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

[Daniel P. Berrange]

On Mon, Dec 07, 2009 at 11:31:47AM +0000, Richard W.M. Jones wrote:
> On Mon, Dec 07, 2009 at 11:30:14AM +0000, Daniel P. Berrange wrote:
> > On Mon, Dec 07, 2009 at 11:19:54AM +0000, Jamie Lokier wrote:
> > > 
> > > No, the question is whether it makes sense to provide a 'shared'
> > > option on the command line, or simply to always map:
> > > 
> > >      image opened read only => F_FDLCK
> > >      image opened writable => F_WRLCK
> > > 
> > > and provide only a single command line option: 'lock'.
> > 
> > That doesn't work in the case of setting up a clustered filesystem
> > shared between guests. That requires that the disk be opened writable,
> > but with a shared (F_RDLOCK) lock.
> 
> I think Jamie's point is that you might as well use no locking at all
> in this configuration.  It's hard to see what lock=shared is
> protecting you against.

This is saying that there is no need to protect 'shared writers'
from 'exclusive writers', which is not true.

Consider you have a cluster of VMs managed by libvirt with a shared
writable disk running the GFS filesystem, and you've not done any
locking on the disk file.

Now an admin comes along with with libguestfs and attempts to access
the disk containing the GFS volume. libguestfs isn't part of the 
cluster but that doesn't matter because you can happily access a GFS
filesystem in standalone mode provided it is not in use by any other
nodes. 

We need to stop libguestfs opening the disk in exclusive-write mode
if other QEMU VMs are using it in shared-write mode.

If QEMU with the shared-writable disks is been using F_RDLOCK, then
this would have prevent libguestfs opening the disk for write with
F_WRLOCK, since the F_RDLOCK blocks all F_WRLOCK attempts.

We really do have 3 combinations of locking / access mode here

 - read-only  + F_RDLOCK
 - read-write + F_RDLOCK
 - read-write + F_WRLOCK

So a single 'lock' flag is not sufficient, we need the shared/exclusive
semantics of the original patch.

Daniel
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