The cross rename support is up to its third revision for hopeful inclusion soon in the mainline Linux kernel. The kernel work is to ultimately allow for two files to be exchanged in the file renaming process.
Miklos Szeredi, one of the developers working on the cross-rename support, explained the motivation on the mailing list patch series
, "The purpose of extending rename is to add cross-rename, a symmetric variant of rename, which exchanges the two files. This allows interesting things, which were not possible before, for example atomically replacing a directory tree with a symlink, etc... This also allows overlayfs and friends to operate on whiteouts atomically."
With the set of 11 patches there is EXT4 support for cross-rename. Implementing support for other local file-systems is also an easy process since the heavy lifting is done by changes to the kernel's VFS layer, but additional work is required for cross-rename on network file-systems.