  Context: Software developers are increasingly facing the challenges of
writing code that is not only concurrent but also correct.
  Objective: To help these developers, it is necessary to understand
concurrency topics they are interested in, their difficulty in finding answers
for questions in these topics, their sentiment for these topics, and how they
use concurrency tools and techniques to guarantee correctness.
  Method: We conduct a large-scale study on the entirety of Stack Overflow to
understand interests, difficulties, sentiment, and tool usages of concurrency
developers. We discuss the implications of our findings for the practice,
research, and education of concurrent software development, and investigate the
relation of our findings with the findings of the previous work.
  Results: A few findings of our study are: (1) questions that concurrency
developers ask can be grouped into a hierarchy with 27 concurrency topics under
8 major categories, (2) thread safety is among the most popular concurrency
topics and client-server concurrency is among the least popular, (3)
irreproducible behavior is among the most difficult topics and memory
consistency is among the least difficult, (4) data scraping is among the most
positive concurrency topics and irreproducible behavior is among the most
negative, (5) root cause identification has the most number of questions for
usage of data race tools and alternative use has the least.
  Conclusion: The results of our study can not only help concurrency developers
but also concurrency educators and researchers to better decide where to focus
their efforts, by trading off one concurrency topic against another.
