linux-kernel - Re: [PATCH] ARM: select HAVE_FUTEX

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Message-ID: <CAK8P3a2JJ=5t16enn2LaZE_zT1ZqNVyi9FQpTUrgVsQWiMtDkw@mail.gmail.com>
Date:   Fri, 17 Sep 2021 09:11:23 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        llvm@...ts.linux.dev, Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Russell King <linux@...linux.org.uk>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Mike Rapoport <rppt@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: select HAVE_FUTEX_CMPXCHG

On Mon, Aug 30, 2021 at 11:38 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> tglx notes:
>   This function [futex_detect_cmpxchg] is only needed when an
>   architecture has to runtime discover whether the CPU supports it or
>   not.  ARM has unconditional support for this, so the obvious thing to
>   do is the below.
>
> Fixes linkage failure from Clang randconfigs:
> kernel/futex.o:(.text.fixup+0x5c): relocation truncated to fit: R_ARM_JUMP24 against `.init.text'
> and boot failures for CONFIG_THUMB2_KERNEL.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/325
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---

Nice! I did remember that we had come up with a number of solutions
for the build error and that at least one of them was an obvious and
nice fix, but I could never figure out what it was. This is clearly better
than any of the other ones I had found when I last looked.

I would add a reference to the commit that introduced the conditional,
and a request for backports as well:

See-also: 03b8c7b623c8 ("futex: Allow architectures to skip
futex_atomic_cmpxchg_inatomic() test")
Cc: stable@...r.kernel.org # v3.14+
Reviewed-by: Arnd Bergmann <arnd@...db.de>