Re: Are there real use cases with the Java access modes?

Thank you! 

> I know of a few like things achievable in other languages 

Is there an example of this? 

> Another important clarification needed is what barriers are enforced on failure of CAS 

Currently in our Herd implementation with Java, on a failed CAS, it treated equivalently as a read with its access mode specified. But I agree that it needs further confirmations on whether it is correct. So if it is `compareAndExchangeRelease`, then no barrier is enforced because the read in this case is a plain read. If it is `compareAndSet​`on a failure branch, then it is the same as a volatile read with a full barrier inserted either before or after depends on the compilation scheme. C11 does something slightly different (according to their Herd implementation). In addition to the read (with its specified memory order), it also performs a non-atomic write to `*expected` following that read if the compare fails. Since it is non-atomic, there is no extra barriers inserted for the write. 

I'll look for that email thread that you mentioned. It seems there might be something more to it. 

Best, 
Shuyang 


From: "Alex Otenko" <[email protected]> 
To: "Shuyang Liu" <[email protected]> 
Cc: "concurrency-interest" <[email protected]> 
Sent: Saturday, July 17, 2021 3:09:49 PM 
Subject: Re: [concurrency-interest] Are there real use cases with the Java access modes? 

I think one important case is an atomic increment that does not introduce full barriers (choose opaque, if you are just counting; acquire, if you are acquiring a semaphore; release, when releasing; or rel-acquire). I know of a few like things achievable in other languages, which probably are beneficial on many non-tso architectures. 

Another important clarification needed is what barriers are enforced on failure of CAS. I think there was a discussion about this on this list, but I don't recall if that resulted in a definitive answer about what it should be. 

Alex 

On Sat, 17 Jul 2021, 21:33 Shuyang Liu via Concurrency-interest, < [email protected] > wrote: 



Hello, 

My colleagues and I have been working on a formal model for the Java access modes that is added since Java 9 [1]. Are there any popular use cases of access modes in real world applications/frameworks/libraries? We would like to see how our current formal model works in real examples. 

Thanks you, 
Shuyang Liu 

[1]. Using JDK 9 Memory Order Modes 
http://gee.cs.oswego.edu/dl/html/j9mm.html 
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