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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Sun, 21 May 2023 10:08:46 +0200
To:     Ze Gao <zegao2021@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Hao Luo <haoluo@google.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Song Liu <song@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Yonghong Song <yhs@fb.com>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        kafai@fb.com, kpsingh@chromium.org, netdev@vger.kernel.org,
        paulmck@kernel.org, songliubraving@fb.com,
        Ze Gao <zegao@tencent.com>
Subject: Re:
Message-ID: <ZGnRjkjxWrK8HzNm@krava>
References: <20220515203653.4039075-1-jolsa@kernel.org>
 <20230520094722.5393-1-zegao@tencent.com>
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On Sat, May 20, 2023 at 05:47:24PM +0800, Ze Gao wrote:
> 
> Hi Jiri,
> 
> Would you like to consider to add rcu_is_watching check in
> to solve this from the viewpoint of kprobe_multi_link_prog_run

I think this was discussed in here:
  https://lore.kernel.org/bpf/20230321020103.13494-1-laoar.shao@gmail.com/

and was considered a bug, there's fix mentioned later in the thread

there's also this recent patchset:
  https://lore.kernel.org/bpf/20230517034510.15639-3-zegao@tencent.com/

that solves related problems

> itself? And accounting of missed runs can be added as well
> to imporve observability.

right, we count fprobe->nmissed but it's not exposed, we should allow
to get 'missed' stats from both fprobe and kprobe_multi later, which
is missing now, will check

thanks,
jirka

> 
> Regards,
> Ze
> 
> 
> -----------------
> From 29fd3cd713e65461325c2703cf5246a6fae5d4fe Mon Sep 17 00:00:00 2001
> From: Ze Gao <zegao@tencent.com>
> Date: Sat, 20 May 2023 17:32:05 +0800
> Subject: [PATCH] bpf: kprobe_multi runs bpf progs only when rcu_is_watching
> 
> From the perspective of kprobe_multi_link_prog_run, any traceable
> functions can be attached while bpf progs need specical care and
> ought to be under rcu protection. To solve the likely rcu lockdep
> warns once for good, when (future) functions in idle path were
> attached accidentally, we better paying some cost to check at least
> in kernel-side, and return when rcu is not watching, which helps
> to avoid any unpredictable results.
> 
> Signed-off-by: Ze Gao <zegao@tencent.com>
> ---
>  kernel/trace/bpf_trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 9a050e36dc6c..3e6ea7274765 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2622,7 +2622,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>  	struct bpf_run_ctx *old_run_ctx;
>  	int err;
>  
> -	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> +	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1 || !rcu_is_watching())) {
>  		err = 0;
>  		goto out;
>  	}
> -- 
> 2.40.1
>