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

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Message-ID: <20190307172541.2262ccaxjk43twi4@shell.armlinux.org.uk>
Date:   Thu, 7 Mar 2019 17:25:41 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Joe Perches <joe@...ches.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

On Thu, Mar 07, 2019 at 09:19:04AM -0800, Joe Perches wrote:
> On Thu, 2019-03-07 at 10:14 +0100, Arnd Bergmann 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'
> > 
> > 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.
> 
> Perhaps the __init and __exit #defines should be noinline
> to allow discarding of the code.

How does that help this case?  The problem is futex_detect_cmpxchg()
being placed in .init.text which is too far from .text.fixup.
Whether it is inlined or not is immaterial since both
futex_detect_cmpxchg() and its only caller futex_init() are both __init
functions.

It seems to me to be completely sane to have:

static void __init foo(...)
{
}

static int __init foo_init(...)
{
	foo();
}

and have the expectation that the compiler _can_, if it desires, inline
foo() into foo_init().

Let me re-iterate: the inlining of futex_detect_cmpxchg() into
futex_init() is not the issue here.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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