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

[Coleen Phillimore]

On Sat, 26 Oct 2024 01:51:12 GMT, Dean Long <dlong 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/cpu/aarch64/stackChunkFrameStream_aarch64.inline.hpp line 119:
> 
>> 117:   return  mask.num_oops()
>> 118:         + 1 // for the mirror oop
>> 119:         + (f.interpreter_frame_method()->is_native() ? 1 : 0) // temp oop slot
> 
> Where is this temp oop slot set and used?

It's the offset of the mirror passed to static native calls.  It pre-existed saving the mirror in all frames to keep the Method alive, and is duplicated.  I think this could be cleaned up someday, which would remove this special case.

> src/hotspot/share/runtime/continuationFreezeThaw.cpp line 1411:
> 
>> 1409:     // zero out fields (but not the stack)
>> 1410:     const size_t hs = oopDesc::header_size();
>> 1411:     oopDesc::set_klass_gap(mem, 0);
> 
> Why, bug fix or cleanup?

This might confuse the change for JEP 450 since with CompactObjectHeaders there's no klass_gap, so depending on which change goes first, there will be conditional code here. Good question though, it looks like we only ever want to copy the payload of the object.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819394224
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1823227312