RFR: 8338417: Explicitly pin a virtual thread before acquiring the JFR string pool monitor [v3]

[Markus Grönlund]

> Greetings,
> 
> Explicitly pin a virtual thread before acquiring the JFR string pool monitor because migrating a carrier thread local event writer object onto another carrier thread is impossible.
> 
> During event commit, the thread is in a critical section because it has loaded a carrier thread local event writer object. For virtual threads, a contended monitor, such as a synchronized block, is a point where a thread could become unmounted.
> 
> A monitor guards the JFR string pool, but remounting a virtual thread onto another carrier is impossible because of the event writer.
> 
> Therefore, it's imperative to use explicit pin constructs to prevent unmounting at this location. 
> 
> Testing: jdk_jfr
> 
> Thanks
> Markus

Markus Grönlund has updated the pull request incrementally with one additional commit since the last revision:

  update test comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20588/files
  - new: https://git.openjdk.org/jdk/pull/20588/files/b047b95c..b96b411f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20588&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20588&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20588.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20588/head:pull/20588

PR: https://git.openjdk.org/jdk/pull/20588