Re: memory barriers (was: Yes, WaitLatch is vulnerable to weak-memory-ordering bugs)

On Thu, 2011-09-22 at 19:12 -0400, Robert Haas wrote:
> But since you asked... as I
> understand it, unless you're running on Alpha, you actually don't need
> a barrier here at all, because all currently-used CPUs other than
> alpha "respect data dependencies", which means that if q->num_items is
> used to compute an address to be read from memory, the CPU will ensure
> that the read of that address is performed after the read of the value
> used to compute the address. At least that's my understanding. But
> Alpha won't.

I'm still trying to figure out how it's even possible to read an address
that's not computed yet. Something sounds strange about that...

I think it might have more to do with branch prediction or something
else. In your example, the address is not computed from q->num_items
directly, it's computed using "i". But that branch being followed is
dependent on a comparison with q->num_items. Maybe that's the dependency
that's not respected?

Regards,
Jeff Davis