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Re: Some introductory docs about C level threading

From: Marius Vollmer
Subject: Re: Some introductory docs about C level threading
Date: Thu, 10 Feb 2005 13:05:04 +0100
User-agent: Gnus/5.1003 (Gnus v5.10.3) Emacs/21.3 (gnu/linux)

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

> (precondition: z is a pair, SCM_CAR(z) is a pair)
> allocate cons cell x
> fill in = 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 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

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