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Message-Id: <20230523133019.ce19932f89585eb10d092896@kernel.org>
Date:   Tue, 23 May 2023 13:30:19 +0800
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Ze Gao <zegao2021@...il.com>
Cc:     Jiri Olsa <olsajiri@...il.com>, 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 Mon, 22 May 2023 10:07:42 +0800
Ze Gao <zegao2021@...il.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.

rcu_is_watching() comment says;

 * if the current CPU is not in its idle loop or is in an interrupt or
 * NMI handler, return true.

Thus it returns *fault* if the current CPU is in the idle loop and not
any interrupt(including NMI) context. This means if any tracable function
is called from idle loop, it can be !rcu_is_watching(). I meant, this is
'context' based check, thus fentry can not filter out that some commonly
used functions is called from that context but it can be detected.

Thank you,

> 
> Regards,
> Ze


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>