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

[Wen Congyang]

On 10/13/2015 06:12 PM, Fam Zheng wrote:
> On Tue, 10/13 17:46, Wen Congyang wrote:
>> On 10/13/2015 05:41 PM, Fam Zheng wrote:
>>> On Tue, 10/13 16:59, Wen Congyang wrote:
>>>> On 10/13/2015 12:25 AM, Stefan Hajnoczi wrote:
>>>>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote:
>>>>>> +static void backup_job_completed(void *opaque, int ret)
>>>>>> +{
>>>>>> +    BDRVReplicationState *s = opaque;
>>>>>> +
>>>>>> +    if (s->replication_state != BLOCK_REPLICATION_DONE) {
>>>>>> +        /* The backup job is cancelled unexpectedly */
>>>>>> +        s->error = -EIO;
>>>>>> +    }
>>>>>> +
>>>>>> +    bdrv_op_block(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET,
>>>>>> +                  s->active_disk->backing_blocker);
>>>>>> +    bdrv_op_block(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE,
>>>>>> +                  s->hidden_disk->backing_blocker);
>>>>>> +
>>>>>> +    bdrv_put_ref_bh_schedule(s->secondary_disk);
>>>>>
>>>>> Why is bdrv_put_ref_bh_schedule() necessary?
>>>>
>>>> It is copied from block_job_cb(). According to the comments in 
>>>> bdrv_put_ref_bh_schedule():
>>>> /*
>>>>  * Release a BDS reference in a BH
>>>>  *
>>>>  * It is not safe to use bdrv_unref() from a callback function when the 
>>>> callers
>>>>  * still need the BlockDriverState.  In such cases we schedule a BH to 
>>>> release
>>>>  * the reference.
>>>>  */
>>>>
>>>> If the comment is right, I think it is necessary to call 
>>>> bdrv_put_ref_bh_schedule() here.
>>>> Because the job is created on the BDS s->secondary disk, 
>>>> backup_job_completed() is
>>>> called in block_job_completed(), and we will still use s->secondary_disk 
>>>> in block_job_release().
>>>
>>> Where is the matching bdrv_ref called?
>>
>> It is in block_job_create()....
>>
>> source: we call in bdrv_ref() in block_job_create(), and the user should 
>> unref it.
>> target: the user call bdrv_ref(), and we will unref it in the job
>>
>> I don't know why we design it like this...
>>
> 
> Maybe it's better to unref it in block_job_release. Then we can simply drop
> bdrv_put_ref_bh_schedule.

I agree with it.

Thanks
Wen Congyang

> 
> Fam
> .
>