Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

Russell King
2006-12-08T08:56:53Z

On Fri, Dec 08, 2006 at 12:18:52PM +1100, Nick Piggin wrote:
> Russell King wrote:
> >On Thu, Dec 07, 2006 at 08:31:08PM +1100, Nick Piggin wrote:
>
> >>>Implementing ll/sc based accessor macros allows both ll/sc _and_ cmpxchg
> >>>architectures to produce optimal code.
> >>>
> >>>Implementing an cmpxchg based accessor macro allows cmpxchg architectures
> >>>to produce optimal code and ll/sc non-optimal code.
> >>>
> >>>See my point?
> >>
> >>Wrong. Your ll/sc implementation with cmpxchg is buggy. The cmpxchg
> >>load_locked is not locked at all,
> >
> >
> >Intentional - cmpxchg architectures don't generally have a load locked.
>
> Exactly, so it is wrong -- you can't implement that behaviour with
> load + cmpxchg.

I disagree. I _have_ implemented the required behaviour. I really
don't understand your point saying that it is wrong.

> >>and there can be interleaving writes
> >>between the load and cmpxchg which do not cause the store_conditional
> >>to fail.
> >
> >
> >In which case the cmpxchg fails and we do the atomic operation again,
> >in exactly the same way that we do the operation again if the 'sc'
> >fails in the ll/sc case.
>
> Not if cmpxchg sees the same value, it won't fail, regardless of how
> many writes have hit that memory address.

Don't see anything wrong with that. If that was a problem, atomic
implementations using cmpxchg on x86 would be impossible.

I think you're trying to implement ll/sc semantics on CPUs without
ll/sc which is exactly not what I'm trying to do. I'd argue that's
impossible.

I'm trying to suggest a better implementation for atomic ops rather
than just bowing to this x86-centric "cmpxchg is the best, everyone
must implement it" mentality.

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/