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Date:   Mon, 15 Aug 2022 16:33:09 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, bpf <bpf@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
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        Hao Luo <haoluo@google.com>,
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        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace
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On Mon, Aug 15, 2022 at 07:25:24AM -0700, Alexei Starovoitov wrote:
> On Mon, Aug 15, 2022 at 5:37 AM Peter Zijlstra <peterz@infradead.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@gmail.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.

It is in full control of the 'call __fentry__'. Absolute full NAK on you
trying to make it otherwise.

> Soon we will have nop5 in the middle of the function.
> ftrace must not touch it.

How are you generating that NOP and what for?