netdev - Re: [PATCH net] net: lwtunnel: disable preemption when required

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Message-ID: <CAADnVQK7vNPbMS7T9TUOW7s6HNbfr4H8CWbjPgVXW7xa+ybPsw@mail.gmail.com>
Date: Mon, 7 Apr 2025 10:54:59 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Justin Iurman <justin.iurman@...ege.be>
Cc: Sebastian Sewior <bigeasy@...utronix.de>, Stanislav Fomichev <stfomichev@...il.com>, 
	Network Development <netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...zon.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH net] net: lwtunnel: disable preemption when required

On Sun, Apr 6, 2025 at 1:59 AM Justin Iurman <justin.iurman@...ege.be> wrote:
>
> On 4/4/25 16:19, Sebastian Sewior wrote:
> > Alexei, thank you for the Cc.
> >
> > On 2025-04-03 13:35:10 [-0700], Alexei Starovoitov wrote:
> >> Stating the obvious...
> >> Sebastian did a lot of work removing preempt_disable from the networking
> >> stack.
> >> We're certainly not adding them back.
> >> This patch is no go.
> >
> > While looking through the code, it looks as if lwtunnel_xmit() lacks a
> > local_bh_disable().
>
> Thanks Sebastian for the confirmation, as the initial idea was to use
> local_bh_disable() as well. Then I thought preempt_disable() would be
> enough in this context, but I didn't realize you made efforts to remove
> it from the networking stack.
>
> @Alexei, just to clarify: would you ACK this patch if we do
> s/preempt_{disable|enable}()/local_bh_{disable|enable}()/g ?

You need to think it through and not sprinkle local_bh_disable in
every lwt related function.
Like lwtunnel_input should be running with bh disabled already.

I don't remember the exact conditions where bh is disabled in xmit path.