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From:   Hao Luo <haoluo@google.com>
Date:   Wed, 1 Jul 2020 10:02:48 -0700
Message-ID: <CA+khW7hLL+=sZwCT_6gHHjHTZnmbNk5Pju9vsLOJF4VjyHY-iA@mail.gmail.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.
To:     Yonghong Song <yhs@fb.com>
Cc:     Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
        linux-kselftest@vger.kernel.org,
        Stanislav Fomichev <sdf@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andriin@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Bill Wendling <morbo@google.com>
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On Tue, Jun 30, 2020 at 7:26 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 6/30/20 5:10 PM, Hao Luo wrote:
> > Ok, with the help of my colleague Ian Rogers, I think we solved the
> > mystery. Clang actually inlined hrtimer_nanosleep() inside
> > SyS_nanosleep(), so there is no call to that function throughout the
> > path of the nanosleep syscall. I've been looking at the function body
> > of hrtimer_nanosleep for quite some time, but clearly overlooked the
> > caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and
> > there are many constants, inlining would not be too surprising.
>
> Oh thanks for explanation. inlining makes sense. We have many other
> instances like this in the past where kprobe won't work properly.
>
> Could you reword your commit message then?
>
>  > causing fentry and kprobe to not hook on this function properly on a
>  > Clang build kernel.
>
> The above is a little vague on what happens. What really happens is
> fentry/kprobe does hook on this function but has no effect since
> its caller has inlined the function.

Sure, sending a v2 with a more accurate description of the issue.

Hao