Mount point removal and renaming

I do not know how you are hitting this problem but it sounds like even with my changes you will have a problem. Aka one build environment killing a file or directory the other build environment is using for a mount point.

That said I agree, that this can be a problem already.

One of the points of user namespaces is to take a bunch of kernel functionality we have been allowing non-root users to use for a long time and remove the need for the suid root helpers. Much like what happened with ptys with the introduction of /dev/ptmx.