RFR: 8257831: Suspend with handshakes [v2]

[Robbin Ehn]

On Tue, 30 Mar 2021 13:54:58 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
>> 
>>  - Merge branch 'master' into SuspendInHandshake
>>  - 8257831: Suspend with handshake (review baseline)
>
> src/hotspot/share/prims/jvmtiRawMonitor.cpp line 364:
> 
>> 362:     for (;;) {
>> 363:       simple_enter(jt);
>> 364:       if (!SafepointMechanism::should_process(jt)) {
> 
> It seems to be likely that the condition is false in the first loop iteration and the thread has to do another iteration even if not suspended. Wouldn't it be ok to break from the loop if `!jt->is_suspended()`?
> I'm asking because in ObjectMonitor::enter() L414 there is similar code and the condition there is `SafepointMechanism::should_process(current) && current->suspend_request_pending()`

I didn't add that optimization here because I don't think it is needed, but I can add it.

> src/hotspot/share/runtime/handshake.cpp line 486:
> 
>> 484:         // Asynchronous may block so they may not execute ~PreserveExceptionMark before safepointing
>> 485:         // in outer loop.
>> 486:         op->do_handshake(_handshakee);
> 
> Maybe add PauseNoSafepointVerifier to document that the current thread can transition to a safe state?

Fixed

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PR: https://git.openjdk.java.net/jdk/pull/3191