RFR: 8273300: Check Mutex ranking during a safepoint [v4]

[Coleen Phillimore]

On Wed, 15 Sep 2021 21:53:17 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> If the gap is 2, then leaf-2 will == oopstorage?  I could make it 3 but it doesn't actually matter.
>
> Obviously the gap needs to be big enough to avoid the overlap, but the presence of +1, +3, +6 and +10 just raise questions as to why different values are used. Why not make them all +10 if you need 10 slots in some areas?

I'm going to address this with a future change because we don't have range checking for overlap yet, and they should all be some consistent and not arbitrary range.  If it was less arbitrary, I'd pick another 6 because
runtime/mutexLocker.cpp:  def(Metaspace_lock               , PaddedMutex  , leaf-3,      true,  _safepoint_check_never);
The rank 'service' had 6 because there are at least 4 locks below that.  Again, I should add range checking and maybe making them all 10 makes a lot of sense.  Locks don't nest that deeply.  I'll add this as a comment in: JDK-8176393.

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PR: https://git.openjdk.java.net/jdk/pull/5467