Re: PostgreSQL XAResource & GlassFish 3.1.2.2

Agreed, it's better to be thread-safe in general than just fix the
barrier issue.

Florent

On Tue, Feb 12, 2013 at 7:48 PM, Heikki Linnakangas
<hlinnakangas(at)vmware(dot)com> wrote:
> On 12.02.2013 20:41, Florent Guillaume wrote:
>>
>> On Tue, Feb 12, 2013 at 7:36 PM, Heikki Linnakangas
>> <hlinnakangas(at)vmware(dot)com> wrote:
>>>
>>> I see. I think there's yet another potential explanation: even if
>>> glassfish
>>> is careful to invoke the end() only after start() has fully finished in
>>> the
>>> other thread, the java memory model does not guarantee that the effects
>>> of
>>> the end() in one thread are visible to the other thread doing the
>>> start().
>>> The update of the PGXAConnection's state-variable might be sitting in the
>>> CPU cache of the CPU running the first thread, not yet visible to the
>>> second
>>> thread.
>>
>>
>> That's the whole point of volatile since Java 5, it enforces a barrier
>> and "happens-before" semantics.
>
>>
>>>
>>> However, I'm not sure if glassfish could guarantee that the start()
>>> has really finished before end() is called, without using some operation
>>> that would also force the CPU cache to be flushed, making the effects
>>> visible.
>>>
>>> In any case, it seems like we should add "synchronized" to all the public
>>> methods in PGXAConnection. The performance penalty should be minimal, and
>>> it
>>> would fix any race conditions of that sort.
>>
>>
>> For Java 5+ this is really overkill.
>
>
> Well, volatile will only fix the issue if it's indeed "just" because of weak
> memory ordering issues. However, if the cause is a genuine race condition
> where glassfish calls end() in one thread before the start() in another
> thread has ran to completion, as Bryan suspected, then volatile is not
> enough. Besides, it would be good for PGXAConnection to be thread-safe in
> general, like the rest of the JDBC driver.
>
> - Heikki

--
Florent Guillaume, Director of R&D, Nuxeo
Open Source, Java EE based, Enterprise Content Management (ECM)
http://www.nuxeo.com http://www.nuxeo.org +33 1 40 33 79 87