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

[Dean Long]

On Sat, 26 Oct 2024 07:04:28 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> src/hotspot/cpu/x86/stubGenerator_x86_64.cpp line 3796:
>> 
>>> 3794:   __ movbool(rscratch1, Address(r15_thread, JavaThread::preemption_cancelled_offset()));
>>> 3795:   __ testbool(rscratch1);
>>> 3796:   __ jcc(Assembler::notZero, preemption_cancelled);
>> 
>> If preemption was canceled, then I wouldn't expect patch_return_pc_with_preempt_stub() to get called.  Does this mean preemption can get canceled (asynchronously be a different thread?) even afgter patch_return_pc_with_preempt_stub() is called?
>
> The comment at the `preemption_cancelled` label explains that a second attempt to acquire the monitor succeeded after freezing. The vthread has to continue execution. For that its frames (removed just above) need to be thawed again.

If preemption was cancelled, we skip over the cleanup.  The native frames haven't been unwound yet.  So when we call thaw, does it cleanup the native frames first, or does it copy the frames back on top of the existing frames (overwrite)?  It seems like we could avoid redundant copying if we could somehow throw out the freeze data and use the native frames still on the stack, which would probably involve not patching in this stub until we know that the preemption wasn't canceled.  Some some finalize actions would be delated, like a two-stage commit.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819657858