It appears that cursorsTimedOut() and incrementCursorsTimedOut can be called on different threads, and there is no lock
https://github.com/10gen/mongo/blob/7b470a0d23c5a0e86f398082f88e1f136837d320/src/mongo/s/query/cluster_cursor_manager.h#L518-L524

Here's the writer:
https://github.com/10agen/mongo/blob/7b470a0d23c5a0e86f398082f88e1f136837d320/src/mongo/s/query/cluster_cursor_cleanup_job.cpp#L65-L66

Here's the reader:
https://github.com/10gen/mongo/blob/7b470a0d23c5a0e86f398082f88e1f136837d320/src/mongo/s/cluster_cursor_stats.cpp#L48-L49

In practice, this race is unlikely, but it's worth fixing. Also, it's creating a lot of noise on TSAN builds (e.g., BF-26555). Probably just using a std::atomic for _cursorsTimedOut will solve the issue.