For example, when a usb reset is received (I could reproduce it
running something very similar to this[1] in a loop) it could be
that the device is unregistered while the power_off delayed work
is still scheduled to run.

Backtrace:

WARNING: at lib/debugobjects.c:261 debug_print_object+0x7c/0x8d()
Hardware name: To Be Filled By O.E.M.
ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x26
Modules linked in: nouveau mxm_wmi btusb wmi bluetooth ttm coretemp drm_kms_helper
Pid: 2114, comm: usb-reset Not tainted 3.5.0bt-next #2
Call Trace:
 [<ffffffff8124cc00>] ? free_obj_work+0x57/0x91
 [<ffffffff81058f88>] warn_slowpath_common+0x7e/0x97
 [<ffffffff81059035>] warn_slowpath_fmt+0x41/0x43
 [<ffffffff8124ccb6>] debug_print_object+0x7c/0x8d
 [<ffffffff8106e3ec>] ? __queue_work+0x259/0x259
 [<ffffffff8124d63e>] ? debug_check_no_obj_freed+0x6f/0x1b5
 [<ffffffff8124d667>] debug_check_no_obj_freed+0x98/0x1b5
 [<ffffffffa00aa031>] ? bt_host_release+0x10/0x1e [bluetooth]
 [<ffffffff810fc035>] kfree+0x90/0xe6
 [<ffffffffa00aa031>] bt_host_release+0x10/0x1e [bluetooth]
 [<ffffffff812ec2f9>] device_release+0x4a/0x7e
 [<ffffffff8123ef57>] kobject_release+0x11d/0x154
 [<ffffffff8123ed98>] kobject_put+0x4a/0x4f
 [<ffffffff812ec0d9>] put_device+0x12/0x14
 [<ffffffffa009472b>] hci_free_dev+0x22/0x26 [bluetooth]
 [<ffffffffa0280dd0>] btusb_disconnect+0x96/0x9f [btusb]
 [<ffffffff813581b4>] usb_unbind_interface+0x57/0x106
 [<ffffffff812ef988>] __device_release_driver+0x83/0xd6
 [<ffffffff812ef9fb>] device_release_driver+0x20/0x2d
 [<ffffffff813582a7>] usb_driver_release_interface+0x44/0x7b
 [<ffffffff81358795>] usb_forced_unbind_intf+0x45/0x4e
 [<ffffffff8134f959>] usb_reset_device+0xa6/0x12e
 [<ffffffff8135df86>] usbdev_do_ioctl+0x319/0xe20
 [<ffffffff81203244>] ? avc_has_perm_flags+0xc9/0x12e
 [<ffffffff812031a0>] ? avc_has_perm_flags+0x25/0x12e
 [<ffffffff81050101>] ? do_page_fault+0x31e/0x3a1
 [<ffffffff8135eaa6>] usbdev_ioctl+0x9/0xd
 [<ffffffff811126b1>] vfs_ioctl+0x21/0x34
 [<ffffffff81112f7b>] do_vfs_ioctl+0x408/0x44b
 [<ffffffff81208d45>] ? file_has_perm+0x76/0x81
 [<ffffffff8111300f>] sys_ioctl+0x51/0x76
 [<ffffffff8158db22>] system_call_fastpath+0x16/0x1b

[1] http://cpansearch.perl.org/src/DPAVLIN/Biblio-RFID-0.03/examples/usbreset.cSigned-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>