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

[Daniel D. Daugherty]

On Wed, 29 Mar 2023 21:55:35 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Process lock-stack oops before inspecting them, when in foreign thread and not at safepoint. Add verifications.
>
> src/hotspot/share/runtime/threads.cpp line 1423:
> 
>> 1421: 
>> 1422: JavaThread* Threads::owning_thread_from_object(ThreadsList * t_list, oop obj) {
>> 1423:   assert(SafepointSynchronize::is_at_safepoint(), "not safe outside of safepoint");
> 
> `ObjectSynchronizer::get_lock_owner()` calls `Threads::owning_thread_from_object()`
> and I don't think you can assert that `ObjectSynchronizer::get_lock_owner()`  is only
> called from a safepoint. In particular, `ThreadSnapshot::initialize()` calls
> `ObjectSynchronizer::get_lock_owner()`  and I believe that `initialize()` function can
> be called from non-safepoint places with M&M APIs...

Update: In v35 you put back the `!UseFastLocking` check in `jmm_GetThreadInfo()`
so now the `assert()` won't fire, but you've again changed the behavior of that API
and now it will be able to observe fewer thread state changes than it did before.

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PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1152526559