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

[David Holmes]

On Mon, 13 Jun 2022 07:45:41 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> 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
>
> The only way to become an active handshaker is to handshake another thread (target), when that happens we verify that target is ThreadsList safe.
> Thus active handshaker is guaranteed that the target is already verified on a ThreadsList.
> As long as we are the active handshake the target is blocked, i.e. target is safepoint safe.
> 
> The reason I think handshake safe is good is because we have 3 (4) cases:
> 1: Current != Target (Not 3 and not 4)
> 2: Current == Target
> 3: Current != Target, but already executing a handshake for target
> 4: Current != Target, but we are in a safepoint (still no internally handled)

@robehn can you explain to me how the current thread can both be the active handshaker of the target _and_ at the same time be executing another handshake with the target? This is making my head spin.

This change has deviated quite considerably from the issue that caused a bug to be filed. And Dan still has concerns that the current thread should still be protected by a TLH even if not strictly necessary. Maybe we actually need to backtrack and restore an invariant that there is always a TLH even for the current thread and fix the JVMTI code that did things differently?

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PR: https://git.openjdk.org/jdk/pull/8992