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

[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

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Commit messages:
 - 8338417

Changes: https://git.openjdk.org/jdk/pull/20588/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20588&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338417
  Stats: 173 lines in 3 files changed: 149 ins; 8 del; 16 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