Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return_* variants

[Boqun Feng <boqun.feng@gmail.com>]

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On Fri, Aug 28, 2015 at 05:39:21PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 28, 2015 at 10:16:02PM +0800, Boqun Feng wrote:
<snip>
> > 
> > Ah.. just read through the thread you mentioned, I might misunderstand
> > you, probably because I didn't understand RCpc well..
> > 
> > You are saying that in a RELEASE we -might- switch from smp_lwsync() to
> > smp_mb() semantically, right? I guess this means we -might- switch from
> > RCpc to RCsc, right?
> > 
> > If so, I think I'd better to wait until we have a conclusion for this.
> 
> Yes, the difference between RCpc and RCsc is in the meaning of RELEASE +
> ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does
> not.
> 
> Currently PowerPC is the only arch that (can, and) does RCpc and gives a
> weaker RELEASE + ACQUIRE. Only the CPU who did the ACQUIRE is guaranteed
> to see the stores of the CPU which did the RELEASE in order.
> 
> As it stands, RCU is the only _known_ codebase where this matters, but
> we did in fact write code for a fair number of years 'assuming' RELEASE
> + ACQUIRE was a full barrier, so who knows what else is out there.
> 
> 
> RCsc - release consistency sequential consistency
> RCpc - release consistency processor consistency
> 
> https://en.wikipedia.org/wiki/Processor_consistency (where they have
> s/sequential/causal/)

Thank you for your detailed explanation! Much clear now ;-)

Regards,
Boqun

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