RFR: 7902982: jcstress: Add samples for some mutex algorithms [v3]

[Vladimir Sitnikov]

On Tue, 29 Jun 2021 12:06:32 GMT, Michael Mirwaldt <github.com+6693355+mmirwaldt at openjdk.org> wrote:

>> jcstress-samples/src/main/java/org/openjdk/jcstress/samples/concurreny/mutex/Mutex_01_NoAlgorithm.java line 61:
>> 
>>> 59:             } else {
>>> 60:                 r.r1 = 2;
>>> 61:             }
>> 
>> This kind of code introduces its own synchronization, so it might hide some outcomes. I believe you want to demonstrate the "already taken" detection code works? If so, I suggest doing it this way:
>> 
>> 
>> boolean taken1, taken2;
>> 
>> @Actor
>> public void actor1(ZZ_Result r) {
>>     taken1 = true;
>>     r.r1 = taken2;
>>     taken1 = false;
>> }
>> 
>> @Actor
>> public void actor2(ZZ_Result r) {
>>     taken2 = true;
>>     r.r2 = taken1;
>>     taken2 = false;
>> }
>
> Yes, I tried to find a way to find out whether both actors have entered the critical section at the same time.
> Your solution is brilliant! At least one of both actors will always witness in the result that both actors have entered the critical section if they did. However, I think the booleans taken1 and taken2 must be volatile because 
> a) they run in different threads with their local caches and might not notice changes of the booleans from the other thread
> b) volatile forces the order of those 3 statements. Otherwise, the JVM is allowed to change the order in which they are executed.
> Am I right or wrong with volatile here? 
> If I am wrong, I will remove the volatile again, of course.

`taken1` and `taken2` should not be `volatile` they should be plain fields.
If you make them volatile, you forbid compiler/CPU from making optimizations which might unveil the flaw in the tested algorithms.

For instance, if you add `SynchonizedAlgorithm` that works like `synchronized(String.class) {...}`, then the test must pass even with non-volatile `taken1` and `taken2`. Dekker and other algorithms should work exactly as `synchronized(...)`, so volatile is not needed.

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PR: https://git.openjdk.java.net/jcstress/pull/85