linux-kernel - Re: [RFC][PATCH 07/12] percpu: Wire up cmpxchg128

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Message-Id: <1d88ba9f-5541-4b67-9cc8-a361eef36547@app.fastmail.com>
Date:   Thu, 29 Dec 2022 14:36:32 +0100
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>,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        "Alexander Gordeev" <agordeev@...ux.ibm.com>,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        "Herbert Xu" <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        "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>, joro@...tes.org,
        suravee.suthikulpanit@....com,
        "Robin Murphy" <robin.murphy@....com>, dwmw2@...radead.org,
        baolu.lu@...ux.intel.com, "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, linux-crypto@...r.kernel.org,
        iommu@...ts.linux.dev, Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC][PATCH 07/12] percpu: Wire up cmpxchg128

On Mon, Dec 19, 2022, at 16:35, Peter Zijlstra wrote:
> In order to replace cmpxchg_double() with the newly minted
> cmpxchg128() family of functions, wire it up in this_cpu_cmpxchg().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Does this work on x86 chips without X86_FEATURE_CX16?

As far as I can tell, the new percpu_cmpxchg128_op uses
the cmpxchg16b instruction unconditionally without checking
for the feature bit first, and is now used by this_cpu_cmpxchg()
unconditionally as well.

This is fine for the moment if the only user is mm/slub.c
and that retains the system_has_cmpxchg128() runtime check,
but I think a better interface would be to guarantee that
this_cpu_cmpxchg() always ends up either in a working
inline asm or the generic fallback but not an invalid
opcode.

For consistency, I would also suggest this_cpu_cmpxchg() to
take the same argument types as cmpxchg(): at most 'unsigned
long' sized, with additional this_cpu_cmpxchg64() and
this_cpu_cmpxchg128() macros that take fixed-size arguments.
I have an older patch set that additionally converts all
8-bit and 16-bit cmpxchg()/xchg() calls to cmpxchg_8()/
xchg_8()/cmpxchg_16()/xchg_16() and and leaves only the
fixed-32bit and variable typed 'unsigned long' sized
callers for the weakly typed variant.

       Arnd