Re: Some introductory docs about C level threading

[Marius Vollmer]

Ken Raeburn <address@hidden> writes:

> I think someone might be tempted to do something like:
>
> thread 1, on cpu 1, with guile core mutex locked:

(There is no guile core mutex.  Is it relevant to the discussion
below?)

> (precondition: z is a pair, SCM_CAR(z) is a pair)
> allocate cons cell x
> fill in x.car = v1
> fill in x.cdr = v2
> SCM_SETCAR(z, x)
>
> thread 2, on cpu 2, running without lock:
>
> set t = SCM_CAR(z) # atomic, so we always get before or after, right?
> read SCM_CAR(t) # should Just Work?

Yes, I had planned for this to just work...

> With weak memory ordering, and no locking during this sequence in
> either thread, I don't think we get any guarantee that SCM_CAR(t) will
> get v1 instead of the uninitialized contents of the cons cell as
> pulled off of the free list.

Yes, I now realize that at least initialization needs to have some
kind of memory synchronization after it.  Would declaring x.car and
x.cdr as volatile do it?

> Especially if it's one that cpu 2 happened to examine recently
> (perhaps it ran the GC pass) and thus has it in cache. I think you
> need a memory synchronization operation in each to guarantee this.

Needing to have the synch operation in all threads would be really
bad... is it really necessary?  If it is, I would get serious doubts
whether we can achieve the SCM is always valid' goal with full
concurrency.