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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11274) ConcurrentModificationException in Configuration Copy Constructor
Date Fri, 07 Nov 2014 02:28:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11274?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14201458#comment-14201458
] 

Jing Zhao commented on HADOOP-11274:
------------------------------------

Oops, I did not notice that setQuietly also takes the lock. The deadlock looks very hard to
hit though, since the two configuration objects need to use each other for construction. However,
considering this is a very critical part of the code, maybe we should revert the patch from
trunk, branch-2 and branch-2.6 before we have a clear solution.

> ConcurrentModificationException in Configuration Copy Constructor
> -----------------------------------------------------------------
>
>                 Key: HADOOP-11274
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11274
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: conf
>            Reporter: Junping Du
>            Assignee: Junping Du
>            Priority: Blocker
>             Fix For: 2.6.0
>
>         Attachments: HADOOP-11274-v2.patch, HADOOP-11274.003.patch, HADOOP-11274.patch
>
>
> Exception as below happens in doing some configuration update in parallel:
> {noformat}
> java.util.ConcurrentModificationException
> 	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:922)
> 	at java.util.HashMap$EntryIterator.next(HashMap.java:962)
> 	at java.util.HashMap$EntryIterator.next(HashMap.java:960)
> 	at java.util.HashMap.putAllForCreate(HashMap.java:554)
> 	at java.util.HashMap.<init>(HashMap.java:298)
> 	at org.apache.hadoop.conf.Configuration.<init>(Configuration.java:703)
> {noformat}
> In a constructor of Configuration - public Configuration(Configuration other), the copy
of updatingResource data structure in copy constructor is not synchronized properly. 
> Configuration.get() eventually calls loadProperty() where updatingResource gets updated.
So, whats happening here is one thread is trying to do copy of Configuration as demonstrated
in stack trace and other thread is doing Configuration.get(key) and than ConcurrentModificationException
occurs because copying of updatingResource is not synchronized in constructor. 
> We should make the update to updatingResource get synchronized, and also fix other tiny
synchronized issues there.



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