[jmm-dev] stores

Paul E. McKenney paulmck at linux.vnet.ibm.com
Mon Feb 24 16:20:20 PST 2014

On Sun, Feb 16, 2014 at 06:00:30PM -0500, Doug Lea wrote:
> Memory models can generate a fair amount of excitement.
> See Linus Torvalds's post on the linux kernel list:
>   https://lkml.org/lkml/2014/2/14/492
> and follow-ups with Paul McKenney. (Condolences!)

Heh!  The fun continues...

> I don't think this introduces anything new with respect
> to JMM9 discussions so far though. In general, speculative
> stores and out-of-thin-air reads break basic safety properties.
> Although there still might be some related open cases
> about inserted stores, including "redundant" ones. As in:
>     if (x != 0) x = 0;
> ==> x = 0; ?

One possibly interesting thing from later in the LKML discussion, though
I am not sure that it maps into the Java final-field model -- you guys
can be the judge of that.  I will present it in C just for definiteness.

T1:	p = &nondefault_gp;
	p->a = 42;
	atomic_store_release(&gp, p);

T2:	p = atomic_load_explicit(&gp, memory_order_consume);
	if (p != &default_gp) {
	r1 = p->a;  /* At this point, the compiler knows p == &default_gp. */

If this particular execution has only &default_gp and &nondefault_gp
as values for gp, are we guaranteed that r1==42?  It would be given
the current wording in the C11 and C++11 standards.  Assuming that this
example even makes sense in the context of Java final fields...

							Thanx, Paul

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