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

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Message-ID: <ca95bab8-5805-4c33-8318-72d618bf0a95@uliege.be>
Date: Tue, 15 Apr 2025 19:12:35 +0200
From: Justin Iurman <justin.iurman@...ege.be>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: 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 4/15/25 17:12, Alexei Starovoitov wrote:
> 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

Ack, sorry for the confusion. I'll post -v2 with that solution.