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Date: Sat, 26 Mar 2011 00:33:05 +0000 (UTC)
From: "Matt Foley (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Created] (HDFS-1790) review use of synchronized methods in
 FSNamesystem
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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:
	isOn
	isPopulatingReplQueues
	leave
	initialize
	canInitializeReplQueues
	canLeave
	setBlock
	incrementSafeBlockCount
	decrementSafeBlockCount
	setManual
but these are not:
	enter
	needEnter
	checkMode
	isManual
	isConsistent

- 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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