RFR (S): 8165859: gcTaskThread is missing volatile qualifier and barriers for _time_stamps

[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-8165859
Webrev: http://cr.openjdk.java.net/~eosterlund/8165859/webrev.00/

This specific CR handles the lack of volatile qualifiers for the 
_time_stamps field, and relatedly the lack of acquire barriers when 
accessing it.
Hopefully, the addition of load acquire and volatile should make this a 
bit more robust.

Testing: JPRT

I will need a sponsor to push this.

Thanks,
/Erik