netdev - Re:

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Message-ID: <CAD8CoPAXse1GKAb15O5tZJwBqMt1N_btH+qRe7c_a-ryUMjx7A@mail.gmail.com>
Date: Sun, 21 May 2023 23:10:16 +0800
From: Ze Gao <zegao2021@...il.com>
To: Yonghong Song <yhs@...a.com>
Cc: jolsa@...nel.org, 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:

> 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 :) .