<div dir="ltr">Great, thanks! <br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com">maurizio.cimadamore@oracle.com</a>> 于2023年3月27日周一 18:33写道：<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">

  
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    <p><br>
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    <div>On 27/03/2023 10:56, 刘希晨 wrote:<br>
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      <div dir="ltr">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! (<a href="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$" target="_blank">https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/doc/panama_memaccess.md#segments</a>)
        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<br>
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    <p>I agree that some more clarification on this aspect at the
      javadoc level would be the right way to approach this.</p>
    <p>Thanks<br>
      Maurizio<br>
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      <div class="gmail_quote">
        <div dir="ltr" class="gmail_attr">Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com" target="_blank">maurizio.cimadamore@oracle.com</a>>
          于2023年3月27日周一 17:07写道：<br>
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            <p>Hi,<br>
              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).</p>
            <p>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.<br>
            </p>
            <p>Hope this helps.</p>
            <p>Maurizio<br>
            </p>
            <div>On 27/03/2023 08:48, 刘希晨 wrote:<br>
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              <div dir="ltr">
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">I
                  have a question about using MemorySegment with
                  Arena.openShared().</div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">I
                  don't know if the memory created by Arena.openShared()
                  or any other shared scope would be safely accessed by
                  multiple threads.</div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">Simply
                  put, if a writer thread wrote something into the
                  MemorySegment and then passing it to the reader thread
                  using a BlockingQueue,</div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">I
                  am confused if the writer operation would already be
                  visible to the reader thread.</div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">I
                  have tested its visibility using jcstress and it turns
                  out every Varhandle.get() will successfully read the
                  data changed in another thread, </div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">even
                  without volatile or locks, so I am confused if the jvm
                  has already created memory fence for memorysegment's
                  access.</div>
                <div style="color:rgb(0,0,0);font-family:Arial;font-size:14px;margin:0px">I
                  hope the authorities could offer a suggested way of
                  accessing same memory segment in multiple threads in
                  Panama FFI's documentation.</div>
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