linux-kernel - Re: [PATCH v4] Makefile.compiler: replace cc-ifversion with compiler-specific macros

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Message-ID: <cff33e12-3d80-7e62-1993-55411ccabc01@collabora.com>
Date:   Tue, 23 May 2023 15:57:06 +0530
From:   Shreeya Patel <shreeya.patel@...labora.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Maksim Panchenko <maks@...a.com>,
        Ricardo Cañuelo <ricardo.canuelo@...labora.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Bill Wendling <morbo@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        regressions@...ts.linux.dev,
        "gustavo.padovan@...labora.com" <gustavo.padovan@...labora.com>,
        Guillaume Charles Tucker <guillaume.tucker@...labora.com>,
        denys.f@...labora.com, kernelci@...ts.linux.dev
Subject: Re: [PATCH v4] Makefile.compiler: replace cc-ifversion with
 compiler-specific macros

Hi Nick and Masahiro,

On 23/05/23 01:22, Nick Desaulniers wrote:
> On Mon, May 22, 2023 at 9:52 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>> On Mon, May 22, 2023 at 12:09:34PM +0200, Ricardo Cañuelo wrote:
>>> On vie, may 19 2023 at 08:57:24, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>>>> It could be; if the link order was changed, it's possible that this
>>>> target may be hitting something along the lines of:
>>>> https://isocpp.org/wiki/faq/ctors#static-init-order i.e. the "static
>>>> initialization order fiasco"
>>>>
>>>> I'm struggling to think of how this appears in C codebases, but I
>>>> swear years ago I had a discussion with GKH (maybe?) about this. I
>>>> think I was playing with converting Kbuild to use Ninja rather than
>>>> Make; the resulting kernel image wouldn't boot because I had modified
>>>> the order the object files were linked in.  If you were to randomly
>>>> shuffle the object files in the kernel, I recall some hazard that may
>>>> prevent boot.
>>> I thought that was specifically a C++ problem? But then again, the
>>> kernel docs explicitly say that the ordering of obj-y goals in kbuild is
>>> significant in some instances [1]:
>> Yes, it matters, you can not change it.  If you do, systems will break.
>> It is the only way we have of properly ordering our init calls within
>> the same "level".
> Ah, right it was the initcall ordering. Thanks for the reminder.
>
> (There's a joke in there similar to the use of regexes to solve a
> problem resulting in two new problems; initcalls have levels for
> ordering, but we still have (unexpressed) dependencies between calls
> of the same level; brittle!).
>
> +Maksim, since that might be relevant info for the BOLT+Kernel work.
>
> Ricardo,
> https://elinux.org/images/e/e8/2020_ELCE_initcalls_myjosserand.pdf
> mentions that there's a kernel command line param `initcall_debug`.
> Perhaps that can be used to see if
> 5750121ae7382ebac8d47ce6d68012d6cd1d7926 somehow changed initcall
> ordering, resulting in a config that cannot boot?


Here are the links to Lava jobs ran with initcall_debug added to the 
kernel command line.

1. Where regression happens (5750121ae7382ebac8d47ce6d68012d6cd1d7926)
https://lava.collabora.dev/scheduler/job/10417706
<https://lava.collabora.dev/scheduler/job/10417706>

2. With a revert of the commit 5750121ae7382ebac8d47ce6d68012d6cd1d7926
https://lava.collabora.dev/scheduler/job/10418012 
<https://lava.collabora.dev/scheduler/job/10418012>


Thanks,
Shreeya Patel