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

[Michael Mirwaldt]

On Tue, 29 Jun 2021 13:41:24 GMT, Vladimir Sitnikov <vsitnikov at openjdk.org> wrote:

>> Can you explain me why taken1 and taken2 must not be volatile?
>> I mean actor1 and actor2 run in different threads, don't they?
>> If so, actor1 won't see the current value of taken2 when actor2 changes taken2 or am I wrong?
>
> Writes to plain, non-volatile fields can be observed by other threads as well �� 
> 
> In case you make `taken1` and `taken2` volatiles, you might accidentally re-create IRIW case out of these `taken1`, `taken2`, and a couple of other fields (see https://github.com/openjdk/jcstress/blob/df83b446f187ae0b0fa31fa54decb59db9e955da/tests-custom/src/main/java/org/openjdk/jcstress/tests/volatiles/VolatileIRIWTest.java )
> 
> In other words, `17.4.4. Synchronization Order` reads `A synchronization order is a total order over all of the synchronization actions of an execution`, while `A write to a volatile variable v (§8.3.1.4) synchronizes-with all subsequent reads of v by any thread`. That means, if you make `takenX` volatile, you might create new `synchronizes-with` edges thus forbidding program behaviors.
> 
> Of course, it might be the case that writes to `taken1` and `taken2` is **never** visible to the other threads (I doubt compilers would be able to do such proof soon), so to play safe, both cases needs to be verified: non-volatile `taken1,2` and making one of them volatile. However, if you keep only one case (e.g. only one implementation of Peterson's algorithm), I would vote for non-volatile `taken1/2`

Ah thanks for that answer. I have not got such deep insights into the "secondary effects" of volatile.
I have removed it from all 4 samples.
Just another question: Is it theoretically possible that we get false positives without volatile?
I mean taken1 and taken2 are updated by their actors but the other actor might not see it.
Then both actors might enter the critical section at the same time without any actor noticing it in the results.
Of course, I know there is no free lunch.

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