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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 20 Dec 2022 08:31:19 -0600
Message-ID: <CAHk-=wi493ukLwziiqofe=WCSfUU8Qa+LK0mp_GrGWKV3NnTpQ@mail.gmail.com>
Subject: Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>, corbet@lwn.net, will@kernel.org,
        mark.rutland@arm.com, catalin.marinas@arm.com, dennis@kernel.org,
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On Tue, Dec 20, 2022 at 5:09 AM Peter Zijlstra <peterz@infradead.org> 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