the seqlock taken during qdisc_change_tx_queue_len?
We need to prevent that racing with dequeue.

> diff --git a/include/linux/skb_array.h b/include/linux/skb_array.h
> index a6b6e8bb3d7b..62d9b0a6329f 100644
> --- a/include/linux/skb_array.h
> +++ b/include/linux/skb_array.h
> @@ -97,6 +97,11 @@ static inline bool skb_array_empty_any(struct skb_array *a)
>  	return ptr_ring_empty_any(&a->ring);
>  }
>  
> +static inline struct sk_buff *__skb_array_consume(struct skb_array *a)
> +{
> +	return __ptr_ring_consume(&a->ring);
> +}
> +
>  static inline struct sk_buff *skb_array_consume(struct skb_array *a)
>  {
>  	return ptr_ring_consume(&a->ring);
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index a126f16bc30b..760ab1b09f8b 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -656,7 +656,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
>  		if (__skb_array_empty(q))
>  			continue;
>  
> -		skb = skb_array_consume_bh(q);
> +		skb = __skb_array_consume(q);
>  	}
>  	if (likely(skb)) {
>  		qdisc_qstats_cpu_backlog_dec(qdisc, skb);
> @@ -697,7 +697,7 @@ static void pfifo_fast_reset(struct Qdisc *qdisc)
>  		if (!q->ring.queue)
>  			continue;
>  
> -		while ((skb = skb_array_consume_bh(q)) != NULL)
> +		while ((skb = __skb_array_consume(q)) != NULL)
>  			kfree_skb(skb);
>  	}
>  
> -- 
> 2.14.3