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

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