"device busy" error when registering device

[Ian Pilcher <arequipeno@gmail.com>]

I just finished moving my existing Fedora 20 root filesystem onto a
bcache device (actually LVM on top of a bcache physical volume).

The bcache cache device is /dev/sda2, a partition on my SSD; the backing
device is /dev/md126p5, a partition on an Intel RAID (imsm) volume.

This configuration only boots successfully about 50% of the time.  The
other 50% of the time, the bcache device is not created, and dracut
times out and dumps me into an emergency shell.

After changing the bcache-register script to use /sys/fs/bcache/register
(instead of register_quiet), I see a "device busy" error when udev
attempts to register the backing device:

  [    2.105581] bcache: register_bcache() error opening /dev/md126p5:
device busy

This is kernel 3.5.15, so this doesn't mean that the device is already
registered; something else has it (temporarily) opened.  I say that it's
opened temporarily, because I am able to register the backing device
manually from the dracut shell -- which starts the the bcache device.

Looking at /usr/lib/udev/bcache-register and the bcache_register source
in drivers/md/bcache/super.c, I notice 2 things.

(1) bcache-register gives up immediately when an error occurs because of
    a (possibly temporary) conflict.

(2) Although the driver logs a different message in the already
    registered case ("device already registered" instead of "device
    busy"), it doesn't provide userspace with any way to distinguish the
    two cases; it always returns -EINVAL.

Suggested fix:

(1) Change bcache_register to return -EBUSY in the device busy case
    (while still returning -EINVAL in the already registered case).

(2) Change bcache-register to check the exit code of the registration
    attempt and retry in the EBUSY case.

Does this make sense?

-- 
========================================================================
Ian Pilcher                                         arequipeno@gmail.com
-------- "I grew up before Mark Zuckerberg invented friendship" --------
========================================================================