RFR: 8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes [v6]

[Coleen Phillimore]

On Mon, 1 Nov 2021 15:59:58 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> Update: I've added comments to WB_HandshakeReadMonitors() and
>> WB_HandshakeWalkStack() to clarify their expectations.
>
> Update again: I took a closer look at `WB_AsyncHandshakeWalkStack()`,
> `WB_HandshakeReadMonitors()` and `WB_HandshakeWalkStack()` and
> I realized that those functions were not properly converting a `jobject` into
> a protected JavaThread*. I've updated them to call the proper conversion
> function and I've changed this code to `guarantee()` that the target is not
> `nullptr`.

Great.

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PR: https://git.openjdk.java.net/jdk/pull/4677