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Date:   Mon, 06 Feb 2023 14:20:06 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Peter Zijlstra" <peterz@infradead.org>
Cc:     "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Jonathan Corbet" <corbet@lwn.net>,
        "Will Deacon" <will@kernel.org>,
        "Boqun Feng" <boqun.feng@gmail.com>,
        "Mark Rutland" <mark.rutland@arm.com>,
        "Catalin Marinas" <catalin.marinas@arm.com>, dennis@kernel.org,
        "Tejun Heo" <tj@kernel.org>, "Christoph Lameter" <cl@linux.com>,
        "Heiko Carstens" <hca@linux.ibm.com>, gor@linux.ibm.com,
        "Alexander Gordeev" <agordeev@linux.ibm.com>,
        borntraeger@linux.ibm.com, "Sven Schnelle" <svens@linux.ibm.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
        "Dave Hansen" <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, "Joerg Roedel" <joro@8bytes.org>,
        suravee.suthikulpanit@amd.com,
        "Robin Murphy" <robin.murphy@arm.com>, dwmw2@infradead.org,
        "Baolu Lu" <baolu.lu@linux.intel.com>,
        "Herbert Xu" <herbert@gondor.apana.org.au>,
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        "Pekka Enberg" <penberg@kernel.org>,
        "David Rientjes" <rientjes@google.com>,
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Subject: Re: [PATCH v2 05/10] percpu: Wire up cmpxchg128
Content-Type: text/plain
Precedence: bulk

On Mon, Feb 6, 2023, at 13:48, Peter Zijlstra wrote:
> On Mon, Feb 06, 2023 at 01:14:28PM +0100, Peter Zijlstra wrote:
>> On Mon, Feb 06, 2023 at 12:24:00PM +0100, Peter Zijlstra wrote:
>> Basically, using 64bit percpu ops on 32bit is already somewhat dangerous
>> -- wiring up native cmpxchg64 support in that case seemed an
>> improvement.
>> 
>> Anyway... let me get on with doing explicit
>> {raw,this}_cpu_cmpxchg{64,128}() thingies.
>
> I only converted x86 and didn't do the automagic downgrade...
>
> Opinions?

I think that's much better, it keeps the interface symmetric
between cmpxchg and this_cpu_cmp_cmpxchg, and makes it harder
to run into the subtle corner case on old x86 CPUs.

For the M486/M586/MK8/MPSC/MATOM/MCORE2 configs, I would
probably want to go with a compile-time check and have most
distro kernels build with at least M586TSC for 32-bit or
an upgraded CONFIG_GENERIC_CPU for x86_64 that assumes cmpxchg16b
(nehalem, second-geneneration k8, silverthorne, ...) in
order to turn system_has_cmpxchg* into a constant value on
all architectures. That can be a separate discussion though
and shouldn't block your series as you just keep the current
state.

> -#ifdef system_has_freelist_aba
> +#ifdef syste_has_freelist_aba

Typo

    Arnd