rtc: hang on boot during unregistration

[Sasha Levin <sasha.levin@oracle.com>]

Hi all,

I got the following spew while booting. My VM proceeded to hang and die.

[   16.620530] =============================================
[   16.621059] [ INFO: possible recursive locking detected ]
[   16.622024] 3.14.0-next-20140403-sasha-00019-g7474aa9-dirty #376 Not tainted
[   16.624145] ---------------------------------------------
[   16.625261] kworker/0:1/1069 is trying to acquire lock:
[   16.626300] ((&(&kobj->release)->work)){+.+.+.}, at: flush_work (kernel/workqueue.c:2880)
[   16.628258]
[   16.628258] but task is already holding lock:
[   16.629415] ((&(&kobj->release)->work)){+.+.+.}, at: process_one_work (include/linux/workqueue.h:186 kernel/workqueue.c:611 kernel/workqueue.c:638 kernel/workqueue.c:2214)
[   16.630106]
[   16.630106] other info that might help us debug this:
[   16.630106]  Possible unsafe locking scenario:
[   16.630106]
[   16.630106]        CPU0
[   16.630106]        ----
[   16.630106]   lock((&(&kobj->release)->work));
[   16.630106]   lock((&(&kobj->release)->work));
[   16.630106]
[   16.630106]  *** DEADLOCK ***
[   16.630106]
[   16.630106]  May be due to missing lock nesting notation
[   16.630106]
[   16.630106] 2 locks held by kworker/0:1/1069:
[   16.630106] #0: ("events"){.+.+.+}, at: process_one_work (include/linux/workqueue.h:186 kernel/workqueue.c:611 kernel/workqueue.c:638 kernel/workqueue.c:2214)
[   16.630106] #1: ((&(&kobj->release)->work)){+.+.+.}, at: process_one_work (include/linux/workqueue.h:186 kernel/workqueue.c:611 kernel/workqueue.c:638 kernel/workqueue.c:2214)
[   16.630106]
[   16.630106] stack backtrace:
[   16.630106] CPU: 0 PID: 1069 Comm: kworker/0:1 Not tainted 3.14.0-next-20140403-sasha-00019-g7474aa9-dirty #376
[   16.630106] Workqueue: events kobject_delayed_cleanup
[   16.630106]  ffffffffb59d09d0 ffff88007c6a58d8 ffffffffb24bfb2f 0000000000000000
[   16.630106]  ffffffffb59d09d0 ffff88007c6a59d8 ffffffffaf1c4c8a ffff88007c5a3d98
[   16.630106]  ffff88007c5a3000 ffff880000000001 ffffffffaf1c0e18 ffff88007c5a3d60
[   16.630106] Call Trace:
[   16.630106] dump_stack (lib/dump_stack.c:52)
[   16.630106] __lock_acquire (kernel/locking/lockdep.c:1740 kernel/locking/lockdep.c:1783 kernel/locking/lockdep.c:2115 kernel/locking/lockdep.c:3182)
[   16.630106] ? check_irq_usage (kernel/locking/lockdep.c:1638)
[   16.630106] ? __lock_acquire (kernel/locking/lockdep.c:3189)
[   16.630106] lock_acquire (arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602)
[   16.630106] ? flush_work (kernel/workqueue.c:2880)
[   16.630106] flush_work (kernel/workqueue.c:2884)
[   16.630106] ? flush_work (kernel/workqueue.c:2880)
[   16.630106] ? debug_smp_processor_id (lib/smp_processor_id.c:57)
[   16.630106] ? put_lock_stats.isra.12 (arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
[   16.630106] ? try_to_grab_pending (kernel/workqueue.c:1260 kernel/workqueue.c:1259)
[   16.630106] ? __cancel_work_timer (arch/x86/include/asm/paravirt.h:809 (discriminator 2) kernel/workqueue.c:2913 (discriminator 2))
[   16.630106] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[   16.630106] __cancel_work_timer (arch/x86/include/asm/bitops.h:313 kernel/workqueue.c:610 kernel/workqueue.c:644 kernel/workqueue.c:2916)
[   16.630106] cancel_work_sync (kernel/workqueue.c:2941)
[   16.630106] work_fixup_free (kernel/workqueue.c:479)
[   16.630106] __debug_check_no_obj_freed (lib/debugobjects.c:277 lib/debugobjects.c:700)
[   16.630106] debug_check_no_obj_freed (lib/debugobjects.c:726)
[   16.630106] kfree (mm/slub.c:2679 mm/slub.c:3401)
[   16.630106] ? rtc_device_release (drivers/rtc/class.c:35)
[   16.630106] rtc_device_release (drivers/rtc/class.c:35)
[   16.630106] device_release (drivers/base/core.c:246)
[   16.630106] kobject_delayed_cleanup (lib/kobject.c:629 lib/kobject.c:638)
[   16.630106] process_one_work (kernel/workqueue.c:2221 include/linux/jump_label.h:105 include/trace/events/workqueue.h:111 kernel/workqueue.c:2226)
[   16.630106] ? process_one_work (include/linux/workqueue.h:186 kernel/workqueue.c:611 kernel/workqueue.c:638 kernel/workqueue.c:2214)
[   16.630106] worker_thread (kernel/workqueue.c:2348)
[   16.630106] ? rescuer_thread (kernel/workqueue.c:2297)
[   16.630106] kthread (kernel/kthread.c:219)
[   16.630106] ? kthread_create_on_node (kernel/kthread.c:185)
[   16.630106] ret_from_fork (arch/x86/kernel/entry_64.S:555)
[   16.630106] ? kthread_create_on_node (kernel/kthread.c:185)


Thanks,
Sasha