Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

Paul E. McKenney
2024-05-02T13:33:50Z

On Thu, May 02, 2024 at 07:11:52AM +0200, John Paul Adrian Glaubitz wrote:
> On Wed, 2024-05-01 at 22:06 -0700, Paul E. McKenney wrote:
> > > Does cmpxchg_emu_u8() have any advantages over the native xchg_u8()?
> >
> > That would be 8-bit xchg() rather than 8-byte cmpxchg(), correct?
>
> Indeed. I realized this after sending my reply.

No problem, as I do know that feeling!

> > Or am I missing something subtle here that makes sh also support one-byte
> > (8-bit) cmpxchg()?
>
> Is there an explanation available that explains the rationale behind the
> series, so I can learn more about it?

We have some places in mainline that need one-byte cmpxchg(), so this
series provides emulation for architectures that do not support this
notion.

> Also, I am opposed to removing Alpha entirely as it's still being actively
> maintained in Debian and Gentoo and works well.

Understood, and this sort of compatibility consideration is why this
version of this patchset does not emulate two-byte (16-bit) cmpxchg()
operations. The original (RFC) series did emulate these, which does
not work on a few architectures that do not provide 16-bit load/store
instructions, hence no 16-bit support in this series.

So this one-byte-only series affects only Alpha systems lacking
single-byte load/store instructions. If I understand correctly, Alpha
21164A (EV56) and later *do* have single-byte load/store instructions,
and thus are still just fine. In fact, it looks like EV56 also has
two-byte load/store instructions, and so would have been OK with
the original one-/two-byte RFC series.

Arnd will not be shy about correcting me if I am wrong. ;-)

> Adrian
>
> --
> .''`. John Paul Adrian Glaubitz
> : :' : Debian Developer
> `. `' Physicist
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