Priority Sifting Reader-Writer Lock v13

Hi,

Here is the reworked version of what was initially called "Fair rwlock", then
"writer-biased rwlock". Hopefully the naming should now better represent the
innovation in this reader-writer locking scheme.

Thanks to Linus' patient explanations, it uses a single atomic op on a 32 bit
variable in the fast path. The bright side of this is instruction-wise
compactness of the fast path and that there is practically no limitation on the
number of readers or writers. The downside is added memory ordering complexity
between fast and slow path variables in the slow path.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68