RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v12]

[Patricio Chilano Mateo]

On Sat, 26 Oct 2024 02:15:29 GMT, Dean Long <dlong at openjdk.org> wrote:

> > On failure to acquire a monitor inside `ObjectMonitor::enter` a virtual thread will call freeze to copy all Java frames to the heap. We will add the virtual thread to the ObjectMonitor's queue and return back to Java. Instead of continue execution in Java though, the virtual thread will jump to a preempt stub which will clear the frames copied from the physical stack, and will return to `Continuation.run()` to proceed with the unmount logic.
> 
> During this time, the Java frames are not changing, so it seems like it doesn't matter if the freeze/copy happens immediately or after we unwind the native frames and enter the preempt stub. In fact, it seems like it could be more efficient to delay the freeze/copy, given the fact that the preemption can be canceled.
>
The problem is that freezing the frames can fail. By then we would have already added the ObjectWaiter as representing a virtual thread. Regarding efficiency (and ignoring the previous issue) both approaches would be equal anyways, since regardless of when you freeze, while doing the freezing the monitor could have been released already. So trying to acquire the monitor after freezing can always succeed, which means we don't want to unmount but continue execution, i.e cancel the preemption.

> src/hotspot/cpu/aarch64/continuationFreezeThaw_aarch64.inline.hpp line 133:
> 
>> 131: 
>> 132: inline void FreezeBase::prepare_freeze_interpreted_top_frame(const frame& f) {
>> 133:   assert(*f.addr_at(frame::interpreter_frame_last_sp_offset) == 0, "should be null for top frame");
> 
> Suggestion:
> 
>   assert(f.interpreter_frame_last_sp() == nullptr, "should be null for top frame");

Changed, here and in the other platforms.

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PR Comment: https://git.openjdk.org/jdk/pull/21565#issuecomment-2442387426
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819592799