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Date: Thu, 21 Jun 2007 23:10:26 -0700 (PDT)
From: "dhruba borthakur (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (HADOOP-1513) A likely race condition between the
 creation of a directory and checking for its existence in the DiskChecker
 class
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    [ https://issues.apache.org/jira/browse/HADOOP-1513?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12507135 ] 

dhruba borthakur commented on HADOOP-1513:
------------------------------------------

Two mkdirs() cannot interleave. If you look at FSNamesystem.mkdirsInternal(), it is synchronized with the global FSNamesystem lock. I belive that the scenario Devaraj explained "cannot happen".


> A likely race condition between the creation of a directory and checking for its existence in the DiskChecker class
> -------------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-1513
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1513
>             Project: Hadoop
>          Issue Type: Bug
>          Components: fs
>    Affects Versions: 0.14.0
>            Reporter: Devaraj Das
>            Assignee: Devaraj Das
>            Priority: Critical
>             Fix For: 0.14.0
>
>         Attachments: 1513.patch
>
>
> Got this exception in a job run. It looks like the problem is a race condition between the creation of a directory and checking for its existence. Specifically, the line:
> if (!dir.exists() && !dir.mkdirs()), doesn't seem safe when invoked by multiple processes at the same time. 
> 2007-06-21 07:55:33,583 INFO org.apache.hadoop.mapred.MapTask: numReduceTasks: 1
> 2007-06-21 07:55:33,818 WARN org.apache.hadoop.fs.AllocatorPerContext: org.apache.hadoop.util.DiskChecker$DiskErrorException: can not create directory: /export/crawlspace/kryptonite/ddas/dfs/data/tmp
> 	at org.apache.hadoop.util.DiskChecker.checkDir(DiskChecker.java:26)
> 	at org.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.createPath(LocalDirAllocator.java:211)
> 	at org.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.getLocalPathForWrite(LocalDirAllocator.java:248)
> 	at org.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.createTmpFileForWrite(LocalDirAllocator.java:276)
> 	at org.apache.hadoop.fs.LocalDirAllocator.createTmpFileForWrite(LocalDirAllocator.java:155)
> 	at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.newBackupFile(DFSClient.java:1171)
> 	at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.(DFSClient.java:1136)
> 	at org.apache.hadoop.dfs.DFSClient.create(DFSClient.java:342)
> 	at org.apache.hadoop.dfs.DistributedFileSystem$RawDistributedFileSystem.create(DistributedFileSystem.java:145)
> 	at org.apache.hadoop.fs.ChecksumFileSystem$FSOutputSummer.(ChecksumFileSystem.java:368)
> 	at org.apache.hadoop.fs.ChecksumFileSystem.create(ChecksumFileSystem.java:443)
> 	at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:254)
> 	at org.apache.hadoop.io.SequenceFile$Writer.(SequenceFile.java:675)
> 	at org.apache.hadoop.io.SequenceFile.createWriter(SequenceFile.java:165)
> 	at org.apache.hadoop.examples.RandomWriter$Map.map(RandomWriter.java:137)
> 	at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:46)
> 	at org.apache.hadoop.mapred.MapTask.run(MapTask.java:189)
> 	at org.apache.hadoop.mapred.TaskTracker$Child.main(TaskTracker.java:1740)
> 2007-06-21 07:55:33,821 WARN org.apache.hadoop.mapred.TaskTracker: Error running child

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