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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-566) INode.permissions should be marked as volatile to avoid synchronization problems
Date Tue, 25 Aug 2009 10:37:59 GMT
INode.permissions should be marked as volatile to avoid synchronization problems
--------------------------------------------------------------------------------

                 Key: HDFS-566
                 URL: https://issues.apache.org/jira/browse/HDFS-566
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: name-node
    Affects Versions: 0.21.0
            Reporter: Steve Loughran
            Priority: Minor


Looking at INode, I can see that the long permissions field is updated in the synchronized
updatePermissions, read in other non-synchronized contexts

I believe that to avoid race conditions and other synchronisation problems, the field should
be marked {{volatile}}

# The Java language specification declares that {{long}} and {{double}} can be written as
two 32-bit writes, unless the field is {{volatile}} http://java.sun.com/docs/books/jls/second_edition/html/memory.doc.html#28733
# The JVM is free to re-order accesses to non-volatile data; reads of the permission may pick
up out of date values
# Volatile data may be cached/optimised out, changes may not propagate

I don't think its enough to make the write operation synchronised, as the reads are still
unsynchronized, and in other threads can pick up values midway through the update, or cache
the value.

It's a simple fix: declare permissions as  {{volatile}}.
{code}
private volatile long permissions;
{code}




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