RFR: 8314480: Memory ordering spec updates in java.lang.ref [v15]

[David Holmes]

On Thu, 21 Mar 2024 23:38:30 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:
> 
>   Elaborate on 'surprising and undesirable effects' in reachabilityFence()

src/java.base/share/classes/java/lang/ref/Reference.java line 576:

> 574:      * detects that there is no further need for an object. The garbage collector
> 575:      * may reclaim an object even if values from that object's fields are still
> 576:      * in use, or while a method on the object is still running, so long as the

Suggestion: method _of_ the object

src/java.base/share/classes/java/lang/ref/Reference.java line 579:

> 577:      * object has otherwise become unreachable.
> 578:      * <p>
> 579:      * This may have surprising and undesirable effects, in particular when using

Nit: I don't think a new paragraph is appropriate here as "This may ..." refers back to the subject of the preceding lines.

src/java.base/share/classes/java/lang/ref/Reference.java line 581:

> 579:      * This may have surprising and undesirable effects, in particular when using
> 580:      * a Cleaner or finalizer for cleanup. If an object becomes unreachable while
> 581:      * a method on the object is running, it can lead to a race between the

Again suggest: method _of_ the object

src/java.base/share/classes/java/lang/ref/Reference.java line 585:

> 583:      * Cleaner or finalizer. For instance, the cleanup thread could cleanup the
> 584:      * resource, followed by the program thread (still running the method)
> 585:      * attempting to access the now-already-freed resource.

suggestion: s/cleanup/free/ to match the `now-already-freed` part.

src/java.base/share/classes/java/lang/ref/Reference.java line 586:

> 584:      * resource, followed by the program thread (still running the method)
> 585:      * attempting to access the now-already-freed resource.
> 586:      * {@code reachabilityFence} can prevent this race by ensuring that the

Suggestion: `Use of {@code reachabilityFence ...`

This avoids starting a sentence with a code font word that starts with a lower-case letter.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1534946886
PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1534947996
PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1534948233
PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1534948894
PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1534949965