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

[Simon Schippers <simon.schippers@tu-dortmund.de>]

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!