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

On 5/10/26 17:44, Michael S. Tsirkin wrote:

On Sun, May 10, 2026 at 04:01:39PM +0200, Simon Schippers wrote:

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On 5/10/26 15:40, Michael S. Tsirkin wrote:

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On Sun, May 10, 2026 at 10:55:34AM +0200, Simon Schippers wrote:

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On 5/10/26 09:03, Simon Schippers wrote:

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On 5/10/26 00:44, Michael S. Tsirkin wrote:

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On Sat, May 09, 2026 at 06:31:47PM +0200, Simon Schippers wrote:

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On 5/8/26 17:10, Simon Schippers wrote:

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+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 Its queue_index is no
longer valid.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.

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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?

This avoids waking when ntfile->tx_ring is full. We can not use 
__ptr_ring_can_produce() with consumer locks, therefore I chose
__ptr_ring_empty() instead.

If there are any elements in ntfile->tx_ring we do not have to wake.
This will be done by the consumer in tun_ring_consume() &
__tun_wake_queue() after consuming those elements.