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

[Alan Bateman]

On Wed, 23 Oct 2024 09:53:53 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> This is the implementation of JEP 491: Synchronize Virtual Threads without Pinning. See [JEP 491](https://bugs.openjdk.org/browse/JDK-8337395) for further details.
>> 
>> In order to make the code review easier the changes have been split into the following initial 4 commits:
>> 
>> - Changes to allow unmounting a virtual thread that is currently holding monitors.
>> - Changes to allow unmounting a virtual thread blocked on synchronized trying to acquire the monitor.
>> - Changes to allow unmounting a virtual thread blocked in `Object.wait()` and its timed-wait variants.
>> - Changes to tests, JFR pinned event, and other changes in the JDK libraries.
>> 
>> The changes fix pinning issues for all 4 ports that currently implement continuations: x64, aarch64, riscv and ppc. Note: ppc changes were added recently and stand in its own commit after the initial ones.
>> 
>> The changes fix pinning issues when using `LM_LIGHTWEIGHT`, i.e. the default locking mode, (and `LM_MONITOR` which comes for free), but not when using `LM_LEGACY` mode. Note that the `LockingMode` flag has already been deprecated ([JDK-8334299](https://bugs.openjdk.org/browse/JDK-8334299)), with the intention to remove `LM_LEGACY` code in future releases.
>> 
>> 
>> ## Summary of changes
>> 
>> ### Unmount virtual thread while holding monitors
>> 
>> As stated in the JEP, currently when a virtual thread enters a synchronized method or block, the JVM records the virtual thread's carrier platform thread as holding the monitor, not the virtual thread itself. This prevents the virtual thread from being unmounted from its carrier, as ownership information would otherwise go wrong. In order to fix this limitation we will do two things:
>> 
>> - We copy the oops stored in the LockStack of the carrier to the stackChunk when freezing (and clear the LockStack). We copy the oops back to the LockStack of the next carrier when thawing for the first time (and clear them from the stackChunk). Note that we currently assume carriers don't hold monitors while mounting virtual threads.
>> 
>> - For inflated monitors we now record the `java.lang.Thread.tid` of the owner in the ObjectMonitor's `_owner` field instead of a JavaThread*. This allows us to tie the owner of the monitor to a `java.lang.Thread` instance, rather than to a JavaThread which is only created per platform thread. The tid is already a 64 bit field so we can ignore issues of the counter wrapping around.
>> 
>> #### General notes about this part:
>> 
>> - Since virtual th...
>
> src/hotspot/share/runtime/javaThread.hpp line 166:
> 
>> 164:   // current _vthread object, except during creation of the primordial and JNI
>> 165:   // attached thread cases where this field can have a temporary value.
>> 166:   int64_t _lock_id;
> 
> Following the review I wanted to better understand when `_lock_id` changes. There seems to be another exception to the rule that `_lock_id` is equal to the `tid` of the current `_vthread`. I think they won't be equal when switching temporarily from the virtual to the carrier thread in `VirtualThread::switchToCarrierThread()`.

Right, and we hope this temporary. We had more use of temporary transitions when the feature was initially added in JDK 19, now we mostly down to the nested parking issue. That will go away when we get to replacing the timer code, and we should be able to remove the switchXXX method and avoid the distraction/complexity that goes with them.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1812385061