[sh/jdk8u] RFR 8276201: Shenandoah: Race results degenerated GC to enter wrong entry point

[Zhengyu Gu]

I would like to backport this patch to Shenandoah/8u. This is a day-one bug.

Bug: https://bugs.openjdk.java.net/browse/JDK-8276201
Patch: 
https://github.com/openjdk/jdk/commit/dbf5100dd705fbe4a3aeae49405ca541d581f106

The original patch does not apply cleanly, but fix is simple and can 
easily applied to sh/8u tree.

8u webrev: http://cr.openjdk.java.net/~zgu/JDK-8276201-8u/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu