Defer unlocking just after taking a lock when possible (when unlock is
done at the very end) and when not trivial (the function body is more
than a couple of lines). This simplifies a bit some functions (no need
to unlock before each return) and for the other, it may avoid a bug in
the future in case a return is inserted into the body of a function.
Use of defer has been optimized a lot in Go and it is believed that
simpler defers have zero overhead since Go 1.14:
https://golang.org/doc/go1.14#runtime
> This release improves the performance of most uses of defer to incur
> almost zero overhead compared to calling the deferred function
> directly. As a result, defer can now be used in performance-critical
> code without overhead concerns.
