See doc/notes.txt. Some test programs are available in the tests/ directory.

Design principles and performance measurements are detailed here:

   http://wtarreau.blogspot.com/2018/02/progressive-locks-fast-upgradable.html

Progressive locks are used, among others, by projects like HAProxy (https://www.haproxy.org/)
and Intel's QAT Engine (https://github.com/intel/QAT_Engine/).

A Golang implementation also exists (https://github.com/richardsamuels/go-plock).