Re: Some introductory docs about C level threading

[Ken Raeburn <raeburn@raeburn.org>]

On Feb 9, 2005, at 07:13, Marius Vollmer wrote:
>> And volatile declarations may fix the possibility of storing
>> incomplete values, but I don't think it would deal with the
>> cross-thread memory access ordering issue.
> As long as accesses to SCM values are atomic, the ordering of
> loads/stores across threads can be arbitrary.

I think someone might be tempted to do something like:

thread 1, on cpu 1, with guile core mutex locked:

(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?

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.  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.


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