What were you trying to do that didn't work?

Based on an upstream report from Andrey Drobyshev

https://lists.gnu.org/archive/html/qemu-devel/2025-04/msg04421.html

What is the impact of this issue to you?

The potential to cause qemu deadlocks when doing block jobs can interfere with live migration.

Please provide the package NVR for which the bug is seen:

How reproducible is this bug?:

Sporadic

Steps to reproduce

1. Per Andrey's email:

2. 1. Run QEMU:
   > SRCDIR=/path/to/srcdir
   >
   > $SRCDIR/build/qemu-system-x86_64 -enable-kvm \
   >   -machine q35 -cpu Nehalem \
   >   -name guest=alma8-vm,debug-threads=on \
   >   -m 2g -smp 2 \
   >   -nographic -nodefaults \
   >   -qmp unix:/var/run/alma8-qmp.sock,server=on,wait=off \
   >   -serial unix:/var/run/alma8-serial.sock,server=on,wait=off \
   >   -object iothread,id=iothread0 \
   >   -blockdev node-name=disk,driver=qcow2,file.driver=file,file.filename=/path/to/img/alma8.qcow2 \
   >   -device virtio-blk-pci,drive=disk,iothread=iothread0

   2. Launch IO (random reads) from within the guest:
   > nc -U /var/run/alma8-serial.sock
   > ...
   > [root@alma8-vm ~]# fio --name=randread --ioengine=libaio --direct=1 --bs=4k --size=1G --numjobs=1 --time_based=1 --runtime=300
   +--group_reporting --rw=randread --iodepth=1 --filename=/testfile

   3. Run snapshots creation & removal of lower snapshot operation in a
   loop (script attached):
   > while /bin/true ; do ./remove_lower_snap.sh ; done

3. where remove_lower_snap.sh is:

4. #!/bin/bash

   SRCDIR=/path/to/srcdir
   STORDIR=/path/to/img
   SNAP1=$STORDIR/snap1.qcow2
   SNAP2=$STORDIR/snap2.qcow2
   QMPSHELL=$SRCDIR/scripts/qmp/qmp-shell
   QMPSOCK=/var/run/alma8-qmp.sock

   function qmp_filter()

   Unknown macro: {     sed -r '/^(Welcome|Connected)/d' }

   function waitjob()

   Unknown macro: {     jobid=$1     while /bin/true ; do         qbjout=$($QMPSHELL -p $QMPSOCK <<EOF             query-block-jobs EOF        )     jobstatus=$(echo "$qbjout" | grep '"status"' | head -1 | awk '{print $2}

   ' | sed 's/[",]//g')

           if [ "x${jobstatus}" == "xready" ] ; then
               echo -e "\n######### Complete job $jobid #########\n"
               $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
                   job-complete id=$jobid
   EOF
          elif [ "x${jobstatus}" == "xconcluded" ] ; then
               echo -e "\n######### Dismiss job $jobid #########\n"
               $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
                   job-dismiss id=$jobid
   EOF
          elif [ "x${jobstatus}" == "x" ] ; then
               break
           fi

       sleep 0.5
       done
   }

   echo -e "\n######### Create snapshot images #########\n"

   qemu-img create -f qcow2 $SNAP1 16G
   qemu-img create -f qcow2 $SNAP2 16G

   echo -e "\n######### Create 1st snapshot #########\n"

   $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
       blockdev-add driver=qcow2 node-name=snap1 file={"driver":"file","filename":"$SNAP1"}
       blockdev-snapshot node=disk overlay=snap1
   EOF

   echo -e "\n######### Create 2nd snapshot #########\n"

   $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
       blockdev-add driver=qcow2 node-name=snap2 file={"driver":"file","filename":"$SNAP2"}
       blockdev-snapshot node=snap1 overlay=snap2
   EOF

   echo -e "\n######### Commit lower snapshot #########\n"

   $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
       block-commit device=snap2 top-node=snap1 base-node=disk auto-finalize=true auto-dismiss=false job-id=commit-snap1
   EOF

   waitjob commit-snap1

   echo -e "\n######### Commit remaining snapshot #########\n"

   $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
       block-commit device=snap2 top-node=snap2 base-node=disk auto-finalize=true auto-dismiss=false job-id=commit-snap2
   EOF

   waitjob commit-snap2

   echo -e "\n######### Remove unneeded snapshot nodes #########\n"

   $QMPSHELL -p $QMPSOCK <<EOF | qmp_filter
       blockdev-del node-name=snap1
       blockdev-del node-name=snap2
   EOF

   echo -e "\n######### Done! #########\n"

5.  

Expected results

No hang

Actual results

Running the script in a loop can hit deadlock