Debugging this in the kernel seemed hard ... so I tried to construct a
user level test using the same code from the kernel. See attached. But
this fails so catastrophically with any number of threads greater than
one, that I suspect I made some mistake cut & pasting the relevant bits
of kernel infrastructure.  The goal of the program is to have several
child processes pounding on a lock while the parent looks in every 5
seconds to see if they are making progress.

-Tony