Occasional 'WARNING: mm/backing-dev.c:636 at cgwb_release_workfn' when running mdadm

Hi, I've been told to report this here.

In our CI test that runs some mdadm commands we occasionally (maybe 1
in 100 runs, not reproducible manually) see the following kernel warn:

[ 15.769393] TEST-64-UDEV-STORAGE.sh[245]: + mdadm -v --stop /dev/md/mdparity
[ 15.786033] ------------[ cut here ]------------
[ 15.788632] !list_empty(&wb->b_attached)
[ 15.788634] WARNING: mm/backing-dev.c:636 at
cgwb_release_workfn+0x142/0x1f0, CPU#0: kworker/0:1/11
[ 15.789235] Modules linked in: raid456 async_raid6_recov async_memcpy
async_pq async_xor async_tx raid1 intel_rapl_msr intel_rapl_common
kvm_amd ccp kvm joydev iTCO_wdt irqbypass mousedev intel_pmc_bxt
ghash_clmulni_intel vfat i2c_i801 fat aesni_intel psmouse i2c_smbus
intel_agp lpc_ich pcspkr i2c_mux virtio_balloon intel_gtt serio_raw
mac_hid tun md_mod dm_mod qemu_fw_cfg vsock_loopback
vmw_vsock_virtio_transport vmw_vsock_virtio_transport_common vsock
virtio_rng
[ 15.790846] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted
7.0.10-arch1-1 #1 PREEMPT(full)
b38726df0ec1c5aec6f05d4eab858505a5944d02
[ 15.791457] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
Arch Linux 1.17.0-2-2 04/01/2014
[ 15.791842] Workqueue: cgwb_release cgwb_release_workfn
[ 15.792166] RIP: 0010:cgwb_release_workfn+0x142/0x1f0
[ 15.792411] Code: f6 43 d8 01 75 2c 48 83 c4 08 48 89 df 48 c7 c6 00
b2 a8 ba 5b 5d e9 0d 5d dc ff e8 f8 fd ff ff 48 8d 43 b8 48 39 43 b8
74 d6 <0f> 0b f6 43 d8 01 74 d4 0f 0b eb d0 48 89 04 24 e8 69 e6 db ff
48
[ 15.793129] RSP: 0018:ffffccd280063e50 EFLAGS: 00010293
[ 15.793401] RAX: ffff89eecbae4eb8 RBX: ffff89eecbae4f00 RCX: 0000000000000008
[ 15.795755] RDX: ffff89eecbae4d80 RSI: 0000000000000202 RDI: ffff89eecae8a040
[ 15.796172] RBP: ffff89eecae8a000 R08: 0000000000000001 R09: ffffccd28018e010
[ 15.796560] R10: 0000000000000001 R11: fefefefefefefeff R12: ffff89ef3bc322c0
[ 15.796889] R13: ffff89eec083c405 R14: 0000000000000000 R15: ffff89eecbae4f00
[ 15.797298] FS: 0000000000000000(0000) GS:ffff89ef7e367000(0000)
knlGS:0000000000000000
[ 15.797637] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 15.797961] CR2: 000055cd8ab1d960 CR3: 0000000109878004 CR4: 0000000000370ef0
[ 15.798319] Call Trace:
[ 15.798454] <TASK>
[ 15.798572] process_one_work+0x19c/0x3a0
[ 15.798732] worker_thread+0x1b1/0x310
[ 15.798896] ? __pfx_worker_thread+0x10/0x10
[ 15.799164] kthread+0xe1/0x120
[ 15.799330] ? __pfx_kthread+0x10/0x10
[ 15.799536] ret_from_fork+0x2bc/0x350
[ 15.799713] ? __pfx_kthread+0x10/0x10
[ 15.800486] ret_from_fork_asm+0x1a/0x30
[ 15.800666] </TASK>
[ 15.800795] Kernel panic - not syncing: kernel: panic_on_warn set ...
[ 15.801089] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted
7.0.10-arch1-1 #1 PREEMPT(full)
b38726df0ec1c5aec6f05d4eab858505a5944d02
[ 15.801553] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
Arch Linux 1.17.0-2-2 04/01/2014
[ 15.801863] Workqueue: cgwb_release cgwb_release_workfn
[ 15.802060] Call Trace:
[ 15.802161] <TASK>
[ 15.802264] dump_stack_lvl+0x5d/0x80
[ 15.802440] vpanic+0x20a/0x410
[ 15.802568] ? cgwb_release_workfn+0x142/0x1f0
[ 15.802761] panic+0x6b/0x70
[ 15.802905] check_panic_on_warn.cold+0xf/0x1e
[ 15.803082] __warn.cold+0xa7/0x10c
[ 15.803204] ? cgwb_release_workfn+0x142/0x1f0
[ 15.803361] __report_bug+0x93/0x110
[ 15.803492] ? cgwb_release_workfn+0x142/0x1f0
[ 15.803681] ? cgwb_release_workfn+0x142/0x1f0
[ 15.804541] report_bug+0x29/0x70
[ 15.804701] handle_bug+0x167/0x360
[ 15.804862] ? cgwb_release_workfn+0x142/0x1f0
[ 15.805096] exc_invalid_op+0x1d/0x90
[ 15.805255] asm_exc_invalid_op+0x1a/0x20
[ 15.805449] RIP: 0010:cgwb_release_workfn+0x142/0x1f0
[ 15.806398] Code: f6 43 d8 01 75 2c 48 83 c4 08 48 89 df 48 c7 c6 00
b2 a8 ba 5b 5d e9 0d 5d dc ff e8 f8 fd ff ff 48 8d 43 b8 48 39 43 b8
74 d6 <0f> 0b f6 43 d8 01 74 d4 0f 0b eb d0 48 89 04 24 e8 69 e6 db ff
48
[ 15.807217] RSP: 0018:ffffccd280063e50 EFLAGS: 00010293
[ 15.808511] RAX: ffff89eecbae4eb8 RBX: ffff89eecbae4f00 RCX: 0000000000000008
[ 15.808785] RDX: ffff89eecbae4d80 RSI: 0000000000000202 RDI: ffff89eecae8a040
[ 15.809056] RBP: ffff89eecae8a000 R08: 0000000000000001 R09: ffffccd28018e010
[ 15.809397] R10: 0000000000000001 R11: fefefefefefefeff R12: ffff89ef3bc322c0
[ 15.809687] R13: ffff89eec083c405 R14: 0000000000000000 R15: ffff89eecbae4f00
[ 15.809978] process_one_work+0x19c/0x3a0
[ 15.810154] worker_thread+0x1b1/0x310
[ 15.810325] ? __pfx_worker_thread+0x10/0x10
[ 15.812450] kthread+0xe1/0x120
[ 15.812594] ? __pfx_kthread+0x10/0x10
[ 15.812738] ret_from_fork+0x2bc/0x350
[ 15.812878] ? __pfx_kthread+0x10/0x10
[ 15.813026] ret_from_fork_asm+0x1a/0x30
[ 15.813179] </TASK>
[ 15.813607] Kernel Offset: 0x39400000 from 0xffffffff81000000
(relocation range: 0xffffffff80000000-0xffffffffbfffffff)

I don't have hard data for when it started, but I don't think I've
seen it for more than a couple of months tops. This one was on 7.0.10
on Arch, but I think I've seen it on Rawhide too.

The test script that triggers it:
https://github.com/systemd/systemd/blob/d096763827718083918244eb3b7796bf27876d56/test/units/TEST-64-UDEV-STORAGE.sh#L1191

Is this a known issue? Thanks

-- 
Kind regards,
Luca Boccassi