Kernel page fault with the following non-sleepable locks

[Farhan Khan]

Hi all,

I received this error following what appeared to be a PCI interrupt.

Kernel page fault with the following non-sleepable locks held:
exclusive sleep mutex rtwn (network driver) r = 0 (0xfffffe000920c000) 
locked @ /usr/src/sys/dev/rtwn/pci/rtwn_pci_rx.c:290

This is followed by a stack trace, with the rtwn_pci_rx_frame as the 
last driver-relevant function. The fault code is "supervisor read 
instruction, page not present".

I understand that this happens after a mutex lock is held beyond a 
period of time, as described here:
https://www.freebsd.org/doc/en/books/faq/troubleshoot.html#idp59180136

My questions are:
A) What defines long periods of time?
B) What common patterns might cause this issue? Is there something in 
specific I should look for? Would it be a PCI read request that takes 
too long to return?

Thank you,
Farhan Khan