Pi 400 WARNs when shutting down

Reproduce shutdown on the Raspberry Pi 400 with the UART enabled, then trace the call chain from gpio_poweroff_do_poweroff through rpi_exp_gpio_dir_out and rpi_firmware_property. Check the related kernel power-off and GPIO paths first; done means shutdown still powers off normally without emitting the RCU warning.

Describe the bug

With a serial cable attached reveals that the Pi 400 kernel is emitting a WARN message when shutting down:

[   57.499952] ------------[ cut here ]------------
[   57.504630] Voluntary context switch within RCU read-side critical section!
[   57.504646] WARNING: CPU: 0 PID: 1 at kernel/rcu/tree_plugin.h:320 rcu_note_context_switch+0x4b4/0x5ac
[   57.521154] Modules linked in: rfcomm snd_seq_dummy snd_hrtimer snd_seq snd_seq_device cmac algif_hash aes_arm64 aes_generic algif_skcipher af_alg bnep binfmt_misc brcmfmac brcmutil hci_uart btbcm bluetooth cfg80211 rpivid_hevc(C) bcm2835_codec(C) v4l2_mem2mem bcm2835_isp(C) bcm2835_v4l2(C) ecdh_generic ecc bcm2835_mmal_vchiq(C) videobuf2_dma_contig videobuf2_vmalloc videobuf2_memops libaes raspberrypi_hwmon videobuf2_v4l2 rfkill videobuf2_common videodev raspberrypi_gpiomem vc_sm_cma(C) snd_bcm2835(C) mc nvmem_rmem uio_pdrv_genirq uio i2c_dev fuse dm_mod ip_tables x_tables ipv6 vc4 v3d gpu_sched drm_shmem_helper snd_soc_hdmi_codec drm_display_helper cec drm_dma_helper i2c_brcmstb drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm drm_panel_orientation_quirks snd_soc_core snd_compress snd_pcm_dmaengine snd_pcm snd_timer snd backlight
[   57.597444] CPU: 0 PID: 1 Comm: systemd-shutdow Tainted: G         C         6.1.0-rpi4-rpi-v8 #1  Debian 1:6.1.54-1+rpt2
[   57.608559] Hardware name: Raspberry Pi 400 Rev 1.0 (DT)
[   57.613941] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   57.620999] pc : rcu_note_context_switch+0x4b4/0x5ac
[   57.626031] lr : rcu_note_context_switch+0x4b4/0x5ac
[   57.631064] sp : ffffffc00802b7d0
[   57.634418] x29: ffffffc00802b7d0 x28: ffffff80401e0000 x27: ffffff80409decc0
[   57.641656] x26: 0000000000000000 x25: ffffff80401e0000 x24: ffffff80401e0000
[   57.648895] x23: 0000000000000000 x22: ffffffd845a68d28 x21: ffffff80401e0000
[   57.656132] x20: ffffffd845557f40 x19: ffffff80fb782f40 x18: 0000000000000006
[   57.663370] x17: 0000000000000000 x16: ffffff80fb7d92c0 x15: ffffffc00802b230
[   57.670607] x14: 0000000000000000 x13: 216e6f6974636573 x12: 206c616369746972
[   57.677844] x11: 6320656469732d64 x10: ffffffd8458e77e8 x9 : ffffffd8444fb258
[   57.685080] x8 : 00000000ffffefff x7 : ffffffd8458e4800 x6 : 00000000000001ff
[   57.692319] x5 : 000000000000bff4 x4 : 40000000fffff1ff x3 : 0000000000000000
[   57.699555] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffffff80401e0000
[   57.706792] Call trace:
[   57.709266]  rcu_note_context_switch+0x4b4/0x5ac
[   57.713946]  __schedule+0xc4/0xa10
[   57.717397]  schedule+0x60/0x100
[   57.720667]  schedule_timeout+0xa4/0x1c0
[   57.724641]  wait_for_completion_timeout+0x7c/0x150
[   57.729585]  mbox_send_message+0xa8/0x12c
[   57.733651]  rpi_firmware_property_list+0xf8/0x270
[   57.738510]  rpi_firmware_property+0x78/0x114
[   57.742925]  rpi_exp_gpio_get_polarity+0x64/0xec
[   57.747604]  rpi_exp_gpio_dir_out+0x68/0x110
[   57.751930]  gpiod_direction_output_raw_commit+0x64/0x1dc
[   57.757406]  gpiod_direction_output+0xb8/0x134
[   57.761908]  gpio_poweroff_do_poweroff+0x40/0xf0
[   57.766595]  legacy_pm_power_off+0x24/0x40
[   57.770752]  sys_off_notify+0x3c/0x64
[   57.774462]  atomic_notifier_call_chain+0x68/0xa0
[   57.779231]  do_kernel_power_off+0x5c/0x74
[   57.783383]  machine_power_off+0x54/0x60
[   57.787360]  kernel_power_off+0x78/0x84
[   57.791247]  __do_sys_reboot+0x200/0x23c
[   57.795222]  __arm64_sys_reboot+0x2c/0x3c
[   57.799286]  invoke_syscall+0x50/0x120
[   57.803084]  el0_svc_common.constprop.0+0x68/0x124
[   57.807941]  do_el0_svc_compat+0x24/0x4c
[   57.811915]  el0_svc_compat+0x30/0x94
[   57.815625]  el0t_32_sync_handler+0x98/0x140
[   57.819952]  el0t_32_sync+0x190/0x194
[   57.823661] ---[ end trace 0000000000000000 ]---

It then proceeds to power off as expected - I'm just logging this so it doesn't get forgotten.

Steps to reproduce the behaviour

1. Attach a serial cable.
2. Enable the UART.
3. Press the power button to shut it down.
4. Splat.

Device (s)

Raspberry Pi 400

System

6.1.0-rpi4-rpi-v8 #1 Debian 1:6.1.54-1+rpt2

Logs

No response

Additional context

No response