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From "Vinod Kumar Vavilapalli (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HADOOP-11274) ConcurrentModificationException in Configuration Copy Constructor
Date Sat, 08 Nov 2014 00:36:34 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-11274?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Vinod Kumar Vavilapalli updated HADOOP-11274:
---------------------------------------------
    Attachment: HADOOP-11274-v3.patch

Working on this to take a light-handed approach instead of global locks: Making updatedResources
a ConcurrentHashMap. The reasons why this is sufficient are
 - The updatedResources is only used in displays to show the lineage of the configuration
properties
 - Further, the lineage information is useful to distinguish which files the properties are
coming from: yarn-site.xml vs core-site.xml vs programmatic set. Multiple programmatic set
calls still imply a programmatic lineage.
 - Even in the non-concurrent updates case, once the clone is done, all future updates in
the old object are not tracked and rightly so. Consequently, there is no reason to have a
prefect snapshot during the clone.

Overall, we don't need the updatedResources snapshot during copy to be strictly consistent
- making it ConcurrentHashMap should be good enough.

> 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-v3.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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