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

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Date:   Mon, 25 Jun 2018 16:25:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Byungchul Park <max.byungchul.park@...il.com>,
        jiangshanlai@...il.com, josh@...htriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com, luto@...nel.org
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks

On Mon, 25 Jun 2018 09:39:51 -0700
Joel Fernandes <joel@...lfernandes.org> 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