Permission error when run block-commit

[Jing-Wei Su]

Hi, all

I was trying to do block-commit, and the disk layout is the following diagram.

        +----------------------+
         |      top-disk         |
         +---------------------+
                    | (backing)
        +----------------------+
         |     middle-disk    |
         +---------------------+
                    | (backing)
        +----------------------+
         |      base-disk      |
         +---------------------+

The script for running the vm is
qemu-system-x86_64 -name vm -enable-kvm -cpu qemu64,+kvmclock -m 1024 \
-drive 
if=none,id=middle_disk0,driver=qcow2,file.filename=${dir}/middle.qcow2,file.locking=off,\
backing.driver=qcow2,backing.file.filename=${dir}/down.qcow2,backing.file.locking=off
\
-drive 
if=ide,id=top-disk0,driver=qcow2,file.filename=${dir}/top.qcow2,backing=middle_disk0
\
-qmp stdio -vnc :0

After booting the vm, I issued the block-commit command:
{
    "execute": "block-commit",
    "arguments": {
        "device": "top-disk0",
        "job-id": "job0",
        "top": "/mnt/NFV/qemu/top.qcow2",
        "base": "/mnt/NFV/qemu/down.qcow2"
    }
}

But, I got the ERROR
{"error": {"class": "GenericError", "desc": "Conflicts with use by
middle_disk0 as 'root', which uses 'consistent read' on #block116"}}

I cannot figure out what makes this ERROR.
Is any idea about the root cause and how to solve the error?

Thanks,
Jing-Wei