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

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Message-ID: <CAK8P3a3+WaQNyJ6Za2qfu6=0mBgU1hApnRXrdp1b1=P7wwyRUg@mail.gmail.com>
Date:   Tue, 15 Dec 2020 12:26:10 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Marco Elver <elver@...gle.com>, Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        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" <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        sparclinux <sparclinux@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

On Tue, Dec 15, 2020 at 7:09 AM Guo Ren <guoren@...nel.org> wrote:
> On Mon, Dec 14, 2020 at 9:15 PM Arnd Bergmann <arnd@...nel.org> 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
  arch_futex_atomic_op_inuser().

- 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.

         Arnd