RFR: Use timed wait to sleep control thread [v2]

[earthling-amzn]

On Thu, 8 Apr 2021 06:26:22 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> earthling-amzn has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Increase mutex rank of gc waiters
>>    
>>    This lets us take the control thread's lock to notify it without tripping the deadlock detector.
>>  - Remove unnecessary enum namespace
>
> src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp line 54:
> 
>> 52:   ConcurrentGCThread(),
>> 53:   _alloc_failure_waiters_lock(Mutex::leaf, "ShenandoahAllocFailureGC_lock", true, Monitor::_safepoint_check_always),
>> 54:   _gc_waiters_lock(Mutex::safepoint, "ShenandoahRequestedGC_lock", true, Monitor::_safepoint_check_always),
> 
> Look at `src/hotspot/share/runtime/mutexLocker.cpp`: if we need to elevate the lock above/below the `leaf`, the usual thing to do is to say `leaf + 1` or `leaf - 1`. I would prefer to make `_control_lock` into `leaf - 1`, as that one is special, not all other locks.

Yes, good call.

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PR: https://git.openjdk.java.net/shenandoah/pull/28