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

On Wed, Aug 4, 2021 at 3:05 PM Thierry Delisle [off-list ref] wrote:

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.

Thanks, Thierry! Your implementation seems reasonable - I'll test it
and will use it in the future versions of the patchset if everything
is OK.

 > - ``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.

Right! I'll need to roll another atomic_xchg helper for this and the
atomic stack above...