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Date: Thu, 18 Jun 2015 23:17:24 +0800
From: Wen Congyang <ghostwcy@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH COLO-Block v6 06/16] Don't allow a disk use
 backing reference target
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To: Stefan Hajnoczi <stefanha@gmail.com>, 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>

At 2015/6/18 20:47, Stefan Hajnoczi Wrote:
> 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(+)
>>
>> 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(BlockDriverState *bs,
>>       }
>>
>>       backing_hd = blk_bs(backing_blk);
>> +    /* Don't allow a disk use backing reference target */
>> +    ret = blk_attach_dev(backing_hd->blk, bs);
>> +    if (ret < 0) {
>> +        error_setg(errp, "backing_hd %s is used by the other device model",
>> +                   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().

NBD server will write to this BDS(backing reference target). So I cannot use
bdrv_op_block(). If some emulated devices also write to it, the data will
be broken. The purpose is that: this BDS cannot be attached, and any 
emulated
devices will meet some erros if it tries to attach to this BDS.

Thanks
Wen Congyang

>
> Stefan
>