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From: Wen Congyang <wency@cn.fujitsu.com>
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Date: Tue, 13 Oct 2015 16:59:48 +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: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.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>, "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 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().

Thanks
Wen Congyang

> .
>