RFR (XS): G1 - Missing memory barrier in start_cset_region_for_worker

[Axel Siebenborn]

Hi,
we have seen crashes on ppc due to a missing memory barrier in 
G1CollectedHeap::start_cset_region_for_worker.
The code checks, if a HeapRegion is valid by comparing the gc_time_stamp 
with a store value in an array and loads the according HeapRegion from 
an other array. However, these entries are written by an other thread. 
The two loads have to be ordered by a memory barrier, to prevent them 
from being re-ordered and accessing a stale value for the HeapRegion.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8147611

Webrev:
http://cr.openjdk.java.net/~asiebenborn/8147611_0/webrev/

Could I please have reviews and a sponsor for this fix.

thanks,
Axel