bpf: remove unnecessary rcu_read_lock in kprobe_multi_link_prog_run

[PATCH bpf-next] bpf: remove unnecessary rcu_read_lock in kprobe_multi_link_prog_run

Posted by Menglong Dong 1 month, 1 week ago

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.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.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

Re: [PATCH bpf-next] bpf: remove unnecessary rcu_read_lock in kprobe_multi_link_prog_run

Posted by Alexei Starovoitov 1 month ago

On Wed, Aug 27, 2025 at 5:38 AM Menglong Dong <menglong8.dong@gmail.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.

pw-bot: cr

> Signed-off-by: Menglong Dong <dongml2@chinatelecom.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
>