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From: Wen Congyang <wency@cn.fujitsu.com>
Message-ID: <561CD2EA.6070701@cn.fujitsu.com>
Date: Tue, 13 Oct 2015 17:46:18 +0800
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new
 driver for block replication
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To: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>, qemu block <qemu-block@nongnu.org>, Stefan Hajnoczi <stefanha@gmail.com>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, qemu devel <qemu-devel@nongnu.org>, "Michael R.
	Hines" <mrhines@linux.vnet.ibm.com>, 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>

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...

Thanks
Wen Congyang

> 
> Fam
> .
>