RFR (S): 8223177: Data race on JvmtiEnvBase::_tag_map in double-checked locking

[Man Cao]

Hi all,

Can I have reviews for this small change that adds memory fences for
double-checked locking?
We found this race while working on the Java ThreadSanitizer project.

Webrev: https://cr.openjdk.java.net/~manc/8223177/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8223177

-Man
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