linux-kernel - Re: [PATCH net-next v6 3/8] tun/tap: add synchronized ring produce/consume with queue management

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Message-ID: <20251125100655-mutt-send-email-mst@kernel.org>
Date: Tue, 25 Nov 2025 11:54:39 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: willemdebruijn.kernel@...il.com, jasowang@...hat.com,
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, eperezma@...hat.com,
	jon@...anix.com, tim.gebauer@...dortmund.de, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev
Subject: Re: [PATCH net-next v6 3/8] tun/tap: add synchronized ring
 produce/consume with queue management

On Thu, Nov 20, 2025 at 04:29:08PM +0100, Simon Schippers wrote:
> Implement new ring buffer produce and consume functions for tun and tap
> drivers that provide lockless producer-consumer synchronization and
> netdev queue management to prevent ptr_ring tail drop and permanent
> starvation.
> 
> - tun_ring_produce(): Produces packets to the ptr_ring with proper memory
>   barriers and proactively stops the netdev queue when the ring is about
>   to become full.
> 
> - __tun_ring_consume() / __tap_ring_consume(): Internal consume functions
>   that check if the netdev queue was stopped due to a full ring, and wake
>   it when space becomes available. Uses memory barriers to ensure proper
>   ordering between producer and consumer.
> 
> - tun_ring_consume() / tap_ring_consume(): Wrapper functions that acquire
>   the consumer lock before calling the internal consume functions.
> 
> Key features:
> - Proactive queue stopping using __ptr_ring_full_next() to stop the queue
>   before it becomes completely full.
> - Not stopping the queue when the ptr_ring is full already, because if
>   the consumer empties all entries in the meantime, stopping the queue
>   would cause permanent starvation.

what is permanent starvation? this comment seems to answer this
question:


	/* Do not stop the netdev queue if the ptr_ring is full already.
	 * The consumer could empty out the ptr_ring in the meantime
	 * without noticing the stopped netdev queue, resulting in a
	 * stopped netdev queue and an empty ptr_ring. In this case the
	 * netdev queue would stay stopped forever.
	 */


why having a single entry in
the ring we never use helpful to address this?




In fact, all your patch does to solve it, is check
netif_tx_queue_stopped on every consumed packet.


I already proposed:

static inline int __ptr_ring_peek_producer(struct ptr_ring *r)
{
        if (unlikely(!r->size) || r->queue[r->producer])
                return -ENOSPC;
        return 0;
}

And with that, why isn't avoiding the race as simple as
just rechecking after stopping the queue?

__ptr_ring_produce();
if (__ptr_ring_peek_producer())
	netif_tx_stop_queue
	if (!__ptr_ring_peek_producer())
		netif_tx_wake_queue(txq);







-- 
MST