[jira] Commented: (DERBY-4497) Incorrect double checked locking idiom used in VTIResultSet

["Knut Anders Hatlen (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DERBY-4497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12795575#action_12795575 ] 

Knut Anders Hatlen commented on DERBY-4497:
-------------------------------------------

Here's my understanding of this code:

A VTIResultSet instance is never accessed concurrently by two threads because of higher-level synchronization (on the Connection object). Fields of VTIResultSet will therefore be read by the same thread that updated them, and visibility of changes is guaranteed without more synchronization. So the synchronization in this method is there only to protect the savedObjects array (which may be accessed concurrently by multiple threads if the execution plan is shared between many statements), and I believe there is no double-checked locking in this method.

> Incorrect double checked locking idiom used in VTIResultSet
> -----------------------------------------------------------
>
>                 Key: DERBY-4497
>                 URL: https://issues.apache.org/jira/browse/DERBY-4497
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.5.3.0
>         Environment: OS: Redhat-5 Linux 2.6.18-92.el5
> JDK: java version "1.6.0"
> Java(TM) SE Runtime Environment (build pxi3260sr4-20090219_01(SR4))
> IBM J9 VM (build 2.4, J2RE 1.6.0 IBM J9 2.4 Linux x86-32 jvmxi3260-20090215_29883 (JIT enabled, AOT enabled)
> J9VM - 20090215_029883_lHdSMr
> JIT  - r9_20090213_2028
> GC   - 20090213_AA)
> JCL  - 20090218_01
>            Reporter: Daniel Luo
>   Original Estimate: 0.08h
>  Remaining Estimate: 0.08h
>
> In method setSharedState of class VTIResultSet, double checked locking idiom is used. But the field compileTimeConstants involved in the idiom is not declared with volatile modifier which is incorrect. Simply add volatile modifier in field compileTimeConstants declaration can quickly fix the problem. Below link and description explain the details.
> http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl
> "The double-checked locking idiom (also called the multithreaded singleton pattern) is a trick designed to support lazy initialization while avoiding the overhead of synchronization. Sometimes it doesn't work correctly since the writes initializing the object and the write to the field storing the object instance can be reordered by the compiler or the cache, which would have the effect of returning what appears to be a partially constructed object instance. The result would be that we read an uninitialized object. In JVMs 1.5 or above, the use of the volatile keyword in field declaration would eliminate the problems."

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