netdev - Re: [PATCH net-next 12/24] seg6: Use nested-BH locking for seg6_bpf_srh

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Message-ID: <20240112112355.k1vpvtth@linutronix.de>
Date: Fri, 12 Jan 2024 12:23:55 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Paolo Abeni <pabeni@...hat.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	David Ahern <dsahern@...nel.org>, Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Yonghong Song <yonghong.song@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 12/24] seg6: Use nested-BH locking for
 seg6_bpf_srh_states.

On 2023-12-18 09:33:39 [+0100], Paolo Abeni wrote:
> > --- a/net/ipv6/seg6_local.c
> > +++ b/net/ipv6/seg6_local.c
> > @@ -1420,41 +1422,44 @@ static int input_action_end_bpf(struct sk_buff *skb,
> >  	}
> >  	advance_nextseg(srh, &ipv6_hdr(skb)->daddr);
> >  
> > -	/* preempt_disable is needed to protect the per-CPU buffer srh_state,
> > -	 * which is also accessed by the bpf_lwt_seg6_* helpers
> > +	/* The access to the per-CPU buffer srh_state is protected by running
> > +	 * always in softirq context (with disabled BH). On PREEMPT_RT the
> > +	 * required locking is provided by the following local_lock_nested_bh()
> > +	 * statement. It is also accessed by the bpf_lwt_seg6_* helpers via
> > +	 * bpf_prog_run_save_cb().
> >  	 */
> > -	preempt_disable();
> > -	srh_state->srh = srh;
> > -	srh_state->hdrlen = srh->hdrlen << 3;
> > -	srh_state->valid = true;
> > +	scoped_guard(local_lock_nested_bh, &seg6_bpf_srh_states.bh_lock) {
> > +		srh_state = this_cpu_ptr(&seg6_bpf_srh_states);
> > +		srh_state->srh = srh;
> > +		srh_state->hdrlen = srh->hdrlen << 3;
> > +		srh_state->valid = true;
> 
> Here the 'scoped_guard' usage adds a lot of noise to the patch, due to
> the added indentation. What about using directly
> local_lock_nested_bh()/local_unlock_nested_bh() ?

If this is preferred, sure.

> Cheers,
> 
> Paolo

Sebastian