[
https://issues.apache.org/jira/browse/HDFS-10183?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15204345#comment-15204345
]
Daryn Sharp commented on HDFS-10183:
------------------------------------
My reading of the JLS and the footnotes seems to pretty clearly indicate memory
barriers are required. I suspect HADOOP-11969 discovered a (hopefully fixes)
jvm bug, so this patch is probably cosmetic but certainly doesn't hurt anything.
> Prevent race condition during class initialization
> --------------------------------------------------
>
> Key: HDFS-10183
> URL: https://issues.apache.org/jira/browse/HDFS-10183
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: fs
> Affects Versions: 2.9.0
> Reporter: Pavel Avgustinov
> Assignee: Pavel Avgustinov
> Priority: Minor
> Fix For: 2.9.0
>
> Attachments: HADOOP-12944.1.patch, HDFS-10183.2.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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
