Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()

Peter Zijlstra
2022-12-20T11:08:56Z

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.