netdev - Re: [PATCH net-next 06/18] netfilter: nf_dup{4, 6}: Move duplication check to task

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Message-ID: <99214ac9-cff7-4a5c-b439-ed9ec2c6877c@redhat.com>
Date: Mon, 17 Mar 2025 18:29:51 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Valentin Schneider <vschneid@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Pablo Neira Ayuso <pablo@...filter.org>,
 Jozsef Kadlecsik <kadlec@...filter.org>, Ingo Molnar <mingo@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, netdev@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH net-next 06/18] netfilter: nf_dup{4, 6}: Move duplication
 check to task_struct.

Hi,

On 3/9/25 3:46 PM, Sebastian Andrzej Siewior wrote:
> nf_skb_duplicated is a per-CPU variable and relies on disabled BH for its
> locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
> this data structure requires explicit locking.
> 
> Due to the recursion involved, the simplest change is to make it a
> per-task variable.
> 
> Move the per-CPU variable nf_skb_duplicated to task_struct and name it
> in_nf_duplicate. Add it to the existing bitfield so it doesn't use
> additional memory.
> 
> Cc: Pablo Neira Ayuso <pablo@...filter.org>
> Cc: Jozsef Kadlecsik <kadlec@...filter.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

I'm not a super-fan of adding more flags to 'struct task', but in this
specific case I agree is the better option, as otherwise we should
acquire the local lock for a relatively large scope - the whole packet
processing by nft, right?

Still this needs some explicit ack from the relevant maintainers.
@Peter, @Juri, @Valentin: could you please have a look?

Thanks!

Paolo