Re: [PATCH 2/4] netdev queue flow control for TUN

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 02, 2025 at 10:09:55AM +0200, Simon Schippers wrote:
> The netdev queue is stopped in tun_net_xmit after inserting an SKB into
> the ring buffer if the ring buffer became full because of that. If the
> insertion into the ptr_ring fails, the netdev queue is also stopped and
> the SKB is dropped. However, this never happened in my testing. To ensure
> that the ptr_ring change is available to the consumer before the netdev
> queue stop, an smp_wmb() is used.

I think the stop -> wake bounce involves enough barriers already,
no need for us to get cute.


> Then in tun_ring_recv, the new helper wake_netdev_queue is called in the
> blocking wait queue and after consuming an SKB from the ptr_ring. This
> helper first checks if the netdev queue has stopped. Then with the paired
> smp_rmb() it is known that tun_net_xmit will not produce SKBs anymore.
> With that knowledge, the helper can then wake the netdev queue if there is
> at least a single spare slot in the ptr_ring by calling ptr_ring_spare
> with cnt=1.
> 
> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> ---
>  drivers/net/tun.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index cc6c50180663..735498e221d8 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1060,13 +1060,21 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  	nf_reset_ct(skb);
>  
> -	if (ptr_ring_produce(&tfile->tx_ring, skb)) {
> +	queue = netdev_get_tx_queue(dev, txq);
> +	if (unlikely(ptr_ring_produce(&tfile->tx_ring, skb))) {
> +		/* Paired with smp_rmb() in wake_netdev_queue. */
> +		smp_wmb();
> +		netif_tx_stop_queue(queue);
>  		drop_reason = SKB_DROP_REASON_FULL_RING;
>  		goto drop;
>  	}
> +	if (ptr_ring_full(&tfile->tx_ring)) {
> +		/* Paired with smp_rmb() in wake_netdev_queue. */
> +		smp_wmb();
> +		netif_tx_stop_queue(queue);
> +	}
>  
>  	/* dev->lltx requires to do our own update of trans_start */
> -	queue = netdev_get_tx_queue(dev, txq);
>  	txq_trans_cond_update(queue);
>  
>  	/* Notify and wake up reader process */
> @@ -2110,6 +2118,24 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	return total;
>  }
>  
> +static inline void wake_netdev_queue(struct tun_file *tfile)
> +{
> +	struct netdev_queue *txq;
> +	struct net_device *dev;
> +
> +	rcu_read_lock();
> +	dev = rcu_dereference(tfile->tun)->dev;
> +	txq = netdev_get_tx_queue(dev, tfile->queue_index);
> +
> +	if (netif_tx_queue_stopped(txq)) {
> +		/* Paired with smp_wmb() in tun_net_xmit. */
> +		smp_rmb();
> +		if (ptr_ring_spare(&tfile->tx_ring, 1))
> +			netif_tx_wake_queue(txq);
> +	}
> +	rcu_read_unlock();
> +}
> +
>  static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>  {
>  	DECLARE_WAITQUEUE(wait, current);
> @@ -2139,7 +2165,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>  			error = -EFAULT;
>  			break;
>  		}
> -
> +		wake_netdev_queue(tfile);
>  		schedule();
>  	}
>  
> @@ -2147,6 +2173,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>  	remove_wait_queue(&tfile->socket.wq.wait, &wait);
>  
>  out:
> +	wake_netdev_queue(tfile);
>  	*err = error;
>  	return ptr;
>  }
> -- 
> 2.43.0