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Re: Volatile keyword

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On 12/14/11 10:39 AM, Robert Klemme wrote:
> On 12/13/2011 08:54 AM, Peter Duniho wrote:
>> On 12/12/11 11:37 PM, Robert Klemme wrote:
>>> [...]Bottom line is that the JVM spec does not make
>>> any guarantees here (§17.7 see Lew's reference).
>>
>> I think it's worth being careful about "does not make any guarantees
>> here". The implication of 17.7 is that there _is_ in fact a guarantee,
>> for certain 64-bit fields marked as "volatile". Specifically, those
>> which are of the type "double" or "long". This is true even on 32-bit
>> systems.
>
> And what guarantee is there? Can you unambiguously formulate that and
> provide reference to the JLS so everybody can check it?
>
>>>> In 32 bit, there is an assembler instruction cmpxchg8b designed to let
>>>> you implement a light weight atomic 64-bit read.
>>>
>>> Since you do not know on what JVM your Java program will run when you
>>> write it (or at least someone can choose to use a different JVM model)
>>> it is safer to code under the assumption that a long and double write is
>>> two operations i.e. not atomic.
>>
>> There is no need to make that assumption provided the field involved is
>> marked "volatile".
>
> I beg to differ - but I am eagerly waiting to be convinced otherwise.'
The JLS does specifically say that volatile will ensure atomic 
operations on long and double.

The last paragraph of section 17.7:

"The load, store, read, and write actions on volatile variables are 
atomic, even if the type of the variable is double or long."

<http://java.sun.com/docs/books/jls/second_edition/html/memory.doc.html#28330>

So unless you use a non-conforming "JVM" implementation (which by 
definition isn't really a JVM), then you will *always* have atomic reads 
and atomic writes for volatile doubles and volatile longs.

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Volatile keyword "raphfrk@gmail.com" <raphfrk@gmail.com> - 2011-12-12 04:51 -0800
  Re: Volatile keyword Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-12 14:31 +0000
    Re: Volatile keyword "raphfrk@gmail.com" <raphfrk@gmail.com> - 2011-12-12 06:52 -0800
  Re: Volatile keyword Lew <lewbloch@gmail.com> - 2011-12-12 06:34 -0800
  Re: Volatile keyword markspace <-@.> - 2011-12-12 10:00 -0800
    Re: Volatile keyword Tom Anderson <twic@urchin.earth.li> - 2011-12-12 18:41 +0000
      Re: Volatile keyword Patricia Shanahan <pats@acm.org> - 2011-12-12 11:43 -0800
    Re: Volatile keyword Roedy Green <see_website@mindprod.com.invalid> - 2011-12-12 11:15 -0800
      Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-13 08:37 +0100
        Re: Volatile keyword Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-12 23:54 -0800
          Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-14 19:39 +0100
            Re: Volatile keyword Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:59 -0800
            Re: Volatile keyword Patricia Shanahan <pats@acm.org> - 2011-12-14 11:00 -0800
              Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-14 22:35 +0100
  Re: Volatile keyword Roedy Green <see_website@mindprod.com.invalid> - 2011-12-12 10:55 -0800

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