On Wed, Dec 02, 2015 at 04:11:41PM -0800, Paul E. McKenney wrote:

This looks architecture-agnostic to me:

a.	TSO systems have smp_mb__after_unlock_lock() be a no-op, and
	have a read-only implementation for spin_unlock_wait().

b.	Small-scale weakly ordered systems can also have
	smp_mb__after_unlock_lock() be a no-op, but must instead
	have spin_unlock_wait() acquire the lock and immediately 
	release it, or some optimized implementation of this.

c.	Large-scale weakly ordered systems are required to define
	smp_mb__after_unlock_lock() as smp_mb(), but can have a
	read-only implementation of spin_unlock_wait().

This would still require all relevant spin_lock() sites to be annotated
with smp_mb__after_unlock_lock(), which is going to be a painful (no
warning when done wrong) exercise and expensive (added MBs all over the
place).

But yes, I think the proposal is technically sound, just not quite sure
how far we'll want to push this.