linux-kernel - Re: [PATCH bpf-next] bpf: remove unnecessary rcu_read_lock in kprobe_multi_link_prog

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Message-ID: <2797045.mvXUDI8C0e@7940hx>
Date: Thu, 28 Aug 2025 09:03:07 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Menglong Dong <menglong8.dong@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject:
 Re: [PATCH bpf-next] bpf: remove unnecessary rcu_read_lock in
 kprobe_multi_link_prog_run

On 2025/8/28 00:21 Alexei Starovoitov <alexei.starovoitov@...il.com> write:
> On Wed, Aug 27, 2025 at 5:38 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > Preemption is disabled in ftrace graph, which indicate rcu_read_lock. So
> > the rcu_read_lock is not needed in fprobe_entry(), and it is not needed
> > in kprobe_multi_link_prog_run() neither.
> 
> kprobe_busy_begin() doing preempt_disable() is an implementation
> detail that might change.
> Having explicit rcu_read_lock() doesn't hurt.
> It's a nop anyway in PREEMPT_NONE.

Ok, I see. Thanks~

> 
> pw-bot: cr
> 
> > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > ---
> >  kernel/trace/bpf_trace.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 606007c387c5..0e79fa84a634 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2741,12 +2741,10 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> >                 goto out;
> >         }
> >
> > -       rcu_read_lock();
> >         regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
> >         old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
> >         err = bpf_prog_run(link->link.prog, regs);
> >         bpf_reset_run_ctx(old_run_ctx);
> > -       rcu_read_unlock();
> >
> >   out:
> >         __this_cpu_dec(bpf_prog_active);
> > --
> > 2.51.0
> >
> 
>