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Date:   Mon, 25 Jun 2018 13:47:08 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Byungchul Park <byungchul.park@lge.com>,
        Byungchul Park <max.byungchul.park@gmail.com>,
        jiangshanlai@gmail.com, josh@joshtriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, kernel-team@lge.com, luto@kernel.org
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks
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On Mon, Jun 25, 2018 at 04:25:57PM -0400, Steven Rostedt wrote:
> On Mon, 25 Jun 2018 09:39:51 -0700
> Joel Fernandes <joel@joelfernandes.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.

Last I checked with Andy Lutomirski, there are a number of things that,
though not NMIs, act like NMIs and that can interrupt each others'
handlers.  This is on x86.

							Thanx, Paul