Re: [RFC PATCH 3/4 v0.3] sched/umcg: RFC: add userspace sll helpers

The helpers below can work with userspace single-linked lists concurrently
without indefinitely spinning in the kernel. Specifically:

push (add to the head of the list):

  step = 0
  while (++step < N)
     old = *head
     *next = old
     cmp_xchng(head, &old, next)

pop (remove the first element from the list):
    mark the node as deleted by flipping its lowest bit without
    actually removing the node from the list:

  curr = *head
  step = 0

  while (curr && ++step < N)

    next = *curr
    if (next & 1)
        curr = next & ~1
        continue

    if (cmp_xchng(curr, next, next | 1))
        return curr
    else
        curr = next & ~1

It is the userspace's responsibility to actually remove the
nodes marked as deleted from the list.

I believe the subsystem called Nemesis introduced a MCS-based queue that could
be useful here. The original paper is https://doi.org/10.1109/CCGRID.2006.31

You can also look at this repo for an implementation of the queue.
https://git.uwaterloo.ca/mkarsten/libfibre/-/blob/master/src/runtime/LockFreeQueues.h

While it uses 2 pointers, I believe it would fit well for idle_workers_ptr
since the push operation is wait-free, which avoids problems for the kernel.
The pop operation requires a lock in userspace *if* multiple servers can
consume from the same queue, but I don't believe it's a requirement in general.