dmsetup remove gives EBUSY (even though it shouldn't be busy)

[Johannes Bauer <dfnsonfsduifb@gmx.de>]

Hi list,

apologies if I'm at the wrong place, but I think this is dm related
(even though in practice the device I'm talking about is using the
dm-crypt target).

This is my problem: I have a open dm-crypt mapping (LUKS) on which I do
perform some block operations (copying). There are no mounted file
systems on the copied device handles at any time. Let's say my device is
called under /dev/mapper/foobar and I'm writing to that block device.

The file descriptor that points to the /dev/mapper/foobar is then closed
via close(2). Then I call sync(2).

Afterwards I fork/exec the command "dmsetup remove foobar". This yields:

device-mapper: remove ioctl on foobar failed: Device or resource busy
Command failed

And results in return code 1. I'm only getting this behavior with slow
disks (i.e. cannot reproduce with a loop device that points to /dev/shm
as underlying block device). And I'm guessing there's some
buffers/caches still in use (i.e. pending writes) which is why dmsetup
returns EBUSY. What I don't understand is why the sync(2) call does not
block until all pending writes have been flushed.

If I do insert sleep(10) before my "dmsetup remove" call, it works
smoothly -- but this is hardly a solution.

Do you have any pointers on why this happens and how I can cleanly solve
this?

Thank you very much,
Cheers,
Johannes