Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Anthony Liguori]

Daniel P. Berrange wrote:
> > Better to stick with on/off.  That gives much easier to understand 
> > semantics.
> >     
>
> The 3rd invocation is not changing the backing file, so it is not causing
> data corruption on the master file. Sure the 3rd OS instance will probably
> crash & get confused because its backing file is changing under its feet
> but as long as 'commit' isn't used, it is still safe.

The 3rd instance is getting data corruption.  The corruption is 
transient because we're using -snapshot but if you change the scenario 
to use a persistent cow overlay, then the corruption is permanent.

The semantics of shared/exclusive are non-obvious to anything but 
someone who understands the implementation.

>  'commit' should 
> require that the disk have been locked in exclusive  mode from the time
> it started. It is not safe to merely let it upgrade the lock from
> shared to exclusive at time of committing because of this exact scenario.
>   
I don't understand why it isn't safe to temporarily acquire a write lock 
with commit.  Can you give an example of how it would cause corruption?

Regards,

Anthony Liguori

> Daniel
>