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

[Thomas Schatzl]

Hi,

On Tue, 2016-01-19 at 15:30 +0100, Axel Siebenborn wrote:
> 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.

  looks good.

Mikael has been faster than me about sponsoring :)

Thanks,
  Thomas