RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v29]

[David Holmes]

On Mon, 2 Jun 2025 21:53:38 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 117:
>> 
>>> 115: 
>>> 116: bool JfrCPUTimeTraceQueue::is_empty() const {
>>> 117:   return Atomic::load(&_head) == 0;
>> 
>> Not entirely clear what is the memory semantics for accessing `_head`. Does it need to be acq/rel? If so, this one should be `::load_acquire`?
>
> Many of the accesses to head do not appear to synchronize with anything and so do not need acquire semantics. But the overall concurrency properties of this code are very unclear to me.

To be clear, you only need acquire semantics here if after seeing the value 0 you need to access fields that were written before `_head` was set to 0. Similarly for most of the other access to `_head`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2122374152