RFR: 8253064: monitor list simplifications and getting rid of TSM [v4]

[Coleen Phillimore]

On Sat, 7 Nov 2020 17:17:01 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> src/hotspot/share/runtime/synchronizer.cpp line 136:
>> 
>>> 134: 
>>> 135:       // Honor block request.
>>> 136:       ThreadBlockInVM tbivm(self->as_Java_thread());
>> 
>> ThreadBlockInVM is generally used to wrap blocking code, not to cause the current thread to block (which it will do as a side-effect if a safepoint/handshake is requested). Surely this should just be call to `process_if_requested` (or the new `process_if_requested_with_exit_check`)?
>
> This kind of use of ThreadBlockInVM predates this changeset so while
> the location is new, then code style is old, very old... I'll hold off changing
> this for now.

I'd rather see ThreadBlockInVM as the convention of allowing the thread to block if a safepoint is requested.  The calls like process_if_requested are becoming alphabet soup and keep changing, so having TBIVM is better in my opinion.
That said, this is a strange usage.  This code appears three times.  It should be a function like allow_safepoint_block(LogStream* ls, timer), with some comment above.  Then it's clear that it's checking for a safepoint in a loop that could take a long time and the logging is ancillary.

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PR: https://git.openjdk.java.net/jdk/pull/642