Pavel Avgustinov created HADOOP-12944:
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Summary: Prevent race condition during class initialization
Key: HADOOP-12944
URL: https://issues.apache.org/jira/browse/HADOOP-12944
Project: Hadoop Common
Issue Type: Bug
Components: fs
Reporter: Pavel Avgustinov
Priority: Minor
Attachments: HADOOP-12944.1.patch
In HADOOP-11969, [~busbey] tracked down a non-deterministic
{{NullPointerException}} to an oddity in the Java memory model: When multiple
threads trigger the loading of a class at the same time, one of them wins and
creates the {{java.lang.Class}} instance; the others block during this
initialization, but once it is complete they may obtain a reference to the
{{Class}} which has non-{{final}} fields still containing their default (i.e.
{{null}}) values. This leads to runtime failures that are hard to debug or
diagnose.
HADOOP-11969 observed that {{ThreadLocal}} fields, by their very nature, are
very likely to be accessed from multiple threads, and thus the problem is
particularly severe there. Consequently, the patch removed all occurrences of
the issue in the code base.
Unfortunately, since then HDFS-7964 has [reverted one of the fixes during a
refactoring|https://github.com/apache/hadoop/commit/2151716832ad14932dd65b1a4e47e64d8d6cd767#diff-0c2e9f7f9e685f38d1a11373b627cfa6R151],
and introduced a [new instance of the
problem|https://github.com/apache/hadoop/commit/2151716832ad14932dd65b1a4e47e64d8d6cd767#diff-6334d0df7d9aefbccd12b21bb7603169R43].
The attached patch addresses the issue by adding the missing {{final}} modifier
in these two cases.
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