RFR: 8307236: Rendezvous GC threads under STS for monitor deflation

[Aleksey Shipilev]

On Thu, 4 May 2023 08:28:03 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> src/hotspot/share/runtime/synchronizer.cpp line 1546:
>> 
>>> 1544:       // being afraid that the object-monitor is going away.
>>> 1545:       VM_RendezvousGCThreads sync_gc;
>>> 1546:       VMThread::execute(&sync_gc);
>> 
>> Not sure why this block is under `current->is_Java_thread()` block. Shouldn't we rendezvous with GC threads regardless?
>
>> Not sure why this block is under `current->is_Java_thread()` block. Shouldn't we rendezvous with GC threads regardless?
> 
> Deflation is done either concurrently by the service thread, or in a safepoint by the VM thread. The current->is_Java_thread() means it's the concurrent case from the service thread. That's when it's needed. Inside of safepoints, the GC threads have already been synchronized.

Okay, I see. Can we then move this block into separate `current->is_service_thread()`-checked block, and `assert(current->is_VM_thread())` on `else` branch? This would make this distinction very clear.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13773#discussion_r1184722928