Question about multi threads visiting shared arena

[刘希晨]

Great, thanks!

Maurizio Cimadamore <maurizio.cimadamore at oracle.com> 于2023年3月27日周一 18:33写道：

>
> 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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