Re: Finding an available fss device

["J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>]

> On 12. Aug 2018, at 10:07, Emmanuel Dreyfus <manu%netbsd.org@localhost> wrote:
> 
> On Sun, Aug 12, 2018 at 09:55:27AM +0200, J. Hannken-Illjes wrote:
>>> You mean you cannot at the same tme snapshot /mount0 on fss0 and 
>>> /mount1 on fss1?
>> 
>> Yes, you have to create the snapshot on /mount0 and once it has been
>> created you create the snapshot on /mount1.
> 
> Where is that limitation? I would not exepect to get such a limitation
> when working on both a different mount and fss device.

The simplest deadlock is:

While creating a snapshot "/mount0" lookup "/mount0/file", it will block
as "/mount0" is suspended.  The lookup holds a lock on "/".

Now snapshot "/ "and trying to suspend "/" will block as the lookup
has the root vnode locked.

There are too many other deadlock scenarios this could ever work.

--
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig (Germany)