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

[Linus Torvalds]

On Tue, Dec 20, 2022 at 5:09 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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