Subject: Re: [PATCH net-next v11 1/4] tun/tap: add ptr_ring consume helper with netdev queue wakeup

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 10 May 2026 11:44:32 -0400

On Sun, May 10, 2026 at 04:01:39PM +0200, Simon Schippers wrote:
> On 5/10/26 15:40, Michael S. Tsirkin wrote:
> > On Sun, May 10, 2026 at 10:55:34AM +0200, Simon Schippers wrote:
> >> On 5/10/26 09:03, Simon Schippers wrote:
> >>> On 5/10/26 00:44, Michael S. Tsirkin wrote:
> >>>> On Sat, May 09, 2026 at 06:31:47PM +0200, Simon Schippers wrote:
> >>>>> On 5/8/26 17:10, Simon Schippers wrote:
> >>>>>> +static void tun_queue_purge(struct tun_struct *tun, struct tun_file *tfile)
> >>>>>>  {
> >>>>>>  	void *ptr;
> >>>>>>  
> >>>>>> -	while ((ptr = ptr_ring_consume(&tfile->tx_ring)) != NULL)
> >>>>>> +	while ((ptr = tun_ring_consume(tun, tfile)) != NULL)
> >>>>>>  		tun_ptr_free(ptr);
> >>>>>>  
> >>>>>>  	skb_queue_purge(&tfile->sk.sk_write_queue);
> >>>>>
> >>>>> Sashiko is right once again. tun_ring_consume() in tun_queue_purge()
> >>>>> operates on a tfile that is being torn down. Its queue_index is no
> >>>>> longer valid. After the swap in __tun_detach(), it points to the
> >>>>> netdev subqueue of a different tfile.
> >>>>> --> We should not wake there.
> >>>>
> >>>> Does it not exactly point at ntfile which is what we want to wake?
> >>>>
> >>>
> >>> I see your point. But calling tun_ring_consume() as done here is
> >>> wrong, because it does not wake if the tx_ring of the tfile 
> >>> (that is currently torn down) is empty. We could change
> >>> tun_ring_consume() to call __tun_wake_queue()
> >>> with consumed=0 if !ptr but I think this would slow down the consumer
> >>> path.
> >>>
> >>
> >> My statement is wrong:
> >> There is no way that the tx_ring is empty and the queue is stopped
> >> at the same time. So we do not need to touch tun_ring_consume() and
> >> this works just fine.
> >>
> >>>>
> >>>>> I will swap tun_ring_consume() with ptr_ring_consume() again and
> >>>>> submit a v12 :)
> >>>>
> >>>> If so then maybe
> >>>> netif_tx_wake_queue(netdev_get_tx_queue(tun->dev, index));
> >>>>
> >>>
> >>> But we should only do this if there is space in the ntfile.
> >>> My approach:
> >>>
> >>> @@ -586,12 +588,18 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
> >>>  		BUG_ON(index >= tun->numqueues);
> >>>  
> >>>  		rcu_assign_pointer(tun->tfiles[index],
> >>>  				   tun->tfiles[tun->numqueues - 1]);
> >>>  		ntfile = rtnl_dereference(tun->tfiles[index]);
> >>> +		spin_lock(&ntfile->tx_ring.consumer_lock);
> >>>  		ntfile->queue_index = index;
> >>>  		ntfile->xdp_rxq.queue_index = index;
> >>> +		ntfile->cons_cnt = 0;
> >>> +		if (__ptr_ring_empty(&ntfile->tx_ring)) {
> >>> +			netif_wake_subqueue(tun->dev, index);
> >>> +		}
> >>> +		spin_unlock(&ntfile->tx_ring.consumer_lock);
> >>>  		rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
> >>>  				   NULL);
> >>>
> >>> ntfile->cons_cnt is unvalid, because the new queue might not be stopped.
> >>> That is the reason why I reset it to 0.
> >>
> >> However, I still prefer this approach because the code is easier to
> >> understand.
> > 
> > 
> > So do you want me to finish review of this one and ack, or want to
> > post v12?
> > 
> 
> I will post a v12 with the proposed changes for patch 1.
> No other changes.
> 
> Thanks!

actually can you clarify? why only when ntfile ring is empty?