On Fri, Oct 09, 2015 at 10:40:39AM +0100, Will Deacon wrote:
> Stepping back a second, I believe that there are three cases:
>
>
> RELEASE X -> ACQUIRE Y (same CPU)
> * Needs a barrier on TSO architectures for full ordering
+PPC
> UNLOCK X -> LOCK Y (same CPU)
> * Needs a barrier on PPC for full ordering
> RELEASE X -> ACQUIRE X (different CPUs)
* Fully ordered everywhere...
* ... but needs a barrier on TSO + PPC to become a full barrier
> UNLOCK X -> ACQUIRE X (different CPUs)
s/ACQUIRE/LOCK/ ?
> * Fully ordered everywhere...
> * ... but needs a barrier on PPC to become a full barrier
If you really meant ACQUIRE, then x86 also needs a barrier in order to
upgrade, seeing how our unlock is equivalent to smp_store_release(). Our
LOCK otoh is far heavier than smp_load_acquire() and would result in
different rules.
And I'm not sure the "(different CPUs)" bit makes sense, as the same is
true if they're on the same CPU.
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