RFR: 8291555: Implement alternative fast-locking scheme [v35]

[David Holmes]

On Thu, 30 Mar 2023 14:30:28 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Please explain why you think this is "not safe". Yes, you can observe state that is in
>> the process of changing, but do you think that we'll see a crash with allowing
>> `Threads::owning_thread_from_object()` to be called from a non-safepoint place?
>
> I don't think we'd see a crash, but we might get false results when we are scanning the lock-stack of a foreign thread, when that thread does not hold still. I'm not even comfortable doing that cross-stack lock query with the old code.

Given the owner could release the monitor the moment after we check I don't see how false results are an issue here. The existing code should be safe when not executed at a safepoint..

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PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1156792723