On Mon, 2 Jun 2025 21:53:38 GMT, David Holmes <dhol...@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
