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Date:   Tue, 20 Dec 2022 12:08:16 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     torvalds@linux-foundation.org, corbet@lwn.net, will@kernel.org,
        mark.rutland@arm.com, catalin.marinas@arm.com, dennis@kernel.org,
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        linux-arch@vger.kernel.org
Subject: Re: [RFC][PATCH 05/12] arch: Introduce
 arch_{,try_}_cmpxchg128{,_local}()
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 <Y6DEfQXymYVgL3oJ@boqun-archlinux>
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On Mon, Dec 19, 2022 at 12:07:25PM -0800, Boqun Feng wrote:
> On Mon, Dec 19, 2022 at 04:35:30PM +0100, Peter Zijlstra wrote:
> > For all architectures that currently support cmpxchg_double()
> > implement the cmpxchg128() family of functions that is basically the
> > same but with a saner interface.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> >  arch/arm64/include/asm/atomic_ll_sc.h |   38 +++++++++++++++++++++++
> >  arch/arm64/include/asm/atomic_lse.h   |   33 +++++++++++++++++++-
> >  arch/arm64/include/asm/cmpxchg.h      |   26 ++++++++++++++++
> >  arch/s390/include/asm/cmpxchg.h       |   33 ++++++++++++++++++++
> >  arch/x86/include/asm/cmpxchg_32.h     |    3 +
> >  arch/x86/include/asm/cmpxchg_64.h     |   55 +++++++++++++++++++++++++++++++++-
> >  6 files changed, 185 insertions(+), 3 deletions(-)
> > 
> > --- a/arch/arm64/include/asm/atomic_ll_sc.h
> > +++ b/arch/arm64/include/asm/atomic_ll_sc.h
> > @@ -326,6 +326,44 @@ __CMPXCHG_DBL(   ,        ,  ,         )
> >  __CMPXCHG_DBL(_mb, dmb ish, l, "memory")
> >  
> >  #undef __CMPXCHG_DBL
> > +
> > +union __u128_halves {
> > +	u128 full;
> > +	struct {
> > +		u64 low, high;
> > +	};
> > +};
> > +
> > +#define __CMPXCHG128(name, mb, rel, cl)					\
> > +static __always_inline u128						\
> > +__ll_sc__cmpxchg128##name(volatile u128 *ptr, u128 old, u128 new)	\
> > +{									\
> > +	union __u128_halves r, o = { .full = (old) },			\
> > +			       n = { .full = (new) };			\
> > +									\
> > +	asm volatile("// __cmpxchg128" #name "\n"			\
> > +	"	prfm	pstl1strm, %2\n"				\
> > +	"1:	ldxp	%0, %1, %2\n"					\
> > +	"	eor	%3, %0, %3\n"					\
> > +	"	eor	%4, %1, %4\n"					\
> > +	"	orr	%3, %4, %3\n"					\
> > +	"	cbnz	%3, 2f\n"					\
> > +	"	st" #rel "xp	%w3, %5, %6, %2\n"			\
> > +	"	cbnz	%w3, 1b\n"					\
> > +	"	" #mb "\n"						\
> > +	"2:"								\
> > +	: "=&r" (r.low), "=&r" (r.high), "+Q" (*(unsigned long *)ptr)	\
> 
> 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.