RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning

[Patricio Chilano Mateo]

On Wed, 23 Oct 2024 20:34:48 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> If the virtual thread is un-mounting from the carrier, why do we need to set the "lock id" back to the virtual thread's id? Sorry I'm finding this quite confusing.
>> 
>> Also `JavaThread::_lock_id` in the VM means "the java.lang.Thread thread-id to use for locking" - correct?
>
> Sorry, I should add context on why this is needed. The problem is that inside this temporal transition we could try to acquire some monitor. If the monitor is not inflated we will try to use the LockStack, but the LockStack might be full from monitors the virtual thread acquired before entering this transition. Since the LockStack is full we will try to make room by inflating one or more of the monitors in it [1]. But when inflating the monitors we would be using the j.l.Thread.tid of the carrier (set into _lock_id when switching the identity), which is wrong. We need to use the j.l.Thread.tid of the virtual thread, so we need to change _lock_id back.
> We are not really unmounting the virtual thread, the only thing that we want is to set the identity to the carrier thread so that we don't end up in this nested calls to parkNanos.
> 
> [1] https://github.com/openjdk/jdk/blob/afb62f73499c09f4a7bde6f522fcd3ef1278e526/src/hotspot/share/runtime/lightweightSynchronizer.cpp#L491

> Also JavaThread::_lock_id in the VM means "the java.lang.Thread thread-id to use for locking" - correct?
>
Yes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1813507846