Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Fri, 2014-02-07 at 10:02 +0100, Torsten Duwe wrote:

quoted

quoted

quoted

Can you pair lwarx with sthcx ? I couldn't immediately find the answer
in the PowerISA doc. If so I think you can do better by being able to
atomically load both tickets but only storing the head without affecting
the tail.

Can I simply write the half word, without a reservation, or will the HW caches
mess up the other half? Will it ruin the cache coherency on some (sub)architectures?

Yes, you can, I *think*

quoted

Plus, sthcx doesn't exist on all PPC chips.

Which ones are lacking it? Do all have at least a simple 16-bit store?

half word atomics (and byte atomics) are new, they've been added in architecture
2.06 I believe so it's fairly recent, but it's still worthwhile to investigate a
way to avoid atomics on unlock on recent processors (we can use instruction patching
if necessary based on CPU features) because there's definitely a significant cost
in doing a larx/stcx. sequence on powerpc, way higher than our current unlock path
of barrier + store.

Cheers,
Ben.