RFR: 8324983: race in CompileBroker::possibly_add_compiler_threads [v2]

[Tom Rodriguez]

> The number of active compiler threads is decremented before the compiler thread has actually activated so possibly_add_compiler_thread might start a new thread on the existing JavaThread.  This adds a check that it's really exiting before proceeding and some new guarantees that ensure threads aren't started on top running threads.

Tom Rodriguez has updated the pull request incrementally with one additional commit since the last revision:

  Convert guarantee to assert

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17662/files
  - new: https://git.openjdk.org/jdk/pull/17662/files/93145119..7bc1ab8c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17662&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17662&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17662.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17662/head:pull/17662

PR: https://git.openjdk.org/jdk/pull/17662