The root of all this: When Nick Piggin posted his first suggestion for
ticket spinlocks on LKML, I immediately liked the idea. For details
check LWN [1], in a nutshell: This algorithm enforces strict FIFO order
for the admission to contended spinlocks, thus it improves the
determinism on SMP systems with more than 2 CPUs.

Meanwhile, ticket spinlocks are mainline (2.6.25). But that version has
to drawbacks for us: it doesn't support nesting like xnlock does, and it
is x86-only so far.

So I designed a version for Xenomai which is both nestable and
arch-independent. It is certainly not as optimal as mainline's version,
but our code path stresses the locking code differently anyway.

This thing here /seems/ to work, but I'm lacking CPUs at home to test.
You can't truly stress ticket locks with only a single dual-core :-/.
QEMU runs into a live-lock with -smp 2, this patch applied and two
moderate latency loops, but that might be an artifact of its
single-threaded VCPU scheduling. And kvm currently locks up under SMP
even without any change, but kvm and SMP is a story of its own. There is
hope: 16-way is waiting at work... :)

Jan

[1] http://lwn.net/Articles/267331