kobject: 'þ€' (00000000269885e6): kobject_cleanup, parent           (null)
======================================================
WARNING: possible circular locking dependency detected
4.20.0 #387 Not tainted
------------------------------------------------------
kworker/0:2/3428 is trying to acquire lock:
0000000069572781 (&mdev->req_queue_mutex){+.+.}, at: v4l2_release+0x1d7/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:455

but task is already holding lock:
------------[ cut here ]------------
00000000a66677b2 ((delayed_fput_work).work){+.+.}, at: process_one_work+0xb9a/0x1c40 kernel/workqueue.c:2128
downgrading a read lock
WARNING: CPU: 1 PID: 15561 at kernel/locking/lockdep.c:3556 __lock_downgrade kernel/locking/lockdep.c:3556 [inline]
WARNING: CPU: 1 PID: 15561 at kernel/locking/lockdep.c:3556 lock_downgrade+0x4d7/0x900 kernel/locking/lockdep.c:3819

which lock already depends on the new lock.

Kernel panic - not syncing: panic_on_warn set ...

the existing dependency chain (in reverse order) is:
CPU: 1 PID: 15561 Comm: blkid Not tainted 4.20.0 #387

-> #3 ((delayed_fput_work).work){+.+.}:
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
       process_one_work+0xc0a/0x1c40 kernel/workqueue.c:2129
Call Trace:
       worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1d3/0x2c6 lib/dump_stack.c:113
       kthread+0x35a/0x440 kernel/kthread.c:246
       ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
 panic+0x2ad/0x55c kernel/panic.c:188

-> #2 ((wq_completion)"events"){+.+.}:
       flush_workqueue+0x30a/0x1e10 kernel/workqueue.c:2655
       flush_scheduled_work include/linux/workqueue.h:599 [inline]
       vim2m_stop_streaming+0x7c/0x2c0 drivers/media/platform/vim2m.c:811
       __vb2_queue_cancel+0x14f/0xd50 drivers/media/common/videobuf2/videobuf2-core.c:1843
       vb2_core_queue_release+0x26/0x80 drivers/media/common/videobuf2/videobuf2-core.c:2255
 __warn.cold.8+0x20/0x45 kernel/panic.c:540
       vb2_queue_release+0x15/0x20 drivers/media/common/videobuf2/videobuf2-v4l2.c:842
       v4l2_m2m_ctx_release+0x1e/0x35 drivers/media/v4l2-core/v4l2-mem2mem.c:930
 report_bug+0x254/0x2d0 lib/bug.c:186
       vim2m_release+0xe6/0x150 drivers/media/platform/vim2m.c:977
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
       v4l2_release+0x224/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:456
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:290
       __fput+0x385/0xa30 fs/file_table.c:278
       ____fput+0x15/0x20 fs/file_table.c:309
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
       task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
RIP: 0010:__lock_downgrade kernel/locking/lockdep.c:3556 [inline]
RIP: 0010:lock_downgrade+0x4d7/0x900 kernel/locking/lockdep.c:3819
       tracehook_notify_resume include/linux/tracehook.h:188 [inline]
       exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
Code: 00 00 fc ff df 41 c6 44 05 00 f8 e9 1b ff ff ff 48 c7 c7 a0 67 2b 88 4c 89 9d 58 ff ff ff 48 89 85 60 ff ff ff e8 29 4b e7 ff <0f> 0b 48 8b 85 60 ff ff ff 4c 8d 4d d8 4c 89 e9 48 ba 00 00 00 00
       prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
       syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
       do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
RSP: 0018:ffff88818041fb70 EFLAGS: 00010086
       entry_SYSCALL_64_after_hwframe+0x49/0xbe
RAX: 0000000000000000 RBX: 1ffff11030083f74 RCX: 0000000000000000

-> #1 (&dev->dev_mutex){+.+.}:
RDX: 0000000000000000 RSI: ffffffff8164e045 RDI: 0000000000000006
RBP: ffff88818041fc28 R08: ffff88817fd2c100 R09: fffffbfff12b211c
       __mutex_lock_common kernel/locking/mutex.c:925 [inline]
       __mutex_lock+0x166/0x1700 kernel/locking/mutex.c:1072
R10: fffffbfff12b211c R11: ffffffff895908e3 R12: ffffffff8b1007e0
       mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
R13: ffff88818041fbc0 R14: 0000000000000003 R15: ffff88817fd2c100
       vim2m_release+0xbc/0x150 drivers/media/platform/vim2m.c:976
       v4l2_release+0x224/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:456
       __fput+0x385/0xa30 fs/file_table.c:278
       ____fput+0x15/0x20 fs/file_table.c:309
       task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
 downgrade_write+0x76/0x270 kernel/locking/rwsem.c:147
       tracehook_notify_resume include/linux/tracehook.h:188 [inline]
       exit_to_usermode_loop+0x318/0x380 arch/x86/entry/common.c:166
       prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
       syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
       do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&mdev->req_queue_mutex){+.+.}:
       lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
 __do_munmap+0xcb0/0xf50 mm/mmap.c:2812
       __mutex_lock_common kernel/locking/mutex.c:925 [inline]
       __mutex_lock+0x166/0x1700 kernel/locking/mutex.c:1072
 __vm_munmap+0x138/0x1f0 mm/mmap.c:2837
       mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
       v4l2_release+0x1d7/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:455
       __fput+0x385/0xa30 fs/file_table.c:278
       delayed_fput+0x55/0x80 fs/file_table.c:304
 __do_sys_munmap mm/mmap.c:2862 [inline]
 __se_sys_munmap mm/mmap.c:2859 [inline]
 __x64_sys_munmap+0x65/0x80 mm/mmap.c:2859
       process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
       worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
       kthread+0x35a/0x440 kernel/kthread.c:246
       ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352

other info that might help us debug this:

Chain exists of:
  &mdev->req_queue_mutex --> (wq_completion)"events" --> (delayed_fput_work).work

 Possible unsafe locking scenario:

       CPU0                    CPU1
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
       ----                    ----
RIP: 0033:0x7f7bb0f7a417
  lock((delayed_fput_work).work);
Code: f0 ff ff 73 01 c3 48 8d 0d 8a ad 20 00 31 d2 48 29 c2 89 11 48 83 c8 ff eb eb 90 90 90 90 90 90 90 90 90 b8 0b 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8d 0d 5d ad 20 00 31 d2 48 29 c2 89
                               lock((wq_completion)"events");
RSP: 002b:00007ffe68783fb8 EFLAGS: 00000203 ORIG_RAX: 000000000000000b
                               lock((delayed_fput_work).work);
RAX: ffffffffffffffda RBX: 00007f7bb11851c8 RCX: 00007f7bb0f7a417
  lock(&mdev->req_queue_mutex);
RDX: 0000000000106300 RSI: 00000000000033ef RDI: 00007f7bb117d000

 *** DEADLOCK ***

RBP: 00007ffe68784120 R08: 0000000000000001 R09: 0000000000000007
2 locks held by kworker/0:2/3428:
R10: 00007f7bb0f74a0b R11: 0000000000000203 R12: 0000000000a63054
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: __write_once_size include/linux/compiler.h:218 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: atomic64_set include/asm-generic/atomic-instrumented.h:40 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: atomic_long_set include/asm-generic/atomic-long.h:59 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: set_work_data kernel/workqueue.c:617 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0: 00000000667151fd ((wq_completion)"events"){+.+.}, at: process_one_work+0xb43/0x1c40 kernel/workqueue.c:2124
R13: 000000e100a63054 R14: 000000e1009778b5 R15: 00007f7bb117a740
 #1: 00000000a66677b2 ((delayed_fput_work).work){+.+.}, at: process_one_work+0xb9a/0x1c40 kernel/workqueue.c:2128

stack backtrace:
CPU: 0 PID: 3428 Comm: kworker/0:2 Not tainted 4.20.0 #387
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events delayed_fput
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1d3/0x2c6 lib/dump_stack.c:113
 print_circular_bug.isra.33.cold.54+0x1bd/0x27d kernel/locking/lockdep.c:1221
 check_prev_add kernel/locking/lockdep.c:1863 [inline]
 check_prevs_add kernel/locking/lockdep.c:1976 [inline]
 validate_chain kernel/locking/lockdep.c:2347 [inline]
 __lock_acquire+0x3360/0x4c20 kernel/locking/lockdep.c:3341
 lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
 __mutex_lock_common kernel/locking/mutex.c:925 [inline]
 __mutex_lock+0x166/0x1700 kernel/locking/mutex.c:1072
 mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
 v4l2_release+0x1d7/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:455
 __fput+0x385/0xa30 fs/file_table.c:278
 delayed_fput+0x55/0x80 fs/file_table.c:304
 process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
 worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
 kthread+0x35a/0x440 kernel/kthread.c:246
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..