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

Ok, I meant

if (...getAndIncrement...) // acquire ordering

for(....addAndGet...) // release ordering

Alex

On Sun, 18 Jul 2021, 07:08 Alex Otenko, <[email protected]> wrote:

> The example for fetch_add with memory order parameter is available in Rust
> and C.
>
> The different examples of code would be: counting watermark. Here the
> loads and releases done by fetch-add I presume are not "normal", but opaque.
>
> A case with release semantics is the common serialization routine: if
> fetch-add with release ordering returns zero, you enter the critical
> section; if non-zero, then you can assume your changes to shared data
> structure will be observed by the contender that observed 0 for this
> fetch-add. Then before exiting the critical section it suffices to do
> fetch-add with acquire ordering. If this fetch-add results in non-zero,
> some contender attempted to enter.
>
> if (contenders.getAndIncrement() >0) return; // release ordering
>
> for(int c=1; c>0; c=contenders.addAndGet(-c)) // acquire ordering
> // process changes to shared data structure, and, say, write to socket
>
>
> Of course, there are some assumptions about shared state modified outside
> this loop, and shared state accessible inside the loop.
>
> Alex
>
> On Sun, 18 Jul 2021, 00:33 Shuyang Liu, <[email protected]> wrote:
>
>> 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
>>> _______________________________________________
>>> Concurrency-interest mailing list
>>> [email protected]
>>> http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>>>
>>
>>

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