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Message-ID: <ZGp+fW855gmWuh9W@krava>
Date: Sun, 21 May 2023 22:26:37 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Ze Gao <zegao2021@...il.com>
Cc: Yonghong Song <yhs@...a.com>, Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>, Hao Luo <haoluo@...gle.com>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Masami Hiramatsu <mhiramat@...nel.org>, Song Liu <song@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>, Yonghong Song <yhs@...com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, kafai@...com,
	kpsingh@...omium.org, netdev@...r.kernel.org, paulmck@...nel.org,
	songliubraving@...com, Ze Gao <zegao@...cent.com>
Subject: Re:

On Sun, May 21, 2023 at 11:10:16PM +0800, Ze Gao wrote:
> > kprobe_multi/fprobe share the same set of attachments with fentry.
> > Currently, fentry does not filter with !rcu_is_watching, maybe
> > because this is an extreme corner case. Not sure whether it is
> > worthwhile or not.
> 
> Agreed, it's rare, especially after Peter's patches which push narrow
> down rcu eqs regions
> in the idle path and reduce the chance of any traceable functions
> happening in between.
> 
> However, from RCU's perspective, we ought to check if rcu_is_watching
> theoretically
> when there's a chance our code will run in the idle path and also we
> need rcu to be alive,
> And also we cannot simply make assumptions for any future changes in
> the idle path.
> You know, just like what was hit in the thread.
> 
> > Maybe if you can give a concrete example (e.g., attachment point)
> > with current code base to show what the issue you encountered and
> > it will make it easier to judge whether adding !rcu_is_watching()
> > is necessary or not.
> 
> I can reproduce likely warnings on v6.1.18 where arch_cpu_idle is
> traceable but not on the latest version
> so far. But as I state above, in theory we need it. So here is a
> gentle ping :) .

hum, this change [1] added rcu_is_watching check to ftrace_test_recursion_trylock,
which we use in fprobe_handler and is coming to fprobe_exit_handler in [2]

I might be missing something, but it seems like we don't need another
rcu_is_watching call on kprobe_multi level

jirka


[1] d099dbfd3306 cpuidle: tracing: Warn about !rcu_is_watching()
[2] https://lore.kernel.org/bpf/20230517034510.15639-4-zegao@tencent.com/