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https://issues.apache.org/jira/browse/HADOOP-1970?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12531411
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Arun C Murthy commented on HADOOP-1970:
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Ok, this is indeed a deadlock... the issue is that there is a differing order
of locks of the parent and child Progress objects in Progress.java.
As shown in the stack trace {{Progress.complete}} locks the child first and
then the parent, where as {{Progress.toString(StringBuffer)}} locks the parent
first and then the child... straight-forward fix is to ensure that parent is
always locked first e.g. in {{Progress.complete}}.
> tasktracker hang in reduce. Deadlock between main and comm thread
> -----------------------------------------------------------------
>
> Key: HADOOP-1970
> URL: https://issues.apache.org/jira/browse/HADOOP-1970
> Project: Hadoop
> Issue Type: Bug
> Components: mapred
> Affects Versions: 0.14.1
> Reporter: Koji Noguchi
> Assignee: Vivek Ratan
> Priority: Blocker
> Fix For: 0.14.2
>
>
> Saw one reduce task stuck on copy.
> jstack on the reduce task(task_200709272248_0001_r_000150_0) process showed
> {noformat}
> Found one Java-level deadlock:
> =============================
> "Comm thread for task_200709272248_0001_r_000150_0":
> waiting to lock monitor 0x08144020 (object 0xd4e30aa8, a
> org.apache.hadoop.util.Progress),
> which is held by "main"
> "main":
> waiting to lock monitor 0x08144084 (object 0xd4e30958, a
> org.apache.hadoop.util.Progress),
> which is held by "Comm thread for task_200709272248_0001_r_000150_0"
> Java stack information for the threads listed above:
> ===================================================
> "Comm thread for task_200709272248_0001_r_000150_0":
> at org.apache.hadoop.util.Progress.toString(Progress.java:113)
> - waiting to lock <0xd4e30aa8> (a org.apache.hadoop.util.Progress)
> at org.apache.hadoop.util.Progress.toString(Progress.java:116)
> - locked <0xd4e30958> (a org.apache.hadoop.util.Progress)
> at org.apache.hadoop.util.Progress.toString(Progress.java:108)
> at org.apache.hadoop.mapred.Task$1.run(Task.java:268)
> at java.lang.Thread.run(Thread.java:619)
> "main":
> at org.apache.hadoop.util.Progress.startNextPhase(Progress.java:58)
> - waiting to lock <0xd4e30958> (a org.apache.hadoop.util.Progress)
> at org.apache.hadoop.util.Progress.complete(Progress.java:70)
> - locked <0xd4e30aa8> (a org.apache.hadoop.util.Progress)
> at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:253)
> at
> org.apache.hadoop.mapred.TaskTracker$Child.main(TaskTracker.java:1777)
> {noformat}
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