linux-kernel - Re: [PATCH v3 00/11] Introduce cmpxchg128() -- aka. the demise of cmpxchg

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Message-Id: <4975b92f-92f6-4d50-8386-9add12ddfd61@app.fastmail.com>
Date:   Mon, 15 May 2023 11:42:23 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Peter Zijlstra" <peterz@...radead.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:     "Jonathan Corbet" <corbet@....net>,
        "Will Deacon" <will@...nel.org>,
        "Boqun Feng" <boqun.feng@...il.com>,
        "Mark Rutland" <mark.rutland@....com>,
        "Catalin Marinas" <catalin.marinas@....com>, dennis@...nel.org,
        "Tejun Heo" <tj@...nel.org>, "Christoph Lameter" <cl@...ux.com>,
        "Heiko Carstens" <hca@...ux.ibm.com>, gor@...ux.ibm.com,
        "Alexander Gordeev" <agordeev@...ux.ibm.com>,
        borntraeger@...ux.ibm.com, "Sven Schnelle" <svens@...ux.ibm.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, "Joerg Roedel" <joro@...tes.org>,
        suravee.suthikulpanit@....com,
        "Robin Murphy" <robin.murphy@....com>,
        "David Woodhouse" <dwmw2@...radead.org>,
        "Baolu Lu" <baolu.lu@...ux.intel.com>,
        "Herbert Xu" <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        "Pekka Enberg" <penberg@...nel.org>,
        "David Rientjes" <rientjes@...gle.com>,
        "Joonsoo Kim" <iamjoonsoo.kim@....com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Roman Gushchin" <roman.gushchin@...ux.dev>,
        "Hyeonggon Yoo" <42.hyeyoo@...il.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, iommu@...ts.linux.dev,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v3 00/11] Introduce cmpxchg128() -- aka. the demise of
 cmpxchg_double()

On Mon, May 15, 2023, at 09:56, Peter Zijlstra wrote:
>
> Since v2:
>
>  - reworked this_cpu_cmpxchg() to not implicity do u128 but provide explicit
>    this_cpu_cmpxchg128() (arnd)
>  - added try_cmpxchg12_local() (per the addition of the try_cmpxchg*_local()
>    family of functions)
>  - slight cleanup of the SLUB conversion (due to rebase and having to touch it)
>  - added a 'cleanup' patch for SLUB, since I was staring at that anyway
>

This is clearly an improvement over the previous state, so I'm
happy with that, and the explicit this_cpu_cmpxchg128() interface
addresses most of my previous concerns.

Reviewed-by: Arnd Bergmann <arnd@...db.de>

The need for runtime feature checking in the callers on x86-64 is still
a bit awkward, but this is no worse than before. I understand that
turning this into a compile-time choice would require first settling
a larger debate about raising the default target for distros beyond
the current CONFIG_GENERIC_CPU.

    Arnd