RFR: 8291555: Implement alternative fast-locking scheme [v30]

[Roman Kennke]

On Tue, 28 Mar 2023 21:39:21 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> src/hotspot/share/runtime/threads.cpp line 1433:
>> 
>>> 1431: 
>>> 1432: JavaThread* Threads::owning_thread_from_monitor(ThreadsList* t_list, ObjectMonitor* monitor) {
>>> 1433:   assert(SafepointSynchronize::is_at_safepoint(), "not safe outside of safepoint");
>> 
>> Shouldn't this be gated on UseFastLocking?
>
> Hmmm.... `owning_thread_from_monitor()` is only called from
> `ObjectSynchronizer::get_lock_owner()` when `get_lock_owner()`
> knows that it has an ObjectMonitor in hand. I'm not at all sure that
> we can assert that `ObjectSynchronizer::get_lock_owner()` is
> only called from a safepoint.

There has been a single call path from management.cpp that is not calling this code at a safepoint, and I changed that code to take the safepoint-taking variant code-path when using UseFastLocking. That path already existed and has been used when max_depth == 0.

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PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1153307575