linux-kernel - [PATCH 2/4] netdev queue flow control for TUN

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Message-ID: <75a65d7c-9f09-40ff-b984-83a9c1cda7a5@tu-dortmund.de>
Date: Wed, 3 Sep 2025 20:49:52 +0200
From: Simon Schippers <simon.schippers@...dortmund.de>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: willemdebruijn.kernel@...il.com, jasowang@...hat.com, eperezma@...hat.com,
        stephen@...workplumber.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
        kvm@...r.kernel.org, Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: [PATCH 2/4] netdev queue flow control for TUN

Michael S. Tsirkin wrote:
> 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.
>

Yes, you and Jason are correct, it seems to be unnecessary. I just removed
the barriers, and it tests fine!