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Date: Mon, 18 Apr 2016 09:10:36 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH for-2.7 v2 05/17] raw-posix: Implement
 .bdrv_lockf
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, Markus Armbruster <armbru@redhat.com>, Max Reitz <mreitz@redhat.com>, den@openvz.org, pbonzini@redhat.com, John Snow <jsnow@redhat.com>

On Sun, 04/17 20:27, Richard W.M. Jones wrote:
> On Fri, Apr 15, 2016 at 11:27:55AM +0800, Fam Zheng wrote:
> > virtlockd in libvirt locks the first byte, we lock byte 1 to avoid
> > the intervene.
> > +static int raw_lockf(BlockDriverState *bs, BdrvLockfCmd cmd)
> > +{
> > +
> > +    BDRVRawState *s = bs->opaque;
> > +    int ret;
> > +    struct flock fl = (struct flock) {
> > +        .l_whence  = SEEK_SET,
> > +        /* Locking byte 1 avoids interfereing with virtlockd. */
> > +        .l_start = 1,
> > +        .l_len = 1,
> > +    };
> > +
> > +    switch (cmd) {
> > +    case BDRV_LOCKF_RWLOCK:
> > +        fl.l_type = F_WRLCK;
> > +        break;
> > +    case BDRV_LOCKF_ROLOCK:
> > +        fl.l_type = F_RDLCK;
> > +        break;
> > +    case BDRV_LOCKF_UNLOCK:
> > +        fl.l_type = F_UNLCK;
> > +        break;
> 
> My understanding is this prevents libguestfs from working on live disk
> images -- we want to be able to read live disk images (using a
> writable overlay and the real disk image as a read-only backing file).

Do you lock the live image or the backing file? If not, you can just read/write
as before, as what the -L option does in this series. Otherwise, you should use
an RO lock on the backing image, which still works.

Fam