Re: The code example in onSpinWait() Javadoc

What's the difference between this example and what's in javadocs?

Alex

On Thu, 14 Nov 2019, 16:29 Francesco Nigro via Concurrency-interest, <
[email protected]> wrote:

> Totally right, sorry again (writing by phone is a terrible idea :( ):
> yes , poll should  use  "o = this.obj".
>
> > Can we get rid of "done"?  Since offerOnce() does not allow null, then
> having the done flag does not seem to help anything.
> Absolutely yes: I've prepared this as a "simplified" (and wrong due to the
> hurry, sorry for that) version of what we do on many queues in JCtools,
> that's why it contains some unnecessary bits as you've rightly pointed
> out.
> Code that is spin awaiting on a condition, sure that it will happen at
> some point in the future, is something that could help to understand the
> rare cases
> where using onSpinWait could be beneficial IMO.
>
>
>
> Il giorno gio 14 nov 2019 alle ore 17:18 Nathan and Ila Reynolds <
> [email protected]> ha scritto:
>
>> In poll(), should "this.obj = o" be "o = this.obj"?  Without this change,
>> it seems poll() could spin forever.
>>
>> Should "poll" be renamed to "take" since the method could block until a
>> value is available?
>>
>> Can we get rid of "done"?  Since offerOnce() does not allow null, then
>> having the done flag does not seem to help anything.
>>
>> Is the compiler and execution engine in my head misinterpreting and
>> mis-executing the code?
>>
>> Here is the code with my suggested changes.
>>    volatile E obj = null;
>>
>>    public void offerOnce(E o) {
>>       Objects.checkNonNull(o);
>>       this.obj = o;
>>    }
>>
>>   public boolean isDone() {
>>        return obj != null;
>>   }
>>
>>    public E take() {
>>       E o;
>>
>>       while (true) {
>>          o = obj;
>>
>>          if (o != null)
>>             return o;
>>
>>          java.lang.Thread.onSpinWait();
>>       }
>>    }
>>
>> -Nathan
>>
>> On 11/14/2019 8:54 AM, Francesco Nigro wrote:
>>
>> Sorry I've written on the email text, I forgot an important part, let me
>> write it properly:
>>
>>    volatile E obj = null;
>>    volatile boolean done = false;
>>
>>    public void offerOnce(E o) {
>>       Objects.checkNonNull(o);
>>       this.done = true;
>>       this.obj = o;
>>    }
>>
>>   public boolean isDone() {
>>        return done;
>>   }
>>
>>    public E poll() {
>>       E o = this.obj;
>>       if (o == null && !this.done) {
>>          return null;
>>       }
>>       //o will be !null at some point
>>       do {
>>          if (o != null)
>>             return o;
>>          java.lang.Thread.onSpinWait();
>>          this.obj = o;
>>       } while(true);
>>    }
>>
>> Similarly to the queue API: poll should return null iff !done, but offer
>> update first done and then this.obj:
>> poll need to read obj, but has to stay consistent to the ordering, so at
>> some point, obj will be visible if done == true.
>> On JCtools we have some queues with a similar behaviour (producer
>> sequence is advanced before writing the actual element in the queue)
>> and we need to spin wait the element apprearence to stay consistent with
>> the isEmpty behaviour: that's a good use case for onSpinWait (when it works
>> :P).
>>
>>
>>
>> Il giorno gio 14 nov 2019 alle ore 16:43 Andrew Haley <[email protected]>
>> ha scritto:
>>
>>> On 11/14/19 3:31 PM, Francesco Nigro via Concurrency-interest wrote:
>>> >     E o = this.obj;
>>> >     if (o == null && !done) {
>>> >         return null;
>>> >      }
>>> >      //o will be !null at some point
>>> >      do {
>>> >          if (o != null)
>>> >             return o;
>>> >          java.lang.Thread.onSpinWait();
>>> >      } while(true);
>>> > }
>>> >
>>> > In case like this is more appropriate, maybe, but much less intuitive
>>> > probably.
>>>
>>> Umm, what? o is a local. This loop spins forever.
>>>
>>> --
>>> Andrew Haley  (he/him)
>>> Java Platform Lead Engineer
>>> Red Hat UK Ltd. <https://www.redhat.com>
>>> https://keybase.io/andrewhaley
>>> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>>>
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