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Date: Thu, 18 Jun 2015 13:47:29 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20150618124729.GD25387@stefanha-thinkpad.redhat.com>
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In-Reply-To: <1434617361-17778-7-git-send-email-wency@cn.fujitsu.com>
Subject: Re: [Qemu-devel] [PATCH COLO-Block v6 06/16] Don't allow a disk use
 backing reference target
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To: Wen Congyang <wency@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, qemu block <qemu-block@nongnu.org>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, qemu devel <qemu-devel@nongnu.org>, Max Reitz <mreitz@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Yang Hongyang <yanghy@cn.fujitsu.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>


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On Thu, Jun 18, 2015 at 04:49:11PM +0800, Wen Congyang wrote:
> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> ---
>  block.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>=20
> diff --git a/block.c b/block.c
> index d1ed227..0b41af4 100644
> --- a/block.c
> +++ b/block.c
> @@ -1294,6 +1294,14 @@ static int bdrv_open_backing_reference_file(BlockD=
riverState *bs,
>      }
> =20
>      backing_hd =3D blk_bs(backing_blk);
> +    /* Don't allow a disk use backing reference target */
> +    ret =3D blk_attach_dev(backing_hd->blk, bs);
> +    if (ret < 0) {
> +        error_setg(errp, "backing_hd %s is used by the other device mode=
l",
> +                   backing_name);
> +        goto free_exit;
> +    }

Can you explain the purpose of this patch?

I'm not sure blk_attach_dev() is the appropriate API.  It is only used
by emulated devices but not by the run-time NBD server, for example.
This means you are not preventing all other users from accessing this
BDS.

The op blockers mechanism is normally used to prevent operations on a
BDS.  See bdrv_op_is_blocked() and bdrv_op_block().

Stefan

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