NoSpammer (no.delete@this.spam.com) on July 14, 2015 2:02 pm wrote:
> Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on July 13, 2015 4:50 pm wrote:
> > NoSpammer (no.delete@this.spam.com) on July 13, 2015 12:34 pm wrote:
> > > With x86 a volatile variable is at least up to date monotonically with respect to what you've seen
> > > so far. That's actually good enough and it's excellent for producer-consumer type of stuff.
> >
> > That's not even true on x86. Loads may be lifted before stores
> > and so you may read an incorrect or "impossible" value.
>
> Let me rephrase it: suppose you have a volatile location you observe, it's possible that you see all changes
> prior to a volatile change, but not yet the volatile change, it's also possible that you see all changes
> and the volatile changed, but it's not possible to see a volatile changed but then in later code not see
> prior changes.

At it's most general, looking at system-wide state, this is untrue. Forwarding from the store queue means that a write can be seen (from one CPU) and then not be seen (from another).