When most public API operations on pthread_t's execute, the per-thread lock
(struct pthread.lock) is acquired to enforce consistency of the kernel and
userspace data structures.

This can cause a problem when a thread (t1) lowers its own priority and some
other thread (t2, high priority) then immediately becomes runnable as a result
of the priority shift. The scenario would look like this:

* There are three threads: T1 (low prio), T2 (mid prio), T3 (high prio)

* T1 is initially running at some value higher than its permanent priority, to
do some startup work

* T2 is executing some CPU-bound job that is always runnable

* T1 finishes initialization, sets itself to its lower (permanent) priority.
This requires grabbing the locking its own per-thread futex ("lock" in struct
pthread). The syscall to alter scheduling parameters will immediately result in
T2 being put on the CPU, so the lock is not yet dropped.

* T3 eventually needs to do some adjustment of T1's scheduling options. So it
tries to grab T1's per-thread lock, but can't since T1 still holds it because
its scheduling syscall hasn't returned to userspace yet.

* Priority inversion. T2 continues to run unchallenged.

Can the pthread.lock be treated as a PI futex instead of a standard futex, in
order to get priority inheritance and work around this inversion?

I'll attach an example program shortly.