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

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Date:   Tue, 20 Dec 2022 08:31:19 -0600
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, corbet@....net, will@...nel.org,
        mark.rutland@....com, catalin.marinas@....com, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, Herbert Xu <herbert@...dor.apana.org.au>,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, joro@...tes.org, suravee.suthikulpanit@....com,
        robin.murphy@....com, dwmw2@...radead.org,
        baolu.lu@...ux.intel.com, Arnd Bergmann <arnd@...db.de>,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        Andrew Morton <akpm@...ux-foundation.org>, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 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@...r.kernel.org
Subject: Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()

On Tue, Dec 20, 2022 at 5:09 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Dec 19, 2022 at 12:07:25PM -0800, Boqun Feng wrote:
> >
> > I wonder whether we should use "(*(u128 *)ptr)" instead of "(*(unsigned
> > long *) ptr)"? Because compilers may think only 64bit value pointed by
> > "ptr" gets modified, and they are allowed to do "useful" optimization.
>
> In this I've copied the existing cmpxchg_double() code; I'll have to let
> the arch folks speak here, I've no clue.

It does sound like the right thing to do. I doubt it ends up making a
difference in practice, but yes, the asm doesn't have a memory
clobber, so the input/output types should be the right ones for the
compiler to not possibly do something odd and cache the part that it
doesn't see as being accessed.

              Linus