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

[Thierry Delisle <tdelisle@uwaterloo.ca>]

 > 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.