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Date: Fri, 30 Jan 2026 16:38:25 -0800
From: Jakub Kicinski <kuba@kernel.org>
To: Eric Dumazet <edumazet@google.com>, Ankit Garg <nktgrg@google.com>
Cc: Joshua Washington <joshwash@google.com>, netdev@vger.kernel.org,
 Harshitha Ramamurthy <hramamurthy@google.com>, Andrew Lunn
 <andrew+netdev@lunn.ch>, "David S. Miller" <davem@davemloft.net>, Paolo
 Abeni <pabeni@redhat.com>, Willem de Bruijn <willemb@google.com>, Praveen
 Kaligineedi <pkaligineedi@google.com>, Catherine Sullivan
 <csully@google.com>, Luigi Rizzo <lrizzo@google.com>, Jon Olson
 <jonolson@google.com>, Sagi Shahar <sagis@google.com>, Bailey Forrest
 <bcf@google.com>, linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH net 0/2] gve: fix crashes on invalid TX queue indices
Message-ID: <20260130163825.3b63222a@kernel.org>
In-Reply-To: <CANn89iJ=Gfjves4qN6hu_VRPSedVRosJsvEQC_irFnEJU_eMLg@mail.gmail.com>
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On Fri, 30 Jan 2026 21:56:07 +0100 Eric Dumazet wrote:
> Jakub, the issue is that before 4.20, calling synchronize_rcu()
> instead of synchronize_rcu_bh()
> was probably a bug. I suspect we had more issues like that.
> 
>  __dev_queue_xmit takes a rcu_read_lock_bh(), while the code (that you
> added in 2018 [1])
> to update the queue netif_set_real_num_tx_queues does synchronize_net()
> (aka synchronize_rcu()) and in earlier times, it would mean that this
> would maybe return too soon (say on preemptible kernels)
> 
> [...]
> 
> So perhaps a fix for pre 4.20 kernel would be: (I kept the
> synchronize_net() to be really cautious and because I really do not
> want to test)

Sounds entirely plausible, FWIW. Ankit, this would mean that you have
to convince RHEL / Rocky to take Eric's patch. Oldest kernel we can
patch upstream is 5.10, AFAIK.