RFR: 8270869: G1ServiceThread may not terminate

[Kim Barrett]

Please review this fix of a rare race in G1ServiceThread termination that
can lead to the thread not terminating in a timely manner.

Testing:
mach5 tier1

-------------

Commit messages:
 - fix race

Changes: https://git.openjdk.java.net/jdk/pull/4819/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4819&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270869
  Stats: 5 lines in 1 file changed: 1 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4819.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4819/head:pull/4819

PR: https://git.openjdk.java.net/jdk/pull/4819