Question about multi threads visiting shared arena

[Maurizio Cimadamore]

On 27/03/2023 10:56, 刘希晨 wrote:
> Thank you very much for the clarification, I really appreciate it. 
> Perhaps you could add some instructions on the recommended way of 
> handling multi threads access over MemorySegment in the documentation, 
> so people could understand it better! 
> (https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/doc/panama_memaccess.md#segments 
> <https://urldefense.com/v3/__https://github.com/openjdk/panama-foreign/blob/foreign-memaccess*2Babi/doc/panama_memaccess.md*segments__;JSM!!ACWV5N9M2RV99hQ!NiffGyRQiYssVQb6SqWG6401Q2n9TvHH-2b3-rLq-x8PSO0w8yXU4jVfFygvKkh4qoTPuy8yGosiS7pYNsWiYFOMF0L2$>) 
> The API in VarHandle are not only provided for MemorySegment and 
> rather low-level, a newbie like me would really got confused of which 
> way to use

I agree that some more clarification on this aspect at the javadoc level 
would be the right way to approach this.

Thanks
Maurizio

>
> Maurizio Cimadamore <maurizio.cimadamore at oracle.com> 于2023年3月27日周一 
> 17:07写道：
>
>     Hi,
>     accessing a shared memory segment is pretty much the same as
>     accessing a byte buffer, or a Java array instance. There are no
>     additional memory fences that are added by our implementation when
>     using "plain" access (as in all the other APIs I have listed), so,
>     in the general case you would probably need to use
>     VarHandle::fullFence() or something similar (to make sure all
>     writes are flushed when one thread is done with the segment).
>
>     It is surprising to see that it worked even w/o adding
>     synchronization/fencing, but that might just be "luck", or the
>     particular system you are using, as is often the case with these
>     matters. From a low-level/implementation perspective there is
>     absolutely no happens-before relationship inserted between writes
>     on a memory segment and reads on the same segment (when using
>     VarHandle::get - that is - a "plain" read/write). That is, plain
>     access really means plain access.
>
>     Hope this helps.
>
>     Maurizio
>
>     On 27/03/2023 08:48, 刘希晨 wrote:
>>     I have a question about using MemorySegment with Arena.openShared().
>>     I don't know if the memory created by Arena.openShared() or any
>>     other shared scope would be safely accessed by multiple threads.
>>     Simply put, if a writer thread wrote something into the
>>     MemorySegment and then passing it to the reader thread using a
>>     BlockingQueue,
>>     I am confused if the writer operation would already be visible to
>>     the reader thread.
>>     I have tested its visibility using jcstress and it turns out
>>     every Varhandle.get() will successfully read the data changed in
>>     another thread,
>>     even without volatile or locks, so I am confused if the jvm has
>>     already created memory fence for memorysegment's access.
>>     I hope the authorities could offer a suggested way of accessing
>>     same memory segment in multiple threads in Panama FFI's
>>     documentation.
>
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