linux-kernel - Re: [RFC v2] net: sched: implement TCQ_F_CAN

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Message-ID: <20210315115332.1647e92b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 15 Mar 2021 11:53:32 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <davem@...emloft.net>, <olteanv@...il.com>, <ast@...nel.org>,
        <daniel@...earbox.net>, <andriin@...com>, <edumazet@...gle.com>,
        <weiwan@...gle.com>, <cong.wang@...edance.com>,
        <ap420073@...il.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>,
        <mkl@...gutronix.de>, <linux-can@...r.kernel.org>
Subject: Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless
 qdisc

On Mon, 15 Mar 2021 11:10:18 +0800 Yunsheng Lin wrote:
> @@ -606,6 +623,11 @@ static const u8 prio2band[TC_PRIO_MAX + 1] = {
>   */
>  struct pfifo_fast_priv {
>  	struct skb_array q[PFIFO_FAST_BANDS];
> +
> +	/* protect against data race between enqueue/dequeue and
> +	 * qdisc->empty setting
> +	 */
> +	spinlock_t lock;
>  };
>  
>  static inline struct skb_array *band2list(struct pfifo_fast_priv *priv,
> @@ -623,7 +645,10 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
>  	unsigned int pkt_len = qdisc_pkt_len(skb);
>  	int err;
>  
> -	err = skb_array_produce(q, skb);
> +	spin_lock(&priv->lock);
> +	err = __ptr_ring_produce(&q->ring, skb);
> +	WRITE_ONCE(qdisc->empty, false);
> +	spin_unlock(&priv->lock);
>  
>  	if (unlikely(err)) {
>  		if (qdisc_is_percpu_stats(qdisc))
> @@ -642,6 +667,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
>  	struct sk_buff *skb = NULL;
>  	int band;
>  
> +	spin_lock(&priv->lock);
>  	for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
>  		struct skb_array *q = band2list(priv, band);
>  
> @@ -655,6 +681,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
>  	} else {
>  		WRITE_ONCE(qdisc->empty, true);
>  	}
> +	spin_unlock(&priv->lock);
>  
>  	return skb;
>  }

I thought pfifo was supposed to be "lockless" and this change
re-introduces a lock between producer and consumer, no?