[libvirt] Use flock() instead of fcntl()

[Daniel P. Berrange]

On Fri, Jul 26, 2013 at 12:31:35PM +0200, David Weber wrote:
> Am Freitag, 26. Juli 2013, 10:14:59 schrieb Daniel P. Berrange:
> > On Fri, Jul 26, 2013 at 10:44:24AM +0200, David Weber wrote:
> > > > Looking again at flock() I see it cannot support locking of ranges, only
> > > > the entire file. This makes it unsuitable as a replacement for libvirt's
> > > > use of fcntl() I'm afraid. I can only sugggest you configure OCFS2 so
> > > > that it supports fcntl(), or setup virtlockd to use separate indirect
> > > > leases on a diffrent shared filesystem, or perhaps try sanlock instead
> > > > which doesn't require any special filesystem support.
> > > 
> > > It's true that flock() doesn't support locking of ranges but I can't see
> > > how this is necessary.
> > 
> > The code may not currently use ranges, but that doesn't mean it'll stay
> > that way. By adding support for flock() we're preventing us from making
> > use of this feature in the future, and I don't want to see that.
> 
> Just curious,  what would be a possible feature which would require range 
> based locking?
> 
> I would really like to see flock() support in virtlockd because all other 
> solutions have major drawbacks for me.

Currently we use locks to protect the content of disk images.

During startup/shutdown, however, libvirt also makes changes to the
metadata of images by setting SELinux labels, uid/gid ownership and
potentially ACLs. Currently we've delibrately crippled some of our
code during shutdown since it isn't safe in the face of multiple
libvirt's running. We need to introduce locking of file metadata
to protect this code. The metadata locks, however, must not conflict
with the content lock. Thus the reason why we only lock a single
byte (range 0-1) for content locks, is that we want to be able to
additional locks (range 1-2 or similar) for the metadata locks
on the same files.

Daniel
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