RFR: 8220293: Deadlock in JFR string pool

[Andrey Petushkov]

Dear All,

As suggested by Mario may I ask to include the backport of the
https://bugs.openjdk.java.net/browse/JDK-8220293 into 8u262 (that is,
critical request). The reason is that this is single severe (VM
deadlock) JFR bug we are aware of
The patch applies cleanly, except for file locations change. Just in
case here is the webrev https://cr.openjdk.java.net/~apetushkov/8220293/
(one actual code change is .hpp guard macro name change due to file
location change)

Verified by the test in the bug (deadlock in about 1 minute without the
fix, running fine for about 15 minutes a few times with the fix)

Thanks,
Andrey