Description of problem:

 * Preparing the images

# qemu-img create -f raw base.img 1g
# qemu-img create -f qcow2 -o backing_file=base.img top1.qcow2
# qemu-img create -f qcow2 -o backing_file=base.img top2.qcow2


 * Running with iothreads

# /usr/libexec/qemu-kvm -m 1g -drive if=none,file=top1.qcow2,format=qcow2,id=drive0 -object iothread,id=iothread0 -device virtio-blk-pci,iothread=iothread0,drive=drive0 -monitor stdio 
QEMU 2.9.0 monitor - type 'help' for more information
(qemu) drive_mirror -n drive0 top2.qcow2
(qemu) info block-jobs
Type mirror, device drive0: Completed 0 of 0 bytes, speed limit 0 bytes/s
(qemu) block_job_complete drive0
qemu-kvm: block.c:1895: bdrv_attach_child: Assertion `bdrv_get_aio_context(parent_bs) == bdrv_get_aio_context(child_bs)' failed.
Aborted (core dumped)


 * Running without iothreads

# /usr/libexec/qemu-kvm -m 1g -drive if=none,file=top1.qcow2,format=qcow2,id=drive0 -device virtio-blk-pci,drive=drive0 -monitor stdio
QEMU 2.9.0 monitor - type 'help' for more information
(qemu) drive_mirror -n drive0 top2.qcow2
(qemu) info block-jobs
Type mirror, device drive0: Completed 0 of 0 bytes, speed limit 0 bytes/s
(qemu) block_job_complete drive0
(qemu) info block-jobs
No active jobs


Version-Release number of selected component (if applicable):

qemu-kvm-rhev-2.9.0-16.el7_4.13.x86_64


How reproducible:

Always


Additional info:

This has been fixed in upstream by this oneliner:

<snip>
[slopezpa@dritchie qemu]$ git show 5ce6bfe255091d532ec4555d0ede816294cb1703
commit 5ce6bfe255091d532ec4555d0ede816294cb1703
Author: sochin.jiang <sochin.jiang>
Date:   Mon Jun 26 19:04:24 2017 +0800

    mirror: Fix inconsistent backing AioContext for after mirroring
    
    mirror_complete opens the backing chain, which should have the same
    AioContext as the top when using iothreads. Make the code guarantee
    this, which fixes a failed assertion in bdrv_attach_child.
    
    Signed-off-by: sochin.jiang <sochin.jiang>
    Message-id: 1498475064-39816-1-git-send-email-sochin.jiang
    [mreitz: Reworded commit message]
    Signed-off-by: Max Reitz <mreitz>

diff --git a/block.c b/block.c
index edfa6b7a2c..b9e5114a27 100644
--- a/block.c
+++ b/block.c
@@ -2185,6 +2185,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
         ret = -EINVAL;
         goto free_exit;
     }
+    bdrv_set_aio_context(backing_hd, bdrv_get_aio_context(bs));
 
     /* Hook up the backing file link; drop our reference, bs owns the
      * backing_hd reference now */
</snip>


Please consider backporting this. Thanks.