linux-kernel - Re: [PATCH v2 05/10] percpu: Wire up cmpxchg128

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Date:   Mon, 6 Feb 2023 12:24:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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 v2 05/10] percpu: Wire up cmpxchg128

On Fri, Feb 03, 2023 at 06:25:04PM +0100, Arnd Bergmann wrote:
> On Thu, Feb 2, 2023, at 15:50, 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>
> 
> I commented on this in the previous version but never got any
> reply from you:
> 
> https://lore.kernel.org/all/1d88ba9f-5541-4b67-9cc8-a361eef36547@app.fastmail.com/

Sorry, seem to have missed that :/

> Unless I have misunderstood what you are doing, my concerns are
> still the same:
> 
> >  #define this_cpu_cmpxchg(pcp, oval, nval) \
> > -	__pcpu_size_call_return2(this_cpu_cmpxchg_, pcp, oval, nval)
> > +	__pcpu_size16_call_return2(this_cpu_cmpxchg_, pcp, oval, nval)
> >  #define this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, 
> > nval2) \
> >  	__pcpu_double_call_return_bool(this_cpu_cmpxchg_double_, pcp1, pcp2, 
> > oval1, oval2, nval1, nval2)
> 
> Having a variable-length this_cpu_cmpxchg() that turns into cmpxchg128()
> and cmpxchg64() even on CPUs where this traps (!X86_FEATURE_CX16) seems
> like a bad design to me.
> 
> I would much prefer fixed-length this_cpu_cmpxchg64()/this_cpu_cmpxchg128()
> calls that never trap but fall back to the generic version on CPUs that
> are lacking the atomics.

You're thinking acidental usage etc..? Lemme see what I can do.