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Date: Thu, 31 Mar 2011 22:24:33 -0700
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: AtomicReferenceArray writes and visibility
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On 3/31/2011 9:58 PM, markspace wrote:
> On 3/31/2011 9:54 PM, Peter Duniho wrote:
>> On 3/31/11 9:22 PM, markspace wrote:
>>> [...]
>>> Read the source code for the AtomicReferenceArray. set() doesn't use any
>>> synchronization. It calls sun.misc.Unsafe#putObjectVolatile(). I don't
>>> see how that's going to create a happens-before.
>>
>> "volatile" creates the happens-before. All writes before a volatile
>> write in the same thread must be visible in a given thread after a
>> volatile read in the same thread from the same location.
>
>
> I didn't notice the keyword volatile there. Did I miss it?

The relevant use of "volatile" is in the java.util.concurrent.atomic
package documentation,
http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/package-summary.html

"The memory effects for accesses and updates of atomics generally follow
the rules for volatiles, as stated in The Java Language Specification,
Third Edition (17.4 Memory Model):"

It goes on to map atomic operations to volatile operations. For example
"get has the memory effects of reading a volatile variable."

> Regardless, the lack of any guarantees in the docs of
> AtomicReferenceArray would kill it for me.  You can't be sure how the
> implementation will change over time.

Does that include the guarantees that it provides indirectly, by
reference to the package documentation?

Patricia