[jira] [Created] (HDFS-14618) Incorrect synchronization of ArrayList field (ArrayList is thread-unsafe).

["Paul Ward (JIRA)" <ji...@apache.org>]

Paul Ward created HDFS-14618:
--------------------------------

             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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