Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication

[Wen Congyang <wency@cn.fujitsu.com>]

On 10/15/2015 10:55 PM, Stefan Hajnoczi wrote:
> On Thu, Oct 15, 2015 at 10:19:17AM +0800, Wen Congyang wrote:
>> On 10/14/2015 10:27 PM, Stefan Hajnoczi wrote:
>>> On Tue, Oct 13, 2015 at 05:08:17PM +0800, Wen Congyang wrote:
>>>> On 10/13/2015 12:27 AM, Stefan Hajnoczi wrote:
>>>>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote:
>>>>>> +        /* start backup job now */
>>>>>> +        bdrv_op_unblock(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET,
>>>>>> +                        s->active_disk->backing_blocker);
>>>>>> +        bdrv_op_unblock(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE,
>>>>>> +                        s->hidden_disk->backing_blocker);
>>>>>
>>>>> Why is it safe to unblock these operations?
>>>>>
>>>>> Why do they have to be blocked for non-replication users?
>>>>
>>>> hidden_disk and secondary disk are opened as backing file, so it is blocked for
>>>> non-replication users.
>>>> What can I do if I don't unblock it and want to do backup?
>>>
>>> CCing Jeff Cody, block jobs maintainer
>>>
>>> You need to explain why it is safe remove this protection.  We can't
>>> merge code that may be unsafe.
>>>
>>> I think we can investigate further by asking: when does QEMU code assume
>>> the backing file is read-only?
>>
>> The backing file is opened in read-only mode. I want to reopen it in read-write
>> mode here in the next version(So the patch 1 will be dropped)
>>
>>>
>>> I haven't checked but these cases come to mind:
>>>
>>> Operations that move data between BDS in the backing chain (e.g. commit
>>> and stream block jobs) will lose or overwrite data if the backing file
>>> is being written to by another coroutine.
>>>
>>> We need to prevent users from running these operations at the same time.
>>
>> Yes, but qemu doesn't provide such API.
> 
> This series can't be merged unless it is safe.
> 
> Have you looked at op blockers and thought about how to prevent unsafe
> operations?

What about this solution:
1. unblock it in bdrv_set_backing_hd()
2. block it in qmp_block_commit(), qmp_block_stream(), qmp_block_backup()..., to
   prevent unsafe operations

Thanks
Wen Congyang

> 
> Stefan
> .
>