[foreign-memaccess+abi] RFR: 8275054: Racy updates when releasing confined scopes

[Maurizio Cimadamore]

As the subject says, there are cases where a close dependency on a confined parent scope can occur from an unrelated thread:

1. the confined scope is acquired by another confined scope featuring implicit cleanup - in which case the cleaner thread will do the release

2. the confined scope is acquired, in the same thread, by a shared scope. Since a shared scope can be closed by any other thread, it follows that release can also be done by any thread.

In such cases we can have races (either between release/release or between acquire/release) and the `lockCount` variable of the updated scope becomes garbage.

The solution is to add an extra variable `asyncReleaseCount`. If a release request occur from the same thread that owns the confined scope, we proceed as usual, otherwise we atomically increment the `asyncReleaseCount`. When the confined scope is later closed, if `lockCount == 0`, nothing needs to be done, so we can just close. Otherwise, we need to check that `lockCount - asyncReleaseCount == 0`. This works because a confined segment can only be acquired by the same thread owning it. So, if `lockCount == 0` there can be no other pending release.

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Commit messages:
 - Handle races between release vs. acquire in confined scopes.

Changes: https://git.openjdk.java.net/panama-foreign/pull/596/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=596&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275054
  Stats: 85 lines in 2 files changed: 83 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/596.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/596/head:pull/596

PR: https://git.openjdk.java.net/panama-foreign/pull/596