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

[Kevin Wolf]

Am 10.05.2016 um 11:23 hat Daniel P. Berrange geschrieben:
> On Tue, May 10, 2016 at 11:14:22AM +0200, Kevin Wolf wrote:
> > Am 10.05.2016 um 10:50 hat Daniel P. Berrange geschrieben:
> > > 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
> > 
> > Why should we do no locking for read-only access by default? If an image
> > is written to, read-only accesses are potentially inconsistent and we
> > should protect users against it.
> > 
> > The only argument I can see in the old versions of this series is
> > "libguestfs does it and usually it gets away with it". For me, that's
> > not an argument for generally allowing this, but at most for providing a
> > switch to bypass the locking.
> > 
> > Because let's be clear about this: If someone lost data because they
> > took an inconsistent backup this way and comes to us qemu developers,
> > all we can tell them is "sorry, what you did is not supported". And
> > that's a pretty strong sign that locking should prevent it by default.
> 
> We have 3 usage scenarios - readonly-share, writable-shared and
> writable-exclusive, and only 2 lock types to play with. This series
> does no locking at all in the writable-shared case, so we still have
> the problem you describe in that someone opening the image for readonly
> access will succeeed even when it is used writable-shared by another
> process.
> 
> So we have to pick a trade-off here. IMHO it is more important to
> ensure we have locks in both the write-shared & write-exclusive case,
> as both of those can definitely result in corrupted images if not
> careful, where as read-only access merely results in your reading
> bad data, no risk of corrupting the original image.

I agree that we want locking for the writable-shared case. That doesn't
mean no locking for read-only, though. I think read-only and writeable-
shared are the same thing as far as locking is concerned.

This is the matrix of the allowed combinations (without a manual lock
override that enables libguestfs to use unsupported cases), as I see it:

                    wr-excl     wr-shared   read-only
write-exclusive     no          no          no
write-shared        no          yes         yes
read-only           no          yes         yes

Do you disagree with any of the entries?

Otherwise, this suggests that write-exclusive is F_WRLCK and both
write-shared and read-only are F_RDLCK.

Kevin