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Date:   Tue, 15 Dec 2020 12:26:10 +0100
From:   Arnd Bergmann <>
To:     Guo Ren <>
Cc:     Thomas Gleixner <>,
        Marco Elver <>, Arnd Bergmann <>,
        Russell King <>,
        Ingo Molnar <>,
        Peter Zijlstra <>,
        Darren Hart <>,
        Nick Desaulniers <>,
        Davidlohr Bueso <>,
        Elena Reshetova <>,
        Greg Kroah-Hartman <>,
        "" <>,,
        sparclinux <>,
        "David S. Miller" <>
Subject: Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

On Tue, Dec 15, 2020 at 7:09 AM Guo Ren <> wrote:
> On Mon, Dec 14, 2020 at 9:15 PM Arnd Bergmann <> wrote:
> > 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.
> The c610, c807, c810 don't support SMP, so futex_cmpxchg_enabled = 1
> with asm-generic's implementation.
> For c860, there is no HAVE_FUTEX_CMPXCHG and cmpxchg_inatomic/inuser
> implementation, so futex_cmpxchg_enabled = 0.
> Thx for point it out, we'll implement cmpxchg_inatomic/inuser for C860
> and still use asm-generic for non-smp CPUs.

Sounds good to me.

With that, I would suggest we actually remove the -ENOSYS fallback
for arch_futex_atomic_op_inuser() and futex_atomic_cmpxchg_inatomic()
in asm-generic/futex.h as well as the HAVE_FUTEX_CMPXCHG Kconfig
symbol, plus these additional fixups:

- for xtensa and mips configurations without ll/sc, fall back to the
  asm-generic version. These are all uniprocessor, while the
  corresponding SMP machines have a working

- Disable SMP support for sun4m/sun4d. From the historic git
  tree, it's unclear how well this ever worked, and very few machines
  of this class ever existed

- Mark SMP for LEON as temporarily broken. As I see in the LEON
  patch set, they have changes to enable compare-and-swap-atomic
  instructions unconditionally, as all SMP Leons have those and
  seem to require this support already for other things.


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