Re: can "rbd unmap" detect if device is mounted?

[Josh Durgin <josh.durgin@inktank.com>]

On 07/16/2012 04:37 PM, Tommi Virtanen wrote:
> On Mon, Jul 16, 2012 at 3:43 PM, Josh Durgin<josh.durgin@inktank.com>  wrote:
>>> I've made this mistake a couple of times now (completely my fault,
>>> when will I learn?), and am wondering if a bit of protection can be
>>> put in place against user errors.
>> Yeah, we've been working on advisory locking. The first step is
>> just adding an option to lock via the rbd command line tool, so you
>> could script lock/map and unmap/unlock.
>
> Is his problem really about the locking?
>
> It sounded to me like, he has something (the mount) referencing a
> block device, and we're letting the block device disappear.
>
> The locking you guys have been talking about sounds like that lock
> would be held whenever the image is mapped, regardless of whether it's
> mounted or not (think mkfs).
>
>
> Should unmap even be possible while the block device is open?
> Shouldn't there be a refcount and an -EBUSY? That's what other block
> device providers do:

That would be the best solution. Looking into it, the rbd driver
is already keeping a refcount just like the loop driver (in 
block_device_operations .open/.release). The rbd driver is only using
it for the struct device though, instead of struct rbd_device. This
shouldn't be too hard to fix.

Josh