linux-kernel - Re: [RFC] ftrace: Add support to keep some functions out of ftrace

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Message-ID: <CAADnVQJfvn2RYydqgO-nS_K+C8WJL7BdCnR44MiMF4rnAwWM5A@mail.gmail.com>
Date:   Mon, 15 Aug 2022 07:25:24 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace

On Mon, Aug 15, 2022 at 5:37 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Aug 15, 2022 at 11:44:32AM +0200, Jiri Olsa wrote:
> > On Mon, Aug 15, 2022 at 10:03:17AM +0200, Peter Zijlstra wrote:
> > > On Sat, Aug 13, 2022 at 03:02:52PM -0400, Steven Rostedt wrote:
> > > > On Fri, 12 Aug 2022 23:18:15 +0200
> > > > Jiri Olsa <olsajiri@...il.com> wrote:
> > > >
> > > > > the patch below moves the bpf function into sepatate object and switches
> > > > > off the -mrecord-mcount for it.. so the function gets profile call
> > > > > generated but it's not visible to ftrace
> > >
> > > Why ?!?
> >
> > there's bpf dispatcher code that updates bpf_dispatcher_xdp_func
> > function with bpf_arch_text_poke and that can race with ftrace update
> > if the function is traced
>
> I thought bpf_arch_text_poke() wasn't allowed to touch kernel code and
> ftrace is in full control of it ?

ftrace is not in "full control" of nop5 and must not be.
Soon we will have nop5 in the middle of the function.
ftrace must not touch it.