Kernel Oops (bCache hangs on registering 3rd device)

[James Sefton <james-3k2nYdb70uTQXOPxS62xeg@public.gmane.org>]

Hi,

Seems I broke something.

I have run into this problem about 3 times now where bCache (or something!) 
seems to hang and I only just thought to check kern.log before trying to reboot 
so was previously assuming something else had just got stuck!

I only had 3 servers available to me and once this problem occurs, the servers 
seems to get stuck rebooting so I cannot do another test to confirm tonight.  (I 
have put a reboot request in for them to be force rebooted but that will not get 
actioned until tomorrow)

Here is what I know about the most recent time this happened:

I had already set-up and been using /dev/bcache0 and /dev/bcache1.  They are 
attached to the cache set and set to writeback.

The problem occurred when I ran "echo /dev/rbd2 >/sys/fs/bcache/register".   
This device had been newly formatted with "make-bcache -B /dev/rbd2"   (rbd2 is 
a SAN based block device)

If it makes any difference - these commands are in a script so the register 
command would have been run *immediately* after the make-bcache command 
returned.  (I am going to try sticking a sleep 3 or something in there tomorrow 
- just in case its related to trying to register the device so quickly after it 
was prepared)

At this point my console just stops.  I cannot CTRL+C to abort the echo command.
top shows increasing load average - presumably it thinks a process is stuck 
waiting for something.  Rebooting server kicks me out of console and then never 
comes back.

/dev/rbd0 and /dev/rbd1 are already registered at this point and were working 
fine.  If I open another console (did this before rebooting) and create 
/dev/rbd3, and then try and register it - the same happens again.

Here is what came up in kern.log:

http://pastebin.com/xNBuv3sj
(Using Gmane to post and it complained about long lines)

Any ideas?

Cheers,

James