netdev - Re: [PATCH net-next 1/4] tun: rcu_deference xdp

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Message-ID: <681bc5f4b261e_20dc6429482@willemb.c.googlers.com.notmuch>
Date: Wed, 07 May 2025 16:43:32 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jon Kohler <jon@...anix.com>, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 davem@...emloft.net, 
 kuba@...nel.org, 
 hawk@...nel.org, 
 john.fastabend@...il.com, 
 netdev@...r.kernel.org, 
 bpf@...r.kernel.org, 
 jon@...anix.com, 
 aleksander.lobakin@...el.com, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, 
 Andrew Lunn <andrew+netdev@...n.ch>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Paolo Abeni <pabeni@...hat.com>, 
 linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH net-next 1/4] tun: rcu_deference xdp_prog only once per
 batch

Jon Kohler wrote:
> Hoist rcu_dereference(tun->xdp_prog) out of tun_xdp_one, so that
> rcu_deference is called once during batch processing.

I'm skeptical that this does anything.

The compiler can inline tun_xdp_one and indeed seems to do so. And
then it can cache the read in a register if that is the best use of
a register.

> 
> No functional change intended.
> 
> Signed-off-by: Jon Kohler <jon@...anix.com>