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NNTP-Posting-Date: Wed, 14 Dec 2011 13:00:55 -0600
Date: Wed, 14 Dec 2011 11:00:50 -0800
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: Volatile keyword
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On 12/14/2011 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?

As indicated in the quoted material, the relevant section is
"17.7 Non-atomic Treatment of double and long"
http://java.sun.com/docs/books/jls/third_edition/html/memory.html#61803

The key paragraph, for this purpose, says:

"For the purposes of the Java programming language memory model, a
single write to a non-volatile long or double value is treated as two
separate writes: one to each 32-bit half. This can result in a situation
where a thread sees the first 32 bits of a 64 bit value from one write,
and the second 32 bits from another write. Writes and reads of volatile
long and double values are always atomic. Writes to and reads of
references are always atomic, regardless of whether they are implemented
as 32 or 64 bit values."

Patricia