you have applied my independent-pointer patch already, but i think your
CLEARTID variant is the most elegant solution: it reuses a clone argument,
thus reduces the number of arguments and it's also a nice conceptual pair
to the existing SETTID call. And the TID field can be used as a 'usage'
field as well, because the TID (PID) can never be 0, reducing the number
of fields in the TCB. And we can change the userspace locking code to use
the TID field no problem.