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From "Matt Foley (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-1790) review use of synchronized methods in FSNamesystem
Date Sat, 26 Mar 2011 00:33:05 GMT
review use of synchronized methods in FSNamesystem

                 Key: HDFS-1790
                 URL: https://issues.apache.org/jira/browse/HDFS-1790
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: name-node
    Affects Versions: 0.23.0
            Reporter: Matt Foley
             Fix For: 0.23.0

While analyzing SafeMode semantics for HDFS-1726, I noticed the following usage of synchronized
methods in FSNamesystem that raised questions:

1. incVolumeFailure() is synchronized on the FSNamesystem instance (monitor lock), for unclear
reasons.  Is this a left-over from the conversion to read/write lock in FSNamesystem?

2. isInSafeMode() and isPopulatingReplQueues() are also synchronized on the FSNamesystem instance
(monitor lock), but the SafeModeInfo methods they call (safeMode.isInSafeMode() and safeMode.isPopulatingReplQueues()
are synchronized on the SafeModeInfo instance.  Is synchronizing on the FSNamesystem instance
necessary?  What does it add?  If it is necessary, should it be using the read/write lock?

3. In SafeModeInfo, these methods are synchronized on the SafeModeInfo instance:
but these are not:

- isOn() asserts isConsistent(), but is otherwise an atomic read operation.
- isPopulatingReplQueues() is an atomic read.  Does it need synchronization?
- leave() is complex, but shouldn't it be synchronized with enter(), which is also a write
operation?  Yet enter() is unsynchronized.
- initializeReplQueues() calls blockManager.processMisReplicatedBlocks(), which can take minutes
to run.  By synchronizing on the SafeModeInfo instance, it prevents essentially all of the
other safeMode methods from running for the duration!  Is this desirable or needed?
- canInitializeReplQueues() is a read-only operation, although it does compare two read values.
 needEnter() compares four read values, and is unsynchronized.  Does either need synchronization?
- canLeave() is a compound operation, it's good that it is synchronized.
- checkMode() is a big compound read/write operation.  Doesn't it need synchronization if
the other methods do?
and so on.

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