RFR: 8275950: Use only _thread_in_vm in ~ThreadBlockInVMPreprocess()

[David Holmes]

On Tue, 26 Oct 2021 18:07:39 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

> Please review this small change. Since _thread_in_vm is already an unsafe state there is no need to use the intermediate _thread_blocked_trans state when transitioning back in ~ThreadBlockInVMPreprocess(). Tested tiers1-3 in mach5.
> 
> Thanks,
> Patricio

Hi Patricio,

This change seems fine.

I note that there are only two callers to `is_a_block_safe_state`:
- ` SafepointSynchronize::block`
- `SafepointMechanism::process`

but ` SafepointSynchronize::block` is itself now only called from `SafepointMechanism::process`, so we don't the assert of `is_a_block_safe_state` in  ` SafepointSynchronize::block`.

Thanks,
David

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Marked as reviewed by dholmes (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6120