linux-kernel - Re: [GIT PULL] tracing: Updates for 5.10

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Message-ID: <20201015222139.1b92a3be@gandalf.local.home>
Date:   Thu, 15 Oct 2020 22:21:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [GIT PULL] tracing: Updates for 5.10

On Thu, 15 Oct 2020 18:54:34 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Oct 15, 2020 at 10:53 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > Updates for tracing and bootconfig:  
> 
> Hmm. I haven't verified that this came from you, but it seems likely..
> Once again my clang build shows something that I don't see in my
> allmodconfig gcc build:
> 
>    WARNING: modpost: vmlinux.o(.text+0x1e5b06): Section mismatch in
> reference from the function __trace_early_add_events() to the function
> .init.text:__trace_early_add_new_event()
>    The function __trace_early_add_events() references
>    the function __init __trace_early_add_new_event().
>    This is often because __trace_early_add_events lacks a __init
>    annotation or the annotation of __trace_early_add_new_event is wrong.
> 
> Hmm?
> 
>                Linus

I see the issue, and I wonder if it has to do with optimization, for gcc
not to warn.

The issue is that we have:

trace_array_create() that can be called at any time. And it has:

	if (trace_instance_dir) {
		ret = trace_array_create_dir(tr);
		if (ret)
			goto out_free_tr;
	} else
		__trace_early_add_events(tr);


Where trace_instance_dir gets set at boot up, and thus the else statement
will never get called after that.

The __trace_early_add_events() then calls __trace_early_add_new_events()
which is __init.

I don't know how gcc didn't trigger this and clang does.

I'll have to think about how to untangle this. Is there some kind of
annotation that makes it show that a path can only be called at boot up and
not later?

-- Steve