RFR(M): Memory ordering in taskqueue.hpp

[Lindenmaier, Goetz]

Hi,

Once more, with webrev:
http://cr.openjdk.java.net/~goetz/webrevs/webrev-taskqueue/
Sorry for that.

I would like to contribute fixes wrt. memory ordering in taskqueue.hpp.

On Platfoms with weak memory ordering the taskqueue is not accessed
properly, as the accesses to the fields _bottom and _age are not ordered
correctly. Volatile is not sufficient to enforce this, because it depends on
what the compiler assumes to be necessary for volatile variables.

The fix is to pull getter/setter routines from Age to TaskQueueSuper and use methods from
OrderAccess to access the fields in _age as well as _bottom.   Further the code must always
use the getter/setter methods to access _bottom, _age or the subfields of _age.
On the other hand we can relax constraints for accesses to locals oldAge and newAge.

The OrderAccess routines do simple load/stores on x86_64.

I want to discuss this change here and would like very much to see it taking
it's way into OpenJDK to support ports on platforms with weak memory
ordering, and, in particular, our PPC port.

Best regards,
  Goetz.