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

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Date:   Thu, 07 Mar 2019 09:42:40 -0800
From:   Joe Perches <joe@...ches.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
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, 2019-03-07 at 17:25 +0000, Russell King - ARM Linux admin wrote:
> 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?

It doesn't particularly.
It does help any other case that might arise.

> 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().

True, but init function performance requirements are
generally trivial and isolating the possibility of
defects like this seems a reasonable trade-off to me.