On Mon, Jun 08, 2015 at 06:20:44PM -0400, Waiman Long wrote:

The current cmpxchg() loop in setting the _QW_WAITING flag for writers
in queue_write_lock_slowpath() will contend with incoming readers
causing possibly extra cmpxchg() operations that are wasteful. This
patch changes the code to do a byte cmpxchg() to eliminate contention
with new readers.

This is very narrow, would not the main cost still be the cacheline
transfers?

Do you have any numbers to back this? I would feel much better about
this if there's real numbers attached.