Test on qemu-kvm-3.1.0-18.module+el8+2834+fa8bb6e2.x86_64, also hit this issue.

Test with qemu-kvm-4.0.0-4.module+el8.1.0+3523+b348b848.x86_64，also hit this issue when do blockdev-backup with data plane enable in transaction mode.

 1.Start guest with qemu cmds:
    /usr/libexec/qemu-kvm \
    -name 'avocado-vt-vm1' \
    -machine q35  \
    -nodefaults \
    -device VGA,bus=pcie.0,addr=0x1  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1-20190602-221944-MrlxVzia,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/var/tmp/monitor-catch_monitor-20190602-221944-MrlxVzia,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=idn20piu  \
    -chardev socket,id=serial_id_serial0,path=/var/tmp/serial-serial0-20190602-221944-MrlxVzia,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20190602-221944-MrlxVzia,path=/var/tmp/seabios-20190602-221944-MrlxVzia,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20190602-221944-MrlxVzia,iobase=0x402 \
    -device pcie-root-port,id=pcie.0-root-port-2,slot=2,chassis=2,addr=0x2,bus=pcie.0 \
    -device qemu-xhci,id=usb1,bus=pcie.0-root-port-2,addr=0x0 \
    -object iothread,id=iothread0 \
    -device pcie-root-port,id=pcie.0-root-port-3,slot=3,chassis=3,addr=0x3,bus=pcie.0 \
    -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pcie.0-root-port-3,addr=0x0,iothread=iothread0 \
    -blockdev driver=file,node-name=file_node,filename=/mnt/aliang/rhel810-64-virtio-scsi.qcow2 \
    -blockdev driver=qcow2,node-name=drive_image1,file=file_node \
    -device scsi-hd,id=image1,drive=drive_image1 \
    -device pcie-root-port,id=pcie.0-root-port-5,slot=5,chassis=5,addr=0x5,bus=pcie.0 \
    -device virtio-scsi-pci,id=virtio_scsi_pci1,bus=pcie.0-root-port-5,addr=0x0,iothread=iothread0 \
    -blockdev driver=file,filename=/home/data.qcow2,node-name=file_data \
    -blockdev driver=qcow2,node-name=drive_data1,file=file_data \
    -device scsi-hd,id=data1,drive=drive_data1 \
    -drive id=drive_cd1,if=none,snapshot=off,aio=threads,cache=none,media=cdrom,file=/home/kvm_autotest_root/iso/linux/RHEL8.1.0-BaseOS-x86_64.iso \
    -device ide-cd,id=cd1,drive=drive_cd1,bus=ide.0,unit=0 \
    -device pcie-root-port,id=pcie.0-root-port-4,slot=4,chassis=4,addr=0x4,bus=pcie.0 \
    -device virtio-net-pci,mac=9a:33:34:35:36:37,id=idj01pFr,vectors=4,netdev=idMgbx8B,bus=pcie.0-root-port-4,addr=0x0  \
    -netdev tap,id=idMgbx8B,vhost=on \
    -m 4096  \
    -smp 4,maxcpus=4,cores=2,threads=1,sockets=2  \
    -cpu 'Skylake-Client',+kvm_pv_unhalt \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm \
    -monitor stdio \
    -qmp tcp:0:3000,server,nowait \

2.Create backup images by blockdev-create
   {'execute':'blockdev-create','arguments':{'options': {'driver':'file','filename':'/root/sn$i','size':21474836480},'job-id':'job1'}}
   {'execute':'blockdev-add','arguments':{'driver':'file','node-name':'drive_sn$i','filename':'/root/sn$i'}}
   {'execute':'blockdev-create','arguments':{'options': {'driver': 'qcow2','file':'drive_sn$i','size':21474836480},'job-id':'job2'}}
   {'execute':'blockdev-add','arguments':{'driver':'qcow2','node-name':'sn$i','file':'drive_sn$i'}}
   {'execute':'job-dismiss','arguments':{'id':'job1'}}
   {'execute':'job-dismiss','arguments':{'id':'job2'}}

3.Do full backup and add bitmap
   { "execute": "transaction", "arguments": { "actions": [ {"type": "blockdev-backup", "data": { "device": "drive_data1", "target": "sn1", "sync": "full", "job-id":"j1" } }, {"type": "block-dirty-bitmap-add", "data": { "node": "drive_data1", "name": "bitmap0" } } ] } }
{"error": {"class": "GenericError", "desc": "Backup between two IO threads is not implemented"}}

After step3, full backup failed.

Expected to be picked up in rebase to qemu 4.1 with commit:

d81e1efbea7d19c2f142d300df56538c73800590 blockdev-backup: don't check aio_context too early

Actually, this is duplicated now because the original bug was moved from 7.7 to 8.1-AV; marking this as a duplicate of that original BZ.

*** This bug has been marked as a duplicate of bug 1683498 ***