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From "Joshua McKenzie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13877) Potential concurrency issue with CDC size calculation
Date Mon, 18 Sep 2017 13:28:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13877?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16170009#comment-16170009
] 

Joshua McKenzie commented on CASSANDRA-13877:
---------------------------------------------

Those volatiles are only written from one thread context each so shouldn't be an issue; single-producer
multi-consumer.

bq. In rare cases we'll not be able to calculate CDC data size correctly
Could you clarify a theoretical scenario in which this would take place?

> Potential concurrency issue with CDC size calculation
> -----------------------------------------------------
>
>                 Key: CASSANDRA-13877
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13877
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Simon Zhou
>            Assignee: Simon Zhou
>
> We're backporting CDC feature and bug fixes to 3.0. There is potential visibility issue
with two variables {{CDCSizeTracker.sizeInProgress}} and {{DirectorySizeCalculator.size}}
. They're declared as volatile however there are cases that when assigning new values to them,
the new values depend on the current value. For example:
> https://github.com/apache/cassandra/blob/e9da85723a8dd40872c4bca087a03b655bd2cacb/src/java/org/apache/cassandra/db/commitlog/CommitLogSegmentManagerCDC.java#L285
> https://github.com/apache/cassandra/blob/e9da85723a8dd40872c4bca087a03b655bd2cacb/src/java/org/apache/cassandra/db/commitlog/CommitLogSegmentManagerCDC.java#L297
> In rare cases we'll not be able to calculate CDC data size correctly. We should change
these two variables back to AtomicLong, as the simplest fix. Java Concurrency In Practice
section 3.1.3 explains well why we shouldn't use volatile in these two cases. I'll provide
patch shortly.
> cc [~JoshuaMcKenzie] [~jay.zhuang]



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