Re: [Qemu-devel] [PATCH for-2.7 v2 05/17] raw-posix: Implement .bdrv_lockf

[Richard W.M. Jones]

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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