Write and test alternative implementations of condition variables

The purpose of the condvar repository is to host code which tests, benchmarks and profiles various implementations of condition variables. The tests intentionally create massive lock contention in a turn-taking system based on Dekker's Algorithm. The contention is large because there are only two kinds of turns, but there are 4 threads that want them. Thus waking a random waiter will activate one which can't run 1/3 of the time.

The Makefile intentionally has compiler optimization disabled. Turning on optimization would require memory barriers and/or atomic variables. In fact, the recent "Formal Verification" by Aurelian Melinte in ACCU's Overload journal mentions that

the Dekker algorithm as detailed below was proven to be unsafe on a machine without atomic reads and writes

which implies that code without atomic reads and writes and consequent memory ordering is also likely unsafe.

A different Overload article, "In an Atomic World" by Lucian Radu Teodorescu inspired a large fraction of the code.

A primary but eventual goal is to test the priority inheritance behavior of these algorithms on a CONFIG_PREEMPT_RT Linux system. See Problems with condition variables in the Linux Foundation's realtime wiki for the motivation.