Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

From: Arnd Bergmann <arnd@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Marco Elver <elver@google.com>, Arnd Bergmann <arnd@arndb.de>,
	Russell King <linux@armlinux.org.uk>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Darren Hart <dvhart@infradead.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Guo Ren <guoren@kernel.org>,
	linux-csky@vger.kernel.org,
	sparclinux <sparclinux@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'
Date: Mon, 14 Dec 2020 14:15:26 +0100	[thread overview]
Message-ID: <CAK8P3a0LWjNgwm605TM4dKCsn078X7NC3sEfdBSgcMNEocQ5iA@mail.gmail.com> (raw)
In-Reply-To: <87czzeg5ep.fsf@nanos.tec.linutronix.de>

On Sat, Dec 12, 2020 at 9:01 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Sat, Dec 12 2020 at 13:26, Marco Elver wrote:
> > On Thu, Mar 07, 2019 at 10:14AM +0100, Arnd Bergmann wrote:
> >> -static void __init futex_detect_cmpxchg(void)
> >> +static noinline void futex_detect_cmpxchg(void)
> >>  {
> >>  #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> >>      u32 curval;
> >
> > What ever happened to this patch?
>
> It obviously fell through the cracks.
>
> > I'm seeing this again with the attached config + next-20201211 (for
> > testing https://bugs.llvm.org/show_bug.cgi?id=48492). Had to apply this
> > patch to build the kernel.
>
> What really bothers me is to remove the __init from a function which is
> clearly only used during init. And looking deeper it's simply a hack.
>
> This function 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.
>

Ah perfect, that is clearly the right solution here.

> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -86,6 +86,7 @@ config ARM
>         select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
>         select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
>         select HAVE_FUNCTION_TRACER if !XIP_KERNEL
> +       select HAVE_FUTEX_CMPXCHG if FUTEX
>         select HAVE_GCC_PLUGINS
>         select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
>         select HAVE_IDE if PCI || ISA || PCMCIA

I had a look at what other architectures always implement
futex_atomic_cmpxchg_inatomic() or can use the asm-generic non-SMP version,
and I found that it's pretty much all of them, the odd ones being just sparc32
and csky, which use asm-generic/futex.h but do have an SMP option,
as well as xtensa

I would guess that for csky, this is a mistake, as the architecture is fairly
new and should be able to implement it. Not sure about sparc32.

       Arnd