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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 25 Jun 2018 13:47:45 -0700
Message-ID: <CALCETrVD6CxY0gDcfNfy8rNK74UZKnVvBJfiK7pLbZLiqgEbOg@mail.gmail.com>
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>, joel@joelfernandes.org,
        Byungchul Park <byungchul.park@lge.com>,
        byungchul park <max.byungchul.park@gmail.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@lge.com,
        Andrew Lutomirski <luto@kernel.org>
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On Mon, Jun 25, 2018 at 1:45 PM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> 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.
>

As a straightforward example, NMI and MCE can nest inside each other.
IIRC we treat #DB somewhat NMI-ish-ly as well.

--Andy


>                                                         Thanx, Paul
>