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From "Uma Maheswara Rao G (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4482) ReplicationMonitor thread can exit with NPE due to the race between delete and replication of same file.
Date Fri, 08 Feb 2013 10:23:13 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4482?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13574364#comment-13574364

Uma Maheswara Rao G commented on HDFS-4482:

Since we wanted to avoid locks here, how about throwing FNFE if inode(traversed parent) is
null in the second loop of  FSDirectory#getFullPathName? can catch the FNFE in computeReplicationWorkForBlocks
specifically and ingnore? [ can check the block collection exist in blocks map or not. if
not just skip from List<ReplicationWork>?]. Not sure whether I missed something here....
> ReplicationMonitor thread can exit with NPE due to the race between delete and replication
of same file.
> --------------------------------------------------------------------------------------------------------
>                 Key: HDFS-4482
>                 URL: https://issues.apache.org/jira/browse/HDFS-4482
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>            Reporter: Uma Maheswara Rao G
>            Priority: Blocker
> Trace:
> {noformat}
> java.lang.NullPointerException
> 	at org.apache.hadoop.hdfs.server.namenode.FSDirectory.getFullPathName(FSDirectory.java:1442)
> 	at org.apache.hadoop.hdfs.server.namenode.INode.getFullPathName(INode.java:269)
> 	at org.apache.hadoop.hdfs.server.namenode.INodeFile.getName(INodeFile.java:163)
> 	at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy.chooseTarget(BlockPlacementPolicy.java:131)
> 	at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWorkForBlocks(BlockManager.java:1157)
> 	at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWork(BlockManager.java:1063)
> 	at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeDatanodeWork(BlockManager.java:3085)
> 	at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$ReplicationMonitor.run(BlockManager.java:3047)
> 	at java.lang.Thread.run(Thread.java:619)
> {noformat}
> What I am seeing here is:
> 1) create a file and write with 2 DNS
> 2) Close the file.
> 3) Kill one DN
> 4) Let replication start.
>   Info:
>     {code}
>  // choose replication targets: NOT HOLDING THE GLOBAL LOCK
>       // It is costly to extract the filename for which chooseTargets is called,
>       // so for now we pass in the block collection itself.
>       rw.targets = blockplacement.chooseTarget(rw.bc,
>           rw.additionalReplRequired, rw.srcNode, rw.liveReplicaNodes,
>           excludedNodes, rw.block.getNumBytes());
> {code}
> Here we are choosing target outside the global lock. Inside we will try to get the src
path from blockCollection(nothing but INodeFile here).
> see the code for FSDirectory#getFullPathName
>  Here it is incrementing the depth until it has parent. and Later it will iterate and
access parent again in next loop.
> 5) before going to secnd loop in FSDirectory#getFullPathName, if file is deleted by client
then that parent would have been set as null. So, here accessing the parent can cause NPE
because it is not under lock.

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