On Thu, Oct 08, 2015 at 02:50:36PM +1100, Michael Ellerman wrote:

On Wed, 2015-10-07 at 08:25 -0700, Paul E. McKenney wrote:

quoted

Currently, we do need smp_mb__after_unlock_lock() to be after the
acquisition on PPC -- putting it between the unlock and the lock
of course doesn't cut it for the cross-thread unlock/lock case.

This ^, that makes me think I don't understand
smp_mb__after_unlock_lock.

How is:

	UNLOCK x
	smp_mb__after_unlock_lock()
	LOCK y

a problem? That's still a full barrier.

quoted

I am with Peter -- we do need the benchmark results for PPC.

Urgh, sorry guys. I have been slowly doing some benchmarks, but time is not
plentiful at the moment.

If we do a straight lwsync -> sync conversion for unlock it looks like that
will cost us ~4.2% on Anton's standard context switch benchmark.

And that does not seem to agree with Paul's smp_mb__after_unlock_lock()
usage and would not be sufficient for the same (as of yet unexplained)
reason.

Why does it matter which of the LOCK or UNLOCK gets promoted to full
barrier on PPC in order to become RCsc?