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

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Message-ID: <20250415073818.06ea327c@kernel.org>
Date: Tue, 15 Apr 2025 07:38:18 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Justin Iurman <justin.iurman@...ege.be>
Cc: Andrea Mayer <andrea.mayer@...roma2.it>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, 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, 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()