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

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Date:   Wed, 16 Dec 2020 12:40:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Guo Ren <guoren@...nel.org>, 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>,
        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 12:26:10PM +0100, Arnd Bergmann wrote:
> 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

Hooray!! what about PA-RISC ? Can we burn that too?