Re: kprobes and rcu_is_watching()

From: Ze Gao <zegao2021@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiri Olsa <olsajiri@gmail.com>, Yonghong Song <yhs@meta.com>,
	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>,
	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: kprobes and rcu_is_watching()
Date: Wed, 24 May 2023 11:51:58 +0800	[thread overview]
Message-ID: <CAD8CoPCHnTMOzqxJT+9Bg7aP=y8Jt4VUV1=xpBopNZv7VkBU8w@mail.gmail.com> (raw)
In-Reply-To: <20230523101041.23ca7cc8@rorschach.local.home>

Thanks Steven, I think we've come to a consensus on this.

The question here is whether bpf tracing fentry i.e.,
__bpf_prog_enter{_sleepable}
needs to check rcu_is_watching as well before using rcu related
calls. And Yonghong suggested making a change when there is
indeed some bad case occurring since it's rare the tracee is in the idle path.

Regards,
Ze

On Tue, May 23, 2023 at 10:10 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> [ Added a subject, as I always want to delete these emails as spam! ]
>
> On Mon, 22 May 2023 10:07:42 +0800
> Ze Gao <zegao2021@gmail.com> wrote:
>
> > Oops, I missed that. Thanks for pointing that out, which I thought is
> > conditional use of rcu_is_watching before.
> >
> > One last point, I think we should double check on this
> >      "fentry does not filter with !rcu_is_watching"
> > as quoted from Yonghong and argue whether it needs
> > the same check for fentry as well.
> >
>
> Note that trace_test_and_set_recursion() (which is used by
> ftrace_test_recursion_trylock()) checks for rcu_is_watching() and
> returns false if it isn't (and the trylock will fail).
>
> -- Steve