Bruce Robbins created SPARK-26851:
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Summary: CachedRDDBuilder only partially implements double-checked
locking
Key: SPARK-26851
URL: https://issues.apache.org/jira/browse/SPARK-26851
Project: Spark
Issue Type: Bug
Components: SQL
Affects Versions: 2.4.0, 3.0.0
Reporter: Bruce Robbins
In CachedRDDBuilder, {{cachedColumnBuffers}} uses double-checked locking to
lazily initialize {{_cachedColumnBuffers}}. Also, clearCache uses
double-checked locking to likely avoid synchronization when
{{_cachedColumnBuffers}} is still null.
However, the resource (in this case, {{_cachedColumnBuffers}}) is not declared
as volatile, which could cause some visibility problems in both methods.
{{cachedColumnBuffers}} may see a null reference when there is actually a
constructed RDD, and {{clearCache}} may pick up a reference to partially
constructed RDD.
>From Java Concurrency in Practice by Brian Goetz et al:
{quote}Subsequent changes in the JMM (Java 5.0 and later) have enabled DCL to
work if resource is made volatile, and the performance impact of this is small
since volatile reads are usually only slightly more expensive than nonvolatile
reads.
{quote}
There are comments in other documentation that volatile is not needed if the
resource in question (again, in this case {{_cachedColumnBuffers}}) is
immutable. While an RDD is immutable from a Spark user's point of view, it may
not be from a JVM's point of view, since not all internal fields are final.
I've marked this as minor since the race conditions are highly unlikely.
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