RFR (XS): 8165860: workgroup classes are missing volatile qualifiers for lock-free code

[Erik Österlund]

Hi,

This bug is a split out of the larger JDK-8033552 for adding volatile 
qualifiers to lock-free code.
The motivation is to make our implementation more robust since these 
kind of issues have struck us a few times too many already.

CR: https://bugs.openjdk.java.net/browse/JDK-8165860
Webrev: http://cr.openjdk.java.net/~eosterlund/8165860/webrev.00/

This specific CR handles the lack of volatile qualifiers in the 
SubTasksDone and SequentialSubTasksDone classes in the workgroup.* files.

SubTasksDone is missing volatile on the _tasks and _threads_completed 
fields that are changed concurrently with atomics.
SequentialSubTasksDone is missing volatile on the _n_claimed and 
_n_completed fields that are changed concurrently with atomics.

Testing: JPRT

I will need a sponsor to push this.

Thanks,
/Erik