RFR: 8259808: Add JFR event to detect GC locker stall

[Denghui Dong]

On Fri, 15 Jan 2021 02:42:20 GMT, Denghui Dong <ddong at openjdk.org> wrote:

> GC locker will stall the operation of GC, resulting in some Java threads can not continue to run until GC locker is released, thus affecting the response time of the application. Add a JFR event to report this information is helpful to detect this issue.
> 
> For the test purpose, I add two Whitebox methods to lock/unlock critical.

GC locker will stall the operation of GC, resulting in some Java threads can not continue to run until GC locker is released, thus affecting the response time of the application. Add a JFR event to report this information is helpful to detect this issue.

For the test purpose, I add two Whitebox methods to lock/unlock critical.

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PR: https://git.openjdk.java.net/jdk/pull/2088