linux-kernel - Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,

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Message-ID: <Y7Vksr9OLZeL3qmU@FVFF77S0Q05N>
Date:   Wed, 4 Jan 2023 11:36:18 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>,
        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, 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 05/12] arch: Introduce
 arch_{,try_}_cmpxchg128{,_local}()

On Tue, Jan 03, 2023 at 05:50:00PM +0100, Arnd Bergmann wrote:
> On Tue, Jan 3, 2023, at 17:19, Mark Rutland wrote:
> > On Tue, Jan 03, 2023 at 02:03:37PM +0000, Mark Rutland wrote:
> >> On Tue, Jan 03, 2023 at 01:25:35PM +0000, Mark Rutland wrote:
> >> > On Tue, Dec 20, 2022 at 12:08:16PM +0100, Peter Zijlstra wrote:
> 
> >> ... makes GCC much happier:
> >
> >> ... I'll go check whether clang is happy with that, and how far back that can
> >> go, otherwise we'll need to blat the high half with a separate constaint that
> >> (ideally) doesn't end up allocating a pointless address register.
> >
> > Hmm... from the commit history it looks like GCC prior to 5.1 might not be
> > happy with that, but that *might* just be if we actually do arithmetic on the
> > value, and we might be ok just using it for memroy effects. I can't currently
> > get such an old GCC to run on my machines so I haven't been able to check.
> 
> gcc-5.1 is the oldest (barely) supported compiler, the minimum was
> last raised from gcc-4.9 in linux-5.15. If only gcc-4.9 and older are
> affected, we're good on mainline but may still want a fix for stable
> kernels.

Yup; I just wanted something that would easily backport to stable, at least as
far as linux-4.9.y (where I couldn't find the minimum GCC version when I looked
yesterday).

> I checked that the cross-compiler binaries from [1] still work, but I noticed
> that this version is missing the native aarch64-to-aarch64 compiler (x86 to
> aarch64 and vice versa are there), and you need to install libmpfr4 [2]
> as a dependency. The newer compilers (6.5.0 and up) don't have these problems.

I was trying the old kernel.org crosstool binaries, but I was either missing a
library (or I have an incompatible version) on my x86_64 host. I'll have
another look today -- thanks for the pointers!

Mark.

>      Arnd
> 
> [1] https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/arm64/5.5.0/
> [2] http://ftp.uk.debian.org/debian/pool/main/m/mpfr4/libmpfr4_3.1.5-1_arm64.deb