RFR: 8291714: Implement a Multi-Reader Single-Writer mutex for Hotspot

[David Holmes]

On Thu, 11 Aug 2022 18:54:50 GMT, Johan Sjölén <duke at openjdk.org> wrote:

>> May I please have a review for this PR which implements a `MRWMutex` class for Hotspot?
>> 
>> This PR does 3 things:
>> 
>> * Adds a port of ZGC's MRSW mutex (see [0]) to Hotspot
>> * Adds some new utilities for writing multi-threaded tests.
>> * Adds some tests for MRSW Mutex using these new utilities
>> 
>> The ticket has some comments which might be worth checking out: https://bugs.openjdk.org/browse/JDK-8291714
>> 
>> [0] Original source code here: https://github.com/openjdk/zgc/blob/zgc_generational/src/hotspot/share/gc/z/zJNICritical.cpp
>
> src/hotspot/share/runtime/mrswMutex.hpp line 105:
> 
>> 103: 
>> 104:         // Do requested transition before blocking
>> 105:         WriterTransition tbivm(JavaThread::current());
> 
> This shouldn't compute the thread, instead `read_lock` and `write_lock` should be given the threads as arguments. This will fail if the current thread isn't a Java one.

It makes sense to pass in the current thread given that the caller already needs to know to select the right transition.

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PR: https://git.openjdk.org/jdk/pull/9838