RFR: 8362282: runtime/logging/StressAsyncUL.java failed with exitValue = 134

[Casper Norrbin]

Hi everyone,

During `AsyncLogWriter` initialization we previously published the global `AsyncLogWriter _instance` before waiting for existing synchronous logging activity to finish and before starting the async consumer thread. As soon as `_instance` became visible, `is_enqueue_allowed()` allowed new log calls to switch to the async path. Those calls bumped `LogOutputList`'s counters while `initialize()` was waiting for `wait_until_no_readers()` to observe no active readers. At the same time, because the consumer thread wasn’t yet running, producers could end up waiting for a consumer that didn’t exist. In rare timing, this created a circular wait that manifested as intermittent lock-ups in `StressAsyncUL.java`.

The initialization order is changed so that we do not make the async writer visible until after all pre-existing synchronous logging has completed. Concretely, `initialize()` now:

1. waits for each `LogTagSet` to report no readers, ensuring synchronous logging is done,
2. publishes `_instance` with a release store so readers observe a fully initialized writer, and
3. immediately starts the async logging thread.

With this ordering, new async producers cannot keep the counters inflated while initialization is waiting. Any messages produced in the brief window between publishing `_instance` and the thread's first run are safely buffered and then consumed as soon as the thread starts.

Testing:
- Oracle tiers 1-3
- Repeated runs of `StressAsyncUL.java` with no observed lock-ups

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Commit messages:
 - init fix

Changes: https://git.openjdk.org/jdk/pull/27169/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362282
  Stats: 5 lines in 1 file changed: 2 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27169.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27169/head:pull/27169

PR: https://git.openjdk.org/jdk/pull/27169