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

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Message-ID: <CAADnVQ+of2aBgmOFGNfixtqgp-spYdvZwHyw_=77S5T_+LXCBw@mail.gmail.com>
Date: Tue, 15 Apr 2025 08:12:16 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Justin Iurman <justin.iurman@...ege.be>, Andrea Mayer <andrea.mayer@...roma2.it>, 
	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>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, bpf <bpf@...r.kernel.org>, 
	Stefano Salsano <stefano.salsano@...roma2.it>, Paolo Lungaroni <paolo.lungaroni@...roma2.it>
Subject: Re: [PATCH net] net: lwtunnel: disable preemption when required

On Tue, Apr 15, 2025 at 7:38 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 15 Apr 2025 11:10:01 +0200 Justin Iurman wrote:
> > > However, there is my opinion an issue that can occur: between the check on
> > > in_softirq() and the call to local_bh_disable(), the task may be scheduled on
> > > another CPU. As a result, the check on in_softirq() becomes ineffective because
> > > we may end up disabling BH on a CPU that is not the one we just checked (with
> > > if (in_softirq()) { ... }).
>
> The context is not affected by migration. The context is fully defined
> by the execution stack.
>
> > Hmm, I think it's correct... good catch. I went for this solution to (i)
> > avoid useless nested BHs disable calls; and (ii) avoid ending up with a
> > spaghetti graph of possible paths with or without BHs disabled (i.e.,
> > with single entry points, namely lwtunnel_xmit() and lwtunnel_output()),
> > which otherwise makes it hard to maintain the code IMO.
> >
> > So, if we want to follow what Alexei suggests (see his last response),
> > we'd need to disable BHs in both ip_local_out() and ip6_local_out().
> > These are the common functions which are closest in depth, and so for
> > both lwtunnel_xmit() and lwtunnel_output(). But... at the "cost" of
> > disabling BHs even when it may not be required. Indeed, ip_local_out()
> > and ip6_local_out() both call dst_output(), which one is usually not
> > lwtunnel_output() (and there may not even be a lwtunnel_xmit() to call
> > either).
> >
> > The other solution is to always call local_bh_disable() in both
> > lwtunnel_xmit() and lwtunnel_output(), at the cost of disabling BHs when
> > they were already. Which was basically -v1 and received a NACK from Alexei.
>
> I thought he nacked preempt_disable()

+1.

imo unconditional local_bh_disable() in tx path is fine.
I didn't like the addition of local_bh_disable() in every lwt related
function without doing home work whether it's needed there or not.
Like input path shouldn't need local_bh_disable