debugfs oddity

[Johannes Berg <johannes@sipsolutions.net>]

Recently, I observed (in bcm43xx) that debugfs seems to keep things
alive when userspace still has a directory open. Consider the following
sequence of events:
 (a) kernel code creates a directory in debugfs
 (b) user changes current directory to that
 (c) kernel code removes that directory in debugfs

Now, consider the equivalent sequence in a regular filesystem (or
tmpfs):
 (a') user creates directory
 (b') user cd's into it
 (c') user deletes directory from a different shell

The same thing should happen, in both cases the directory is kept around
in a way until the process that has the current dir in the dead
directory gives it up.

Now, however, consider
 (d') user creates directory with the same name

This works fine, and the old process sees nothing that happens in the
new directory, as expected. However,
 (d) kernel code tries to create a debugfs directory with the same name

does not work at all.

Is this expected behaviour? It seems that once a driver requested that a
directory is removed it can rightfully expect to be able to recreate it
afterwards even if there's still the need to keep it lingering around
for a bit.

Similar things can probably happen when attributes are kept open, but I
haven't tested this. I have also not tested sysfs or configfs.

johannes