Paul Ward created HDFS-14618:
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Summary: 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
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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