RFR: 8314480: Memory ordering spec updates in java.lang.ref

[Roger Riggs]

On Mon, 13 Nov 2023 22:31:16 GMT, Brent Christian <bchristi at openjdk.org> wrote:

> Classes in the `java.lang.ref` package would benefit from an update to bring the spec in line with how the VM already behaves. The changes would focus on _happens-before_ edges at some key points during reference processing.
> 
> A couple key things we want to be able to say are:
> - `Reference.reachabilityFence(x)` _happens-before_ reference processing occurs for 'x'.
> - `Cleaner.register()` _happens-before_ the Cleaner thread runs the registered cleaning action.
> 
> This will bring Cleaner in line (or close) with the memory visibility guarantees made for finalizers in [JLS 17.4.5](https://docs.oracle.com/javase/specs/jls/se18/html/jls-17.html#jls-17.4.5):
> _"There is a happens-before edge from the end of a constructor of an object to the start of a finalizer (§12.6) for that object."_

src/java.base/share/classes/java/lang/ref/Reference.java line 552:

> 550:      * of this method.
> 551:      * Invocation of this method does not itself initiate reference processing,
> 552:      * garbage collection, or finalization.

My understanding was that it is not the object instance that is being guarded, only that the reference holding the object is considered a strong root and is only used to delimit a range of bytecodes for which the reference is considered to be strong.
In particular, the invocation of the method itself has no semantics, only that a control flow could reach that statement and the reference was considered strong as long as the reference was in a scope that included the reachability fence.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1401310991