Re: RBD images and exclusive locking

[Peter Lieven]

Am 24.03.22 um 12:51 schrieb Peter Lieven:
> Hi,
>
>
> following the thread on the ceph ml about rbd and exclusive locks I was 
> wondering if we should rewrite the rbd driver to
>
> try to acquire an exclusive lock on open and not magically on the first write. 
> This way we could directly terminate qemu
>
> if the rbd image is in use like it is done with ordinary image files on a 
> filesystem for some time now.


Digging a bit into the code and testing it seems that the exclusive-lock on rbd 
image is not that exclusive. We can easily start several

qemu instances accessing the same image. Watching the locks on the image, the 
lock owner of the one exclusive lock

toggles between the instances. This has potentitial for severe corruption.

I am thinking not of the case where a qemu instance gets killed or host dies. I 
am thinking of network issues where the disconnected

old instance of a VM suddenly kicks back in.


However, if I manually acquire an exclusive lock on qemu_rbd_open_image all 
other callers get EROFS.

So whats the difference between the exclusive lock that a write request obtains 
and an exclusive lock created

by rbd_lock_acquire? From the rbd lock ls perspective they are identical.


Best

Peter