An operation for filesystem tucking

If you flip that symlink while a program is starting up it'll get some libraries from A and others from B, which may cause it to crash. So in that sense it's definitely not atomic. The same problem applies to a mount-on-top operation.

On the other hand if you have a tuck-mount you can do an atomic unmount afterwards, but only if there's no current user. That guarantees that you don't run into inconsistent libraries et al. even when you can't control exactly when a program (re)starts.