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https://issues.apache.org/jira/browse/HDFS-14618?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16875287#comment-16875287
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Anu Engineer commented on HDFS-14618:
-------------------------------------
[~paulward24] Thank you for your contribution. I have committed this patch to
the trunk.
> Incorrect synchronization of ArrayList field (ArrayList is thread-unsafe).
> --------------------------------------------------------------------------
>
> Key: HDFS-14618
> URL: https://issues.apache.org/jira/browse/HDFS-14618
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Paul Ward
> Assignee: Paul Ward
> Priority: Critical
> Labels: fix-provided, patch-available
> Attachments: race.patch
>
>
> I submitted a CR for this issue at:
> https://github.com/apache/hadoop/pull/1030
> The field {{timedOutItems}} (an {{ArrayList}}, i.e., not thread safe):
> https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L70
> is protected by synchronization on itself ({{timedOutItems}}):
> https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L167-L168
> https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L267-L268
> https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L178
> However, in one place:
> https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L133-L135
> it is (trying to be) protected by synchronized using
> {{pendingReconstructions}} --- but this cannot protect {{timedOutItems}}.
> Synchronized on different objects does not ensure mutual exclusion with the
> other locations.
> I.e., 2 code locations, one synchronized by {{pendingReconstructions}} and
> the other by {{timedOutItems}} can still executed concurrently.
> This CR adds the synchronized on {{timedOutItems}}.
> Note that this CR keeps the synchronized on {{pendingReconstructions}}, which
> is needed for a different purpose (protect {{pendingReconstructions}})
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