getContentSummary should hold the FSNamesystem readlock
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                 Key: HDFS-1775
                 URL: https://issues.apache.org/jira/browse/HDFS-1775
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Dmytro Molkov
            Assignee: Dmytro Molkov
            Priority: Minor


Right now the getContentSummary call on the namenode only holds the FSDirectory 
lock, but not the FSNamesystem lock. What we are seeing because of that is: 
1) getContentSummary takes the read lock on FSDirectory 
2) the write operation comes and takes a write lock on FSNamesystem and waits 
for getContentSummary to finish to get a write lock on FSDirectory

As a result other read operations can't be executed. Since getContentSummary 
can take a while to execute on large directories, the performance would be 
improved if we hold a readlock while doing that.

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