RFR: 8340586: JdkJfrEvent::get_all_klasses stores non-strong oops in JNI handles [v2]

[Coleen Phillimore]

On Tue, 5 Nov 2024 13:57:04 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> JdkJfrEvent::get_all_klasses walks the class hierarchy of the Event class to find all event sub klasses. The mirrors of these classes are then stored in JNI handles and eventually transfered to an arraylist which is returned to the java caller.
>> 
>> Because the klasses are found by walking the class hierarchy, their is no guarantee that the klass is strongly reachable, and thus that the java_mirror load is strong. So we must explicitly keep the klass alive (and not have a safepoint between the load and the store into the object graph).
>> 
>> Having both `fill_klasses` and `transform_klasses_to_local_jni_handles` made the call to `keep_alive` and the comment explaining the reason why somewhat obscured, so I merged them. (The split seems to have served no purpose except to introduce the need for a lot of const and reinterpret casts.)
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update comment

src/hotspot/share/oops/klass.inline.hpp line 42:

> 40:   // Resolving the holder (a WeakHandle) will keep the klass alive until the next safepoint.
> 41:   // Making the klass's CLD handle oops (e.g. the java_mirror), safe to store in the object
> 42:   // graph and it's roots (e.g. Handles).

typo: its doesn't have a '

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PR Review Comment: https://git.openjdk.org/jdk/pull/21893#discussion_r1829490928