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Date: Sun, 17 Apr 2016 20:27:25 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
Message-ID: <20160417192725.GJ19398@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: Fam Zheng <famz@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 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).

So please don't do this; or suggest how we can ignore the lock for
backing files.

Rich.

-- 
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