After running kernel 5.12-rc1 for short time, it crashed:

..................................
usb 1-1.1.1.5: USB disconnect, device number 11
usb 1-1.1.1.5.4: USB disconnect, device number 12
cp210x ttyUSB0: cp210x converter now disconnected from ttyUSB0
BUG: kernel NULL pointer dereference, address: 0000000000000278
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] SMP NOPTI
CPU: 0 PID: 1712 Comm: kworker/0:0 Tainted: G O 5.12.0-rc1 #1
Hardware name: Hewlett-Packard HP Z220 CMT Workstation/1790, BIOS K51 v01.87 06/10/2019
Workqueue: usb_hub_wq hub_event
RIP: 0010:gpiodevice_release+0xc/0x70
Code: c0 0f b6 c0 5b c3 85 c0 5b 0f 95 c0 0f b6 c0 c3 0f 0b eb b1 b8 fb ff ff ff 5b c3 0f 1f 00 55 48 8b 6f 78 48 c7 c7 d0 24 13 bd <48> 8b 95 78 02 00 00 48 8b 85 80 02 00 00 48 89 42 08 48 89 10 8b
RSP: 0018:ffffa3594277fb10 EFLAGS: 00010282
RAX: ffffffffbc414070 RBX: 00000000ffffffff RCX: 0000000000000282
RDX: ffff916f049e7998 RSI: 0000000000000282 RDI: ffffffffbd1324d0
RBP: 0000000000000000 R08: 000000000000003f R09: 0000000000000282
R10: 0000000000000001 R11: ffffffffbd1646a0 R12: ffff916ec5658c00
R13: ffffffffbd145e60 R14: ffff916ed5064960 R15: 0000000000000002
FS: 0000000000000000(0000) GS:ffff9171cdc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000278 CR3: 000000014431a002 CR4: 00000000001706f0
Call Trace:
device_release+0x2f/0x80
kobject_put+0x63/0xc0
cp210x_disconnect+0x1b/0x30 [cp210x]
usb_serial_disconnect+0xe1/0x130
usb_unbind_interface+0x65/0x1c0
__device_release_driver+0x144/0x1f0
device_release_driver+0x1f/0x30
bus_remove_device+0xcd/0x110
device_del+0x185/0x450
? kobject_put+0x70/0xc0
usb_disable_device+0xac/0x150
usb_disconnect.cold+0x60/0x1a4
usb_disconnect.cold+0x29/0x1a4
usb_disconnect.cold+0x29/0x1a4
hub_event+0x5cf/0x1230
? __switch_to_asm+0x42/0x70
process_one_work+0x1ea/0x340
worker_thread+0x48/0x3c0
? rescuer_thread+0x380/0x380
kthread+0x111/0x130
? __kthread_bind_mask+0x60/0x60
ret_from_fork+0x22/0x30
Modules linked in: rfcomm cmac bnep btusb btrtl btbcm btintel bluetooth ecdh_generic ecc hid_logitech_hidpp joydev cp210x hid_logitech_dj uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_common snd_usb_audio snd_usbmidi_lib videodev snd_rawmidi mc iptable_filter bpfilter vmnet(O) vmw_vsock_vmci_transport vsock vmw_vmci vmmon(O) sunrpc x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel snd_hda_codec_realtek kvm snd_hda_codec_generic ledtrig_audio snd_hda_codec_hdmi irqbypass snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hda_core iTCO_wdt snd_hwdep snd_seq snd_seq_device hp_wmi at24 sparse_keymap rfkill rapl wmi_bmof iTCO_vendor_support snd_pcm intel_cstate i2c_i801 intel_uncore snd_timer i2c_smbus pcspkr snd soundcore lpc_ich wmi drm zram ip_tables x_tables crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel r8169 e1000e fuse
CR2: 0000000000000278
---[ end trace c0708be999dd76ef ]---
RIP: 0010:gpiodevice_release+0xc/0x70
Code: c0 0f b6 c0 5b c3 85 c0 5b 0f 95 c0 0f b6 c0 c3 0f 0b eb b1 b8 fb ff ff ff 5b c3 0f 1f 00 55 48 8b 6f 78 48 c7 c7 d0 24 13 bd <48> 8b 95 78 02 00 00 48 8b 85 80 02 00 00 48 89 42 08 48 89 10 8b
RSP: 0018:ffffa3594277fb10 EFLAGS: 00010282
RAX: ffffffffbc414070 RBX: 00000000ffffffff RCX: 0000000000000282
RDX: ffff916f049e7998 RSI: 0000000000000282 RDI: ffffffffbd1324d0
RBP: 0000000000000000 R08: 000000000000003f R09: 0000000000000282
R10: 0000000000000001 R11: ffffffffbd1646a0 R12: ffff916ec5658c00
R13: ffffffffbd145e60 R14: ffff916ed5064960 R15: 0000000000000002
FS: 0000000000000000(0000) GS:ffff9171cdc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000278 CR3: 000000014431a002 CR4: 00000000001706f0

Trying to reboot afterwards:

# shutdown -r now
Session terminated, killing shell...Bluetooth: hci0: sending frame failed (-19)
alsactl[800]: segfault at 28 ip 000055cd5969d92a sp 00007ffdddeaa340 error 4 in alsactl[55cd5969b000+10000]
Code: 89 ef e8 59 de ff ff 48 8d 15 54 d7 00 00 e9 4a ff ff ff e8 f8 e1 ff ff 0f 1f 84 00 00 00 00 00 41 54 49 89 fc 55 53 48 8b 2f <8b> 55 28 85 d2 7e 1d 31 db 0f 1f 44 00 00 48 8b 45 20 48 8b 3c d8

This was triggered when (re)connecting USB devices via a KVM, so might not occur on stand-alone systems?     More investigation needed, and I’ll have to change nvidia (tainted!) for nouveau if I decide to ‘report’ it!

Robert Gadsdon.   March 1st 2021.