RFR: 8259808: Add JFR event to detect GC locker stall [v6]

[Thomas Schatzl]

On Wed, 27 Jan 2021 02:25:00 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.
>
> Denghui Dong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   improve descriptions

Looks good.

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Marked as reviewed by tschatzl (Reviewer).

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