Re: "device busy" error when registering device

[Ian Pilcher <arequipeno@gmail.com>]

On 08/13/2014 03:24 PM, Rolf Fokkens wrote:
> I'm running a very similar setup, but my system boots 100% of the time.
> So it may be useful to find out what's causing the problems at your
> system. You're using Intel RAID and I'm using Linux software RAID. It
> may be relevant, I don't know.

I suspect that the most significant difference is that my backing device
is a partition on a RAID device, rather than the device itself.

In reality, however, I don't think it's worth spending a lot of time
figuring out exactly what else is accessing the device.  If I understand
udev correctly, this sort of "conflict" is an inevitable consequence of
its asynchronous nature, and helper programs like bcache-register need
to be robust enough to deal with them.

(If I'm wrong about this, then I guess this is a udev bug?)

BTW, this is what the bcache-related dmesg entries look like when I hit
the conflict with my patches applied:

> [    1.761932] bcache: bch_journal_replay() journal replay done, 1972 keys in 75 entries, seq 312536
> [    1.762043] bcache: register_cache() registered cache device sda2
> [    1.762631] bcache: register_bcache() error opening /dev/md126p5: device busy
> [    1.862949] bcache: register_bdev() registered backing device md126p5
> [    1.864268] bcache: bch_cached_dev_attach() Caching md126p5 as bcache0 on set fb8d253b-6144-4b48-8ed6-7b1724ed83a7
> [    3.553053] bcache: register_bcache() error opening /dev/sda2: device already registered
> [    4.182180] bcache: register_bcache() error opening /dev/md126p5: device already registered

You can see that the backing device is busy when it first tries to
register it, but it succeeds 1/10th of a second later.  (I'm assuming
the last two "already registered" messages occur when udev replays
its events later in the boot process.)

Thanks!

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