Context: Code review is a crucial step of the software development life cycle in order to detect possible problems in source code before merging the changeset to the codebase. Although there is no consensus on a formally defined life cycle of the code review process, many companies and open-source software (OSS) communities converge on common rules and best practices. In spite of minor differences in different platforms, the primary purpose of all these rules and practices leads to a faster and more effective code review process. Non-conformance of developers to this process does not only reduce the advantages of the code review but can also introduce waste in later stages of software development.

Objectives: The aim of this study is to provide an empirical understanding of the bad practices followed in the code review process, that are code review (CR) smells.

Methods: We first conduct a multivocal literature review in order to gather code review bad practices discussed in white and gray literature. Then, we conduct a targeted survey with 32 experienced software practitioners and perform follow-up interviews in order to get their expert opinion. Based on this process, a taxonomy of code review smells is introduced. To quantitatively demonstrate the existence of these smells, we analyze226,292 code reviews collected from eight OSS projects.

Results: We observe that a considerable number of code review smells exist in all projects with varying degrees of ratios. The empirical results illustrate that 72.2% of the code reviews among eight projects are affected by at least one code review smell.

Conclusion: The empirical analysis shows that the OSS projects are substantially affected by the code review smells. The provided taxonomy could provide a foundation for best practices and tool support to detect and avoid code review smells in practice.