linux-kernel - Re: [RFC][PATCH 11/12] slub: Replace cmpxchg

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Date:   Tue, 10 Jan 2023 11:28:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Heiko Carstens <hca@...ux.ibm.com>, corbet@....net,
        will@...nel.org, boqun.feng@...il.com, mark.rutland@....com,
        catalin.marinas@....com, dennis@...nel.org, tj@...nel.org,
        cl@...ux.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 11/12] slub: Replace cmpxchg_double()

On Mon, Jan 09, 2023 at 04:02:33PM -0600, Linus Torvalds wrote:
> On Mon, Jan 9, 2023 at 10:29 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > I ran into a ton of casting trouble when compiling kernel/fork.c which
> > uses this_cpu_cmpxchg() on a pointer type and the compiler hates casting
> > pointers to an integer that is not the exact same size.
> 
> Ahh. Yeah - not because that code needs or wants the 128-bit case, but
> because the macro expands to all sizes in a switch statement, so the
> compiler sees all the cases even if only one is then statically
> picked.
> 
> So the silly casts are for all the cases that never matter.
> 
> Annoying.

Yes, very.

This seems to compile (and boot). Let me go update the others and push
it out for the robots to have a go.

#define percpu_cmpxchg128_op(size, qual, _var, _oval, _nval)		\
({									\
	union {								\
		typeof(_var) full;					\
		struct {						\
			u64 low, high;					\
		};							\
	} old__, new__;							\
									\
	old__.full = _oval;						\
	new__.full = _nval;						\
									\
	asm qual ("cmpxchg16b " __percpu_arg([var])			\
		  : [var] "+m" (_var),					\
		    "+a" (old__.low),					\
		    "+d" (old__.high)					\
		  : "b" (new__.low),					\
		    "c" (new__.high)					\
		  : "memory");						\
									\
	old__.full;							\
})