RFR: 8352251: Implement JEP 518: JFR Cooperative Sampling [v34]

[Markus Grönlund]

On Thu, 22 May 2025 11:00:31 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>>> The crit sec to be stored before the poll test is issued.
>>> 
>>> Most likely not needed, because of implicit fences involved in thread suspension.
>> 
>> OK, but reading `sampling_critical_section` doesn't seem to have any synchronization at all, so it doesn't much matter what you do when writing it.
>
> Reading is only done when the thread is suspended. However, I need to know that the critical section is set before the poll.

The suspension mechanism should serialize all memory so that it appears to be set when read upon suspension.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24296#discussion_r2102279740