WARNING in debug_print_object

Hello,

syzbot hit the following crash on upstream commit
255442c93843f52b6891b21d0b485bf2c97f93c3 (Thu Feb 1 03:25:25 2018 +0000)
Merge tag 'docs-4.16' of git://git.lwn.net/linux

So far this crash happened 2 times on mmots, upstream.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: <redacted>
It will help syzbot understand when the bug is fixed. See footer for details.
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------------[ cut here ]------------
ODEBUG: activate active (active state 1) object type: rcu_head hint:           (null)
WARNING: CPU: 0 PID: 5989 at lib/debugobjects.c:291 debug_print_object+0x166/0x220 lib/debugobjects.c:288
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 5989 Comm: syzkaller148927 Not tainted 4.15.0+ #290
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 panic+0x1e4/0x41c kernel/panic.c:183
 __warn+0x1dc/0x200 kernel/panic.c:547
 report_bug+0x211/0x2d0 lib/bug.c:184
 fixup_bug.part.11+0x37/0x80 arch/x86/kernel/traps.c:178
 fixup_bug arch/x86/kernel/traps.c:247 [inline]
 do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
 invalid_op+0x22/0x40 arch/x86/entry/entry_64.S:1097
RIP: 0010:debug_print_object+0x166/0x220 lib/debugobjects.c:288
RSP: 0018:ffff8801d82df400 EFLAGS: 00010082
RAX: dffffc0000000008 RBX: 0000000000000003 RCX: ffffffff815a4e0e
RDX: 0000000000000000 RSI: 1ffff1003b05be3b RDI: ffff8801db41f6d0
RBP: ffff8801d82df440 R08: 0000000000000000 R09: 1ffff1003b05be0d
R10: ffff8801d82df300 R11: ffffffff86b39018 R12: 0000000000000001
R13: ffffffff86b41f20 R14: ffffffff86012ae0 R15: 0000000000000000
 debug_object_activate+0x49b/0x730 lib/debugobjects.c:444
 debug_rcu_head_queue kernel/rcu/rcu.h:129 [inline]
 __call_rcu.constprop.67+0xf2/0xef0 kernel/rcu/tree.c:3021
 call_rcu_sched+0x12/0x20 kernel/rcu/tree.c:3100
 pppol2tp_release+0x34d/0x560 net/l2tp/l2tp_ppp.c:496
 sock_release+0x8d/0x1e0 net/socket.c:595
 sock_close+0x16/0x20 net/socket.c:1149
 __fput+0x327/0x7e0 fs/file_table.c:209
 ____fput+0x15/0x20 fs/file_table.c:243
 task_work_run+0x199/0x270 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x9bb/0x1ad0 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:968
 SYSC_exit_group kernel/exit.c:979 [inline]
 SyS_exit_group+0x1d/0x20 kernel/exit.c:977
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x440cc9
RSP: 002b:00007ffefac52e48 EFLAGS: 00000206 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440cc9
RDX: 0000000000440cc9 RSI: 000000000000002e RDI: 0000000000000000
RBP: 0000000000008561 R08: 0000000000000000 R09: 0000000000001759
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 0000000000000003 R14: 00000000006cf050 R15: 00000000004a260e

======================================================
WARNING: possible circular locking dependency detected
4.15.0+ #290 Not tainted
------------------------------------------------------
syzkaller148927/5989 is trying to acquire lock:
 ((console_sem).lock){..-.}, at: [<00000000c7e85caf>] down_trylock+0x13/0x70 kernel/locking/semaphore.c:136

but task is already holding lock:
 (&obj_hash[i].lock){-.-.}, at: [<000000003d0dc721>] debug_object_activate+0x1d7/0x730 lib/debugobjects.c:432

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&obj_hash[i].lock){-.-.}:
       __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
       _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
       __debug_object_init+0x109/0x1040 lib/debugobjects.c:343
       debug_object_init+0x17/0x20 lib/debugobjects.c:391
       debug_hrtimer_init kernel/time/hrtimer.c:411 [inline]
       debug_init kernel/time/hrtimer.c:459 [inline]
       hrtimer_init+0x8c/0x410 kernel/time/hrtimer.c:1260
       init_dl_task_timer+0x1b/0x50 kernel/sched/deadline.c:1060
       __sched_fork+0x2c4/0xb70 kernel/sched/core.c:2189
       init_idle+0x75/0x820 kernel/sched/core.c:5333
       sched_init+0xb19/0xc43 kernel/sched/core.c:6030
       start_kernel+0x452/0x819 init/main.c:585
       x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:378
       x86_64_start_kernel+0x77/0x7a arch/x86/kernel/head64.c:359
       secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237

-> #2 (&rq->lock){-.-.}:
       __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
       _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:144
       rq_lock kernel/sched/sched.h:1758 [inline]
       task_fork_fair+0x7a/0x690 kernel/sched/fair.c:9450
       sched_fork+0x435/0xc00 kernel/sched/core.c:2405
       copy_process.part.37+0x1758/0x4b60 kernel/fork.c:1746
       copy_process kernel/fork.c:1589 [inline]
       _do_fork+0x1f7/0xfe0 kernel/fork.c:2068
       kernel_thread+0x34/0x40 kernel/fork.c:2130
       rest_init+0x22/0xf0 init/main.c:402
       start_kernel+0x7f1/0x819 init/main.c:716
       x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:378
       x86_64_start_kernel+0x77/0x7a arch/x86/kernel/head64.c:359
       secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237

-> #1 (&p->pi_lock){-.-.}:
       __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
       _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
       try_to_wake_up+0xbc/0x1600 kernel/sched/core.c:1989
       wake_up_process+0x10/0x20 kernel/sched/core.c:2152
       __up.isra.0+0x1cc/0x2c0 kernel/locking/semaphore.c:262
       up+0x13b/0x1d0 kernel/locking/semaphore.c:187
       __up_console_sem+0xb2/0x1a0 kernel/printk/printk.c:245
       console_unlock+0x538/0xd70 kernel/printk/printk.c:2248
       vprintk_emit+0x4ad/0x590 kernel/printk/printk.c:1757
       vprintk_default+0x28/0x30 kernel/printk/printk.c:1796
       vprintk_func+0x57/0xc0 kernel/printk/printk_safe.c:379
       printk+0xaa/0xca kernel/printk/printk.c:1829
       regdb_fw_cb+0x1d7/0x220 net/wireless/reg.c:886
       request_firmware_work_func+0x151/0x2c0 drivers/base/firmware_class.c:1365
       process_one_work+0xbbf/0x1af0 kernel/workqueue.c:2113
       worker_thread+0x223/0x1990 kernel/workqueue.c:2247
       kthread+0x33c/0x400 kernel/kthread.c:238
       ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:542

-> #0 ((console_sem).lock){..-.}:
       lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
       __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
       _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
       down_trylock+0x13/0x70 kernel/locking/semaphore.c:136
       __down_trylock_console_sem+0xa2/0x1e0 kernel/printk/printk.c:228
       console_trylock+0x15/0x100 kernel/printk/printk.c:2065
       vprintk_emit+0x49b/0x590 kernel/printk/printk.c:1756
       vprintk_default+0x28/0x30 kernel/printk/printk.c:1796
       vprintk_func+0x57/0xc0 kernel/printk/printk_safe.c:379
       printk+0xaa/0xca kernel/printk/printk.c:1829
       __warn_printk+0x90/0xf0 kernel/panic.c:599
       debug_print_object+0x166/0x220 lib/debugobjects.c:288
       debug_object_activate+0x49b/0x730 lib/debugobjects.c:444
       debug_rcu_head_queue kernel/rcu/rcu.h:129 [inline]
       __call_rcu.constprop.67+0xf2/0xef0 kernel/rcu/tree.c:3021
       call_rcu_sched+0x12/0x20 kernel/rcu/tree.c:3100
       pppol2tp_release+0x34d/0x560 net/l2tp/l2tp_ppp.c:496
       sock_release+0x8d/0x1e0 net/socket.c:595
       sock_close+0x16/0x20 net/socket.c:1149
       __fput+0x327/0x7e0 fs/file_table.c:209
       ____fput+0x15/0x20 fs/file_table.c:243
       task_work_run+0x199/0x270 kernel/task_work.c:113
       exit_task_work include/linux/task_work.h:22 [inline]
       do_exit+0x9bb/0x1ad0 kernel/exit.c:865
       do_group_exit+0x149/0x400 kernel/exit.c:968
       SYSC_exit_group kernel/exit.c:979 [inline]
       SyS_exit_group+0x1d/0x20 kernel/exit.c:977
       entry_SYSCALL_64_fastpath+0x29/0xa0

other info that might help us debug this:

Chain exists of:
  (console_sem).lock --> &rq->lock --> &obj_hash[i].lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&obj_hash[i].lock);
                               lock(&rq->lock);
                               lock(&obj_hash[i].lock);
  lock((console_sem).lock);

 *** DEADLOCK ***

2 locks held by syzkaller148927/5989:
 #0:  (sk_lock-AF_PPPOX){+.+.}, at: [<0000000075d2b1cc>] lock_sock include/net/sock.h:1461 [inline]
 #0:  (sk_lock-AF_PPPOX){+.+.}, at: [<0000000075d2b1cc>] pppol2tp_release+0x96/0x560 net/l2tp/l2tp_ppp.c:472
 #1:  (&obj_hash[i].lock){-.-.}, at: [<000000003d0dc721>] debug_object_activate+0x1d7/0x730 lib/debugobjects.c:432

stack backtrace:
CPU: 0 PID: 5989 Comm: syzkaller148927 Not tainted 4.15.0+ #290
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_circular_bug.isra.38+0x2cd/0x2dc kernel/locking/lockdep.c:1223
 check_prev_add kernel/locking/lockdep.c:1863 [inline]
 check_prevs_add kernel/locking/lockdep.c:1976 [inline]
 validate_chain kernel/locking/lockdep.c:2417 [inline]
 __lock_acquire+0x30a8/0x3e00 kernel/locking/lockdep.c:3431
 lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
 _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
 down_trylock+0x13/0x70 kernel/locking/semaphore.c:136
 __down_trylock_console_sem+0xa2/0x1e0 kernel/printk/printk.c:228
 console_trylock+0x15/0x100 kernel/printk/printk.c:2065
 vprintk_emit+0x49b/0x590 kernel/printk/printk.c:1756
 vprintk_default+0x28/0x30 kernel/printk/printk.c:1796
 vprintk_func+0x57/0xc0 kernel/printk/printk_safe.c:379
 printk+0xaa/0xca kernel/printk/printk.c:1829
 __warn_printk+0x90/0xf0 kernel/panic.c:599
 debug_print_object+0x166/0x220 lib/debugobjects.c:288
 debug_object_activate+0x49b/0x730 lib/debugobjects.c:444
 debug_rcu_head_queue kernel/rcu/rcu.h:129 [inline]
 __call_rcu.constprop.67+0xf2/0xef0 kernel/rcu/tree.c:3021
 call_rcu_sched+0x12/0x20 kernel/rcu/tree.c:3100
 pppol2tp_release+0x34d/0x560 net/l2tp/l2tp_ppp.c:496
 sock_release+0x8d/0x1e0 net/socket.c:595
 sock_close+0x16/0x20 net/socket.c:1149
 __fput+0x327/0x7e0 fs/file_table.c:209
 ____fput+0x15/0x20 fs/file_table.c:243
 task_work_run+0x199/0x270 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x9bb/0x1ad0 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:968
 SYSC_exit_group kernel/exit.c:979 [inline]
 SyS_exit_group+0x1d/0x20 kernel/exit.c:977
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x440cc9
RSP: 002b:00007ffefac52e48 EFLAGS: 00000206 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440cc9
RDX: 0000000000440cc9 RSI: 000000000000002e RDI: 0000000000000000
RBP: 0000000000008561 R08: 0000000000000000 R09: 0000000000001759
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 0000000000000003 R14: 00000000006cf050 R15: 00000000004a260e
Shutting down cpus with NMI
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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