I'm seeing RCU warnings in Linus's current tree (like 87dfd85c38923acd9517e8df4afc908565df0961) that come from RCU:

WARNING: CPU: 0 PID: 0 at kernel/rcu/tree_exp.h:787 rcu_exp_handler+0x35/0xe0

But they *ONLY* occur on a system with a newer iwlwifi device:

aa:00.0 Network controller: Intel Corporation Wi-Fi 6 AX210/AX211/AX411 160MHz (rev 1a)

and never in a VM or on an older device (like an 8260).  During a bisect the only seem to occur with the "83" version of the firmware.

iwlwifi 0000:aa:00.0: loaded firmware version 83.e8f84e98.0 ty-a0-gf-a0-83.ucode op_mode iwlmvm

The first warning gets spit out within a millisecond of the last printk() from the iwlwifi driver.  They eventually result in a big spew of RCU messages like this:

[   27.124796] rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 0-...D } 125 jiffies s: 193 root: 0x1/.
[   27.126466] rcu: blocking rcu_node structures (internal RCU debug):
[   27.128114] Sending NMI from CPU 3 to CPUs 0:
[   27.128122] NMI backtrace for cpu 0 skipped: idling at intel_idle+0x5f/0xb0
[   27.159757] loop30: detected capacity change from 0 to 8
[   27.204967] rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 0-...D } 145 jiffies s: 193 root: 0x1/.
[   27.206353] rcu: blocking rcu_node structures (internal RCU debug):
[   27.207751] Sending NMI from CPU 3 to CPUs 0:
[   27.207825] NMI backtrace for cpu 0 skipped: idling at intel_idle+0x5f/0xb0

I usually see them at boot.  In that case, they usually hang the system and keep it from booting.  I've also encountered them at reboots and also seen them *not* be fatal at boot.  I suspect it has to do with which CPU gets wedged.