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https://issues.apache.org/jira/browse/HADOOP-8684?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13445078#comment-13445078
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Jing Zhao commented on HADOOP-8684:
-----------------------------------
In the WritableComparator#get() a double check is used to replace the original
synchronized static method. So I keep the synchronized in the double check to
make sure there is no put operation happening during the second check and the
construction of a generic comparator. But the synchronized can be omitted if we
do not need to keep this semantic.
> Deadlock between WritableComparator and WritableComparable
> ----------------------------------------------------------
>
> Key: HADOOP-8684
> URL: https://issues.apache.org/jira/browse/HADOOP-8684
> Project: Hadoop Common
> Issue Type: Bug
> Components: io
> Affects Versions: 1.0.3, 3.0.0
> Reporter: Hiroshi Ikeda
> Assignee: Jing Zhao
> Priority: Minor
> Attachments: Hadoop-8684.patch, Hadoop-8684.patch, Hadoop-8684.patch,
> Hadoop-8684.patch, WritableComparatorDeadLockTestApp.java
>
>
> Classes implementing WriableComparable in Hadoop call the method
> WritableComparator.define() in their static initializers. This means, the
> classes call the method define() while thier class loading, under locking
> their class objects. And, the method WritableComparator.define() locks the
> WritableComaprator class object.
> On the other hand, WritableComparator.get() also locks the WritableComparator
> class object, and the method may create instances of the targeted comparable
> class, involving loading the targeted comparable class if any. This means,
> the method might try to lock the targeted comparable class object under
> locking the WritableComparator class object.
> There are reversed orders of locking objects, and you might fall in deadlock.
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