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From "Daniel Luo (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4498) Incorrect double checked locking idiom used in VTIResultSet
Date Thu, 31 Dec 2009 07:59:29 GMT
Incorrect double checked locking idiom used in VTIResultSet
-----------------------------------------------------------

                 Key: DERBY-4498
                 URL: https://issues.apache.org/jira/browse/DERBY-4498
             Project: Derby
          Issue Type: Bug
          Components: SQL
    Affects Versions: 10.5.3.0
         Environment: [daniel@daniel-desk src]$ uname -a
Linux daniel-desk.dyndns.org 2.6.18-92.el5 #1 SMP Tue Apr 29 13:16:12 EDT 2008 i686 i686 i386
GNU/Linux
[daniel@daniel-desk src]$ java -version
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


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. Declare the field compileTimeConstants with modifier volatile 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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