linux-kernel - Re: [PATCH 1/2] futex: mark futex_detect

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Message-ID: <20190307182102.GA12466@archlinux-ryzen>
Date:   Thu, 7 Mar 2019 11:21:02 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

On Thu, Mar 07, 2019 at 10:12:11AM -0800, Nick Desaulniers wrote:
> On Thu, Mar 7, 2019 at 1:15 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On 32-bit ARM, I got a link failure in futex_init() when building
> > with clang in some random configurations:
> >
> > kernel/futex.o:(.text.fixup+0x5c): relocation truncated to fit: R_ARM_JUMP24 against `.init.text'
> 
> Do we know what function from the fixup text section is calling
> futex_detect_cmpxchg?  I'm curious if this is maybe another case of
> -Wsection where some function may be in the wrong section?
> 

Looks like this is the call stack:

futex_init ->
    futex_detect_cmpxchg ->
        cmpxchg_futex_value_locked ->
            futex_atomic_cmpxchg_inatomic

This is the same issue I reported: https://github.com/ClangBuiltLinux/linux/issues/325

Marking arm's futex_atomic_cmpxchg_inatomic as noinline also fixes this
so maybe that's it?

Cheers,
Nathan

> >
> > As far as I can tell, the problem is that a branch is over 16MB
> > apart in those configurations, but only if it branches back to
> > the init text.
> >
> > Marking the futex_detect_cmpxchg() function as noinline and
> > not __init avoids the problem for me.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  kernel/futex.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index c3b73b0311bc..dda77ed9f445 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -3849,7 +3849,7 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
> >  }
> >  #endif /* CONFIG_COMPAT_32BIT_TIME */
> >
> > -static void __init futex_detect_cmpxchg(void)
> > +static noinline void futex_detect_cmpxchg(void)
> >  {
> >  #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> >         u32 curval;
> > --
> > 2.20.0
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers