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[Benjamin Drung <bdrung@xxxxxxxxx>]

** Bug watch added: Debian Bug tracker #773355
   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=773355

** Also affects: linux (Debian) via
   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=773355
   Importance: Unknown
       Status: Unknown

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https://bugs.launchpad.net/bugs/1403459

Title:
   kernel bug when hotplugging a CPU

Status in linux package in Ubuntu:
  Incomplete
Status in linux package in Debian:
  Unknown
Status in linux package in openSUSE:
  Fix Released

Bug description:
  When I hot-plug a CPU to an Ubuntu 14.10 VM running on KVM, I get
  following kernel bug. Ubuntu 14.04 is not affected by this bug.

  # uname -a
  Linux ubuntu 3.16.0-28-generic #38-Ubuntu SMP Fri Dec 12 17:37:40 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
  # dmesg
  [  109.469094] CPU1 has been hot-added
  [  109.477005] SMP alternatives: switching to SMP code
  [  109.490583] x86: Booting SMP configuration:
  [  109.490586] smpboot: Booting Node 0 Processor 1 APIC 0x1
  [  109.476344] kvm-clock: cpu 1, msr 0:3ffe6041, secondary cpu clock
  [  109.586704] kvm: enabling virtualization on CPU1
  [  109.596330] KVM setup async PF for cpu 1
  [  109.596330] kvm-stealtime: cpu 1, msr 3de2d100
  [  109.596645] microcode: CPU1: patch_level=0x01000065
  [  109.596910] Will online and init hotplugged CPU: 1
  [  109.598469] kobject '(null)' (ffff88003de3b5d8): tried to add an uninitialized object, something is seriously wrong.
  [  109.599255] CPU: 0 PID: 1424 Comm: systemd-udevd Not tainted 3.16.0-28-generic #38-Ubuntu
  [  109.599257] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
  [  109.599259]  ffff880036465250 ffff88003af77c88 ffffffff81781eaa ffff88003de3b5d8
  [  109.599263]  ffff88003af77cf8 ffffffff8139e896 ffff880000000020 ffff88003af77d08
  [  109.599266]  ffff88003af77cb0 ffff88003af77cc8 ffffffff8137967b 0000000000000001
  [  109.599269] Call Trace:
  [  109.599276]  [<ffffffff81781eaa>] dump_stack+0x45/0x56
  [  109.599280]  [<ffffffff8139e896>] kobject_add+0xa6/0xb0
  [  109.599283]  [<ffffffff8137967b>] ? get_first_sibling+0x2b/0x40
  [  109.599287]  [<ffffffff8139d08d>] ? ida_get_new_above+0x1dd/0x210
  [  109.599291]  [<ffffffff81378b66>] blk_mq_register_hctx+0xa6/0xc0
  [  109.599293]  [<ffffffff813794c1>] blk_mq_sysfs_register+0x41/0x60
  [  109.599297]  [<ffffffff81376fc2>] blk_mq_queue_reinit_notify+0x72/0xb0
  [  109.599300]  [<ffffffff8109972d>] notifier_call_chain+0x4d/0x70
  [  109.599303]  [<ffffffff8109983e>] __raw_notifier_call_chain+0xe/0x10
  [  109.599306]  [<ffffffff81070133>] cpu_notify+0x23/0x50
  [  109.599309]  [<ffffffff81070348>] _cpu_up+0x168/0x180
  [  109.599312]  [<ffffffff810703ea>] cpu_up+0x8a/0xb0
  [  109.599315]  [<ffffffff81778ee0>] cpu_subsys_online+0x40/0xa0
  [  109.599319]  [<ffffffff814cf225>] device_online+0x65/0x90
  [  109.599322]  [<ffffffff814cf2cd>] online_store+0x7d/0x90
  [  109.599325]  [<ffffffff814cc658>] dev_attr_store+0x18/0x30
  [  109.599328]  [<ffffffff8125950a>] sysfs_kf_write+0x3a/0x50
  [  109.599331]  [<ffffffff812586ae>] kernfs_fop_write+0xee/0x180
  [  109.599335]  [<ffffffff811e10b7>] vfs_write+0xb7/0x1f0
  [  109.599338]  [<ffffffff811e1c66>] SyS_write+0x46/0xb0
  [  109.599342]  [<ffffffff81789eed>] system_call_fastpath+0x1a/0x1f

  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

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