Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

Arnd Bergmann
2021-03-31T07:24:03Z

On Wed, Mar 31, 2021 at 12:35 AM Stafford Horne <shorne@gmail.com> wrote:
>
> I just want to chime in here, there may be a better spot in the thread to
> mention this but, for OpenRISC I did implement some generic 8/16-bit xchg code
> which I have on my todo list somwhere to replace the other generic
> implementations like that in mips.
>
> arch/openrisc/include/asm/cmpxchg.h
>
> The idea would be that architectures just implement these methods:
>
> long cmpxchg_u32(*ptr,old,new)
> long xchg_u32(*ptr,val)
>
> Then the rest of the generic header would implement cmpxchg.

I like the idea of generalizing it a little further. I'd suggest staying a
little closer to the existing naming here though, as we already have
cmpxchg() for the type-agnostic version, and cmpxchg64() for the
fixed-length 64-bit version.

I think a nice interface between architecture-specific and architecture
independent code would be to have architectures provide
arch_cmpxchg32()/arch_xchg32() as the most basic version, as well
as arch_cmpxchg8()/arch_cmpxchg16()/arch_xchg8()/arch_xchg16()
if they have instructions for those.

The common code can then build cmpxchg16()/xchg16() on top of
either the 16-bit or the 32-bit primitives, and build the cmpxchg()/xchg()
wrapper around those (or alternatively we can decide to have them
only deal with fixed-32-bit and long/pointer sized atomics).

Arnd