linux-kernel - Re: [PATCH 2/2] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_32 && X86

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Date:   Wed, 25 May 2022 09:29:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Waiman.Long@...com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF
 for X86_32 && X86_CMPXCHG64

On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@...il.com> wrote:
>
> +       select ARCH_USE_CMPXCHG_LOCKREF if X86_64 || (X86_32 && X86_CMPXCHG64)

Ugh. That looks pointlessly complicated. X86_64 already enables
X86_CMPXCHG64 afaik, so you can just say

        select ARCH_USE_CMPXCHG_LOCKREF if X86_CMPXCHG64

which is much clearer: CMPXCHG_LOCKREF needs CMPXCHG64, and the
Kconfig option says exactly that.

That said, this also makes me wonder if we should just make cmpxchg8b
requirement unconditional.

Googling around, it looks like Windows NT stopped booting on CPUs
without cmpxchg8b in version 5.1. That was in 2001.

Here we are, 21 years later, and we still ostensibly try to support
CPUs without it, but I doubt it's worth it.

So maybe time to just say "we require cmpxchg8b".

In fact, I think we have effectively done that already for years, since we have

    config X86_CMPXCHG64
            def_bool y
            depends on X86_PAE || ...

iow, enabling X86_PAE will force-enable CMPXCHG8B due to the wider
page table entries.

And I would assume that all distros basically do that anyway (but I do
not have a 32-bit distro around to check).

It would mean that we would finally drop support for i486 (and
possibly some Pentium clones, but afaik a number of them did actually
support cmpxchg8b even if they didn't report it in cpuid).

Comments?

                 Linus