Re: [Qemu-devel] [PATCH v4 00/27] block: Lock images when opening

[Daniel P. Berrange]

On Tue, May 10, 2016 at 09:43:04AM +0100, Richard W.M. Jones wrote:
> On Tue, May 10, 2016 at 09:14:26AM +0100, Richard W.M. Jones wrote:
> > However I didn't test the write-shareable case (the libvirt
> > <shareable/> flag which should map to a shared lock -- is that right Dan?).
> 
> To Dan (mainly): I think setting the <shareable/> flag in libvirt only
> sets cache=unsafe on the qemu drive (it may have other effects for
> virtlockd).  Should there be an extra qemu drive flag to communicate
> that the drive is write-shareable?

Sure, if QEMU had a way to indicate that the disk was used in a
write-sharable mode, libvirt would use it.

I agree with your general point that we should do no locking for
read-only access, F_RDLCK for shared-write and F_WRLCK for
exclusive-write access. I think I made that point similarly against
an earlier version of this patchset

Regards,
Daniel
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