On Thu, Jul 06, 2017 at 08:21:10AM -0700, Paul E. McKenney wrote:

And yes, there are architecture-specific optimizations for an
empty spin_lock()/spin_unlock() critical section, and the current
arch_spin_unlock_wait() implementations show some of these optimizations.
But I expect that performance benefits would need to be demonstrated at
the system level.

I do in fact contended there are any optimizations for the exact
lock+unlock semantics.

The current spin_unlock_wait() is weaker. Most notably it will not (with
exception of ARM64/PPC for other reasons) cause waits on other CPUs.