Comment 25 for bug 1403459

When I hot-plug a CPU to a Fedora 21 VM running on KVM, I get following kernel bug:

$ uname -a
Linux fedora 3.17.6-300.fc21.x86_64 #1 SMP Mon Dec 8 22:29:32 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
$ journalctl
Dez 17 10:40:21 fedora kernel: CPU1 has been hot-added
Dez 17 10:40:21 fedora kernel: SMP alternatives: switching to SMP code
Dez 17 10:40:21 fedora kernel: x86: Booting SMP configuration:
Dez 17 10:40:21 fedora kernel: smpboot: Booting Node 0 Processor 1 APIC 0x1
Dez 17 10:40:21 fedora kernel: kvm-clock: cpu 1, msr 0:3ffca041, secondary cpu clock
Dez 17 10:40:21 fedora kernel: kvm: enabling virtualization on CPU1
Dez 17 10:40:21 fedora kernel: KVM setup async PF for cpu 1
Dez 17 10:40:21 fedora kernel: kvm-stealtime: cpu 1, msr 3de2d100
Dez 17 10:40:21 fedora kernel: kobject '(null)' (ffff88003de3b4d8): tried to add an uninitialized object, something is seriously wrong.
Dez 17 10:40:21 fedora kernel: CPU: 1 PID: 1056 Comm: systemd-udevd Not tainted 3.17.6-300.fc21.x86_64 #1
Dez 17 10:40:21 fedora kernel: Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Dez 17 10:40:21 fedora kernel: 0000000000000000 00000000ffd31f20 ffff88003b593c68 ffffffff8173f8aa
Dez 17 10:40:21 fedora kernel: ffff88003de3b4d8 ffff88003b593ce0 ffffffff813850e6 ffffffff00000020
Dez 17 10:40:21 fedora kernel: ffff88003b593cf0 ffff88003b593c98 00000000ffd31f20 0000000000000002
Dez 17 10:40:21 fedora kernel: Call Trace:
Dez 17 10:40:21 fedora kernel: [<ffffffff8173f8aa>] dump_stack+0x45/0x56
Dez 17 10:40:21 fedora kernel: [<ffffffff813850e6>] kobject_add+0xc6/0xd0
Dez 17 10:40:21 fedora kernel: [<ffffffff81383bd5>] ? ida_get_new_above+0x1f5/0x220
Dez 17 10:40:21 fedora kernel: [<ffffffff81362376>] blk_mq_register_hctx+0xa6/0xc0
Dez 17 10:40:21 fedora kernel: [<ffffffff81362cb1>] blk_mq_sysfs_register+0x41/0x60
Dez 17 10:40:21 fedora kernel: [<ffffffff813606b2>] blk_mq_queue_reinit_notify+0x72/0xb0
Dez 17 10:40:21 fedora kernel: [<ffffffff810b62dd>] notifier_call_chain+0x4d/0x70
Dez 17 10:40:21 fedora kernel: [<ffffffff810b63ee>] __raw_notifier_call_chain+0xe/0x10
Dez 17 10:40:21 fedora kernel: [<ffffffff81097363>] cpu_notify+0x23/0x50
Dez 17 10:40:21 fedora kernel: [<ffffffff81097598>] _cpu_up+0x188/0x1a0
Dez 17 10:40:21 fedora kernel: [<ffffffff8109763a>] cpu_up+0x8a/0xb0
Dez 17 10:40:21 fedora kernel: [<ffffffff81738fa0>] cpu_subsys_online+0x40/0xa0
Dez 17 10:40:21 fedora kernel: [<ffffffff814975b5>] device_online+0x65/0x90
Dez 17 10:40:21 fedora kernel: [<ffffffff81497675>] online_store+0x95/0xa0
Dez 17 10:40:21 fedora kernel: [<ffffffff81494708>] dev_attr_store+0x18/0x30
Dez 17 10:40:21 fedora kernel: [<ffffffff812895fa>] sysfs_kf_write+0x3a/0x50
Dez 17 10:40:21 fedora kernel: [<ffffffff812889de>] kernfs_fop_write+0xee/0x180
Dez 17 10:40:21 fedora kernel: [<ffffffff8120c597>] vfs_write+0xb7/0x1f0
Dez 17 10:40:21 fedora kernel: [<ffffffff81060e21>] ? trace_do_page_fault+0x41/0x100
Dez 17 10:40:21 fedora kernel: [<ffffffff8120d1c5>] SyS_write+0x55/0xd0
Dez 17 10:40:21 fedora kernel: [<ffffffff81746a69>] system_call_fastpath+0x16/0x1b
Dez 17 10:40:21 fedora kernel: microcode: CPU1: patch_level=0x01000065
Dez 17 10:40:21 fedora kernel: Will online and init hotplugged CPU: 1

I initially reported this bug in openSUSE and it was fixed in the Linus tree: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=06a41a9