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https://issues.apache.org/jira/browse/ARROW-7510?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17009941#comment-17009941
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Francois Saint-Jacques commented on ARROW-7510:
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Is this really an issue? Worst case, 2 threads compute the lazy null count and
race to write the same value.
> [C++] Array::null_count() is not thread-compatible
> --------------------------------------------------
>
> Key: ARROW-7510
> URL: https://issues.apache.org/jira/browse/ARROW-7510
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Reporter: Zhuo Peng
> Priority: Minor
>
> ArrayData has a mutable member null_count, that can be updated in a const
> function. However null_count is not atomic, so it's subject to data race.
>
> I guess Arrays are not thread-safe (which is reasonable), but at least they
> should be thread-compatible so that concurrent access to const member
> functions are fine.
> (The race looks "benign", but see [1][2])
> [https://github.com/apache/arrow/blob/dbe708c7527a4aa6b63df7722cd57db4e0bd2dc7/cpp/src/arrow/array.cc#L123]
>
> [1][https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong]
> [2][https://bartoszmilewski.com/2014/10/25/dealing-with-benign-data-races-the-c-way/]
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