RFR: 8314480: Memory ordering spec updates in java.lang.ref [v16]

[Brent Christian]

On Mon, 25 Mar 2024 19:26:41 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."_
>
> Brent Christian has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Small updates to reachabilityFence, per review

src/java.base/share/classes/java/lang/ref/Reference.java line 638:

> 636:      *
> 637:      * <p> It is sometimes possible to better encapsulate use of
> 638:      * {@code reachabilityFence}.  Continuing the above example, if it were

[Per Hans in email,](https://mail.openjdk.org/pipermail/core-libs-dev/2024-March/120747.html) I agree that this is not a good example, and is misleading.

Even if `update()` could be called without harm if the finalizer has already executed:
the user has called `action()` expecting some action (involving the `externalResource`) to take place. With the `reachabilityFence()` call "refactored" in this way, it is unclear whether that action actually takes place.

Better to stick with the try/finally/reachabilityFence() pattern, and have `action()` behave consistently.

This passage/example should be removed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1538183404