Github user JoshRosen commented on the pull request:
https://github.com/apache/spark/pull/2935#issuecomment-60460198
Due to a Hadoop thread-safety issue in `Configuration`'s constructor, we
need to hold a lock in any code that might call `new Configuration()` on the
executor. We don't want to hold the lock for the entire task deserialization
because that won't allow tasks to be launched in parallel. Instead, though, we
could make our own wrapper that holds a Configuration and grabs
CONFIGURATION_INSTANTIATION_LOCK in its `readObject` or `readExternal` method.
For some context on this issue, see #2683 and
https://issues.apache.org/jira/browse/SPARK-2585
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