+++ This bug was initially created as a clone of Bug #1740049 +++

Description:
Libvirt crashed when do blkiotune to running vm with cgroup2 manually mounted

How reproducible:
100%

Versions:
kernel-4.18.0-107.el8.x86_64
libvirt-5.6.0-1.module+el8.1.0+3890+4d3d259c.x86_64
qemu-kvm-4.0.0-6.module+el8.1.0+3736+a2aefea3.x86_64

Steps:
1. adding "cgroup_no_v1=all" in kernel cmd line, and reboot host
$ grub2-editenv - list | grep kernelopts
kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet

$ grub2-editenv - set "kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet cgroup_no_v1=all"

$ reboot

2. having a running vm
[root@dell-per740-07 /]# virsh list
 Id   Name             State
--------------------------------
 2    avocado-vt-vm1   running

3. mount cgroupv2 to some path
[root@dell-per740-07 /]# mount -t cgroup2 cgroup2 /cgroup2/

4. do blkiotune to that vm
[root@dell-per740-07 /]# virsh blkiotune avocado-vt-vm1 --weight 999
error: Disconnected from qemu:///system due to keepalive timeout
error: Unable to change blkio parameters
error: internal error: connection closed due to keepalive timeout



=========================================================
The backtrace info for the crash in step 4 is as follow:
=========================================================
Thread 2 "libvirtd" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffedf06700 (LWP 9137)]
0x00007ffff7424caa in virCgroupBackendForController () from /lib64/libvirt.so.0
Missing separate debuginfos, use: yum debuginfo-install libiscsi-1.18.0-8.module+el8.1.0+3554+1a3a94a6.x86_64 libselinux-2.9-2.1.el8.x86_64 netcf-libs-0.2.8-11.module+el8.1.0+3554+1a3a94a6.x86_64
(gdb) bt
#0  0x00007ffff7424caa in virCgroupBackendForController () from /lib64/libvirt.so.0
#1  0x00007ffff741f645 in virCgroupSetBlkioWeight () from /lib64/libvirt.so.0
#2  0x00007fffae59174b in qemuDomainSetBlkioParameters () from /usr/lib64/libvirt/connection-driver/libvirt_driver_qemu.so
#3  0x00007ffff764a6e6 in virDomainSetBlkioParameters () from /lib64/libvirt.so.0
#4  0x000055555558ec03 in remoteDispatchDomainSetBlkioParameters (args=0x7fffe8002cc0, rerr=0x7fffedf05940, msg=0x55555583ff30, client=<optimized out>, server=0x5555558038a0)
    at remote/remote_daemon_dispatch_stubs.h:10295
#5  remoteDispatchDomainSetBlkioParametersHelper (server=0x5555558038a0, client=<optimized out>, msg=0x55555583ff30, rerr=0x7fffedf05940, args=0x7fffe8002cc0, ret=0x7fffe8002dd0)
    at remote/remote_daemon_dispatch_stubs.h:10262
#6  0x00007ffff758b256 in virNetServerProgramDispatch () from /lib64/libvirt.so.0
#7  0x00007ffff758fee0 in virNetServerProcessMsg () from /lib64/libvirt.so.0
#8  0x00007ffff75901d4 in virNetServerHandleJob () from /lib64/libvirt.so.0
#9  0x00007ffff74a966d in virThreadPoolWorker () from /lib64/libvirt.so.0
#10 0x00007ffff74a89ae in virThreadHelper () from /lib64/libvirt.so.0
#11 0x00007ffff4d512de in start_thread (arg=<optimized out>) at pthread_create.c:486
#12 0x00007ffff4447463 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95