RFR: 8287281: adjust guarantee in Handshake::execute for the case of target thread being current [v6]

[David Holmes]

On Tue, 7 Jun 2022 12:42:05 GMT, Johan Sjölén <duke at openjdk.java.net> wrote:

>> Please review this PR for fixing JDK-8287281.
>> 
>> If a thread is handshake safe we immediately execute the closure, instead of going through the regular Handshake process. 
>> 
>> Finally:  Should `VirtualThreadGetThreadClosure` and its `do_thread()` body be inlined instead? We can do this in this PR, imho, but I'm hoping to get some input on this.
>> 
>> 
>> Passes tier1. Running tier2-5.
>
> Johan Sjölén has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move assert up and remove other assert, remove unused var

I have to agree with Dan. This is supposed to only be about targeting the current thread, but we are now no longer ensuring the target is protected by a TLH when the current thread is the active_handshaker. So I would vote for:

1. Change the is_handshake_safe_for to current == target and be done with it.

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PR: https://git.openjdk.java.net/jdk/pull/8992