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

Nick Piggin
2006-12-08T22:34:08Z

Russell King wrote:
> 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.

Yes, I did think that from reading your emails. It is not a problem
as such, but it is important to be clear on semantics.

> 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.

Even if ARM is able to handle any arbitrary C code between the
"load locked" and store conditional API, other architectures can not
by definition.

--
SUSE Labs, Novell Inc.
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