RFR(XS): make shared variables volatile

[Roman Kennke]

Am Donnerstag, den 22.09.2016, 16:04 +0100 schrieb Andrew Haley:
> On 21/09/16 15:47, Aleksey Shipilev wrote:
> > 
> > On 09/21/2016 04:40 PM, Zhengyu Gu wrote:
> > > 
> > > http://cr.openjdk.java.net/~zgu/volatile/webrev.01/
> > 
> > +1
> > 
> > I wonder if this means we should update/poll these volatile
> > variables
> > with Atomic::* too, at least for the benefit of weak arches like
> > AArch64.
> 
> Always.  volatile is never sufficient for variables accessed racily
> by
> multiple threads, and in C++11 it's undefined behaviour.  I remember
> finding and fixing a couple of these when I did the AArch64
> Shenandoah
> port.

The question that came up while discussing this is how to access such
fields. Atomic and OrderAccess both have no public load() and store()
methods (or similar). There is load_acquire() and release_store() both
those are recommended only to be used in pairs. I believe it would have
the desired effects, but doesn't exactly look clean. Any hints?

Roman