ConcurrentHashMap::computeIfAbsent and synchronized

[mikmoila]

Hi.

As often mentioned in this mailing-list a feedback about preview/incubator
features is appreciated, so here's one:

I was experimenting with a caching system utilising ConcurrentHashMap as
cache store and Structured Concurrency API for refreshing the entries from
multiple sources ( StructuredTaskScope.ShutdownOnSuccess ). The idea was to
make http-requests for getting the fresh values but the first
implementation simply uses UUID::randomUUID for simulating that.

I noticed that the programs halts In a test case where "N" concurrent calls
(where "N" >= number of cpu cores) running on virtual threads end-up
calling the ConcurrentHashMap::computeIfAbsent for the same (non-existing)
key.

"-Djdk.tracePinnedThreads=full" reveals that there is a pinned carrier
thread:

java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1708)
<== monitors:1

The documentation (
https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/util/concurrent/ConcurrentHashMap.html#computeIfAbsent(K,java.util.function.Function)
) says:

  "Some attempted update operations on this map by other threads may be
blocked while computation is in progress, so the computation should be
short and simple."

This is clear but I still found it as a surprise that it uses synchronized
instead of "virtual-thread friendly" constructs.

If needed I can post a small demo program.

JDK used is latest OpenJDK-19 GA, OS is Windows:
  openjdk version "19.0.2" 2023-01-17
  OpenJDK Runtime Environment (build 19.0.2+7-44)
  OpenJDK 64-Bit Server VM (build 19.0.2+7-44, mixed mode, sharing)

Best Regards,
  Mika
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