Comment 1 for bug 1403459

When I hot-plug a CPU to an openSUSE VM running on KVM, I get following kernel oops:

# uname -a
Linux opensuse 3.16.6-2-default #1 SMP Mon Oct 20 13:47:22 UTC 2014 (feb42ea) x86_64 x86_64 x86_64 GNU/Linux
# dmesg
[ 54.927758] CPU2 has been hot-added
[ 54.928610] smpboot: Booting Node 0 Processor 2 APIC 0x2
[ 0.004000] kvm-clock: cpu 2, msr 2:403de081, secondary cpu clock
[ 54.940001] TSC synchronization [CPU#1 -> CPU#2]:
[ 54.940001] Measured 195274448514 cycles TSC warp between CPUs, turning off TSC clock.
[ 54.940001] tsc: Marking TSC unstable due to check_tsc_sync_source failed
[ 54.960222] KVM setup async PF for cpu 2
[ 54.960265] kvm-stealtime: cpu 2, msr 24008d240
[ 54.960342] microcode: CPU2 sig=0x306c1, pf=0x1, revision=0x1
[ 55.012464] Will online and init hotplugged CPU: 2
[ 55.012574] kobject '(null)' (ffff88024009a3d8): tried to add an uninitialized object, something is seriously wrong.
[ 55.012615] CPU: 1 PID: 892 Comm: systemd-udevd Not tainted 3.16.6-2-default #1
[ 55.012649] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_171129-lamiak 04/01/2014
[ 55.012688] ffff8802347cbd98 ffffffff815ca482 ffff88024009a3d8 ffffffff8130e153
[ 55.012729] ffffffff00000020 ffff8802347cbda8 ffff8802347cbd50 00000001334da7a8
[ 55.012766] 000000000000b0a0 0000000001ca6930 0000000000000002 ffff8802334da298
[ 55.012804] Call Trace:
[ 55.012845] [<ffffffff81005096>] dump_trace+0x86/0x330
[ 55.012881] [<ffffffff810053d4>] show_stack_log_lvl+0x94/0x170
[ 55.012919] [<ffffffff81006a31>] show_stack+0x21/0x50
[ 55.012955] [<ffffffff815ca482>] dump_stack+0x41/0x51
[ 55.012990] [<ffffffff8130e153>] kobject_add+0xa3/0xb0
[ 55.013027] [<ffffffff812ed4a1>] blk_mq_register_hctx+0x91/0xb0
[ 55.013062] [<ffffffff812edd5e>] blk_mq_sysfs_register+0x3e/0x50
[ 55.013095] [<ffffffff812eb9c2>] blk_mq_queue_reinit_notify+0x72/0xb0
[ 55.013129] [<ffffffff810806bc>] notifier_call_chain+0x4c/0x70
[ 55.013162] [<ffffffff8105beef>] cpu_notify+0x1f/0x40
[ 55.013195] [<ffffffff8105c0d7>] _cpu_up+0x157/0x170
[ 55.013226] [<ffffffff8105c169>] cpu_up+0x79/0xa0
[ 55.013260] [<ffffffff815bf555>] cpu_subsys_online+0x35/0x80
[ 55.013294] [<ffffffff8140c64d>] device_online+0x5d/0xa0
[ 55.013328] [<ffffffff8140c705>] online_store+0x75/0x80
[ 55.013360] [<ffffffff8122127a>] kernfs_fop_write+0xda/0x150
[ 55.013394] [<ffffffff811b1aa2>] vfs_write+0xb2/0x1f0
[ 55.013426] [<ffffffff811b25e2>] SyS_write+0x42/0xa0
[ 55.013462] [<ffffffff815d0c2d>] system_call_fastpath+0x1a/0x1f
[ 55.013517] [<00007f9fbf3804e0>] 0x7f9fbf3804df

I can reproduce it on a Ubuntu 14.10 host with stock KVM and on Debian host with a custom KVM. The oops sometimes causes the virtual machine to freeze.