Re: [PATCH 4/4 v0.4] sched/umcg: RFC: implement UMCG syscalls

I have attached an atomic stack implementation I wrote. I believe it would
be applicable here. It is very similar except the kernel side no longer
needs a retry loop, the looping is moved to the user-space after the pop.
Using it instead of the code you have in enqueue_idle_worker means the
timeout is no longer needed.

- ``uint64_t idle_server_tid_ptr``: points to a pointer variable in the
  userspace that points to an idle server, i.e. a server in IDLE state waiting
  in sys_umcg_wait(); read-only; workers must have this field set; not used
  in servers.

  When a worker's blocking operation in the kernel completes, the kernel
  changes the worker's state from ``BLOCKED`` to ``IDLE``, adds the worker
  to the list of idle workers, and checks whether
  ``*idle_server_tid_ptr`` is not zero. If not, the kernel tries to cmpxchg()
  it with zero; if cmpxchg() succeeds, the kernel will then wake the server.
  See `State transitions`_ below for more details.

In this case, I believe cmpxchg is not necessary and xchg suffices.