Best thread pool implementation for the base system?

I count 5 thread pool implementations in contrib:
* cddl/compat/opensolaris/misc/thread_pool.c
* contrib/apr-util/misc/apr_thread_pool.c
* contrib/llvm/lib/Support/ThreadPool.cpp
* contrib/openmp/runtime/src/kmp_tasking.cpp
* contrib/ofed/opensm/complib/cl_threadpool.c

However, I can't find any examples outside of contrib.  I'd like to use one
in /sbin/geli.  Shall I roll my own (as everybody else apparently does), or
is there something I'm not aware of?

-Alan