Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks

[Steven Rostedt]

On Mon, 25 Jun 2018 09:39:51 -0700
Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> For whatever its worth, I made some notes of what I understood from reading
> the code and old posts because I was sure I would otherwise forget
> everything:
> http://www.joelfernandes.org/linuxinternals/2018/06/15/rcu-dynticks.html

Nice write up. I may point some people to this ;-)

Anyway "complications due to nested NMIs (yes NMIs can nest!)"

What arch allows for NMIs to nest. Because we don't let that happen on
x86, and there's code that I know of that is called by NMIs that is not
re-entrant, and can crash if we allow for NMIs to nest. For example
"in_nmi()" will not show that we are in_nmi() if we allow for nesting
of NMIs. It has a single bit that gets incremented when we enter NMI
code, and cleared when we leave it.

-- Steve