[libvirt] [PATCH v2 6/7] domain_lock: Implement metadata locking

[Michal Prívozník]

On 08/20/2018 05:07 PM, Daniel P. Berrangé wrote:
> On Tue, Aug 14, 2018 at 01:19:42PM +0200, Michal Privoznik wrote:
>> In order for our drivers to lock resources for metadata change we
>> need set of new APIs. Fortunately, we don't have to care about
>> every possible device a domain can have. We care only about those
>> which can live on a network filesystem and hence can be accessed
>> by multiple daemons at the same time. These devices are covered
>> in virDomainLockMetadataLock() and only a small fraction of
>> those can be hotplugged (covered in the rest of the introduced
>> APIs).
> 
> I'm not sure I understand the rationale behind saying we only care
> about resources on network filesystems.
> 
> If I have 2 locally running guests, and both have a serial port
> backed by a physical serial port, eg
> 
>   <serial type="dev">
>     <source path="/dev/ttyS0"/>
>     <target port="1"/>
>   </serial>
> 
> we *do* care about locking /dev/ttyS0, as libvirtd isn't doing
> mutual exclusion checks anywhere else for the /dev/ttyS0 device
> node.

Ah you mean that the system wide daemon and session daemon could clash
when relabeling /dev/ttyS0? Well, we don't do relabeling for session
daemons and running two system daemons is not supported (you're gonna
hit more serious problems when trying that anyway).

> 
> In general I think we need to lock every single file resource
> that is labelled for a guest, regardless of whether its local
> or remote.

Well this might be feasible iff locking would be done from security driver.

Michal