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

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Message-ID: <20180625181522.53b5d296@gandalf.local.home>
Date:   Mon, 25 Jun 2018 18:15:22 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Byungchul Park <byungchul.park@....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 13:47:08 -0700
"Paul E. McKenney" <paulmck@...ux.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@...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.  
> 
> 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.
>

Perhaps things like MCEs, but they don't call nmi_enter(). And usually
when something does, it probably puts the machine into an unstable
state. Getting RCU right, may be the least of the worries.

You may want to ask Andy if there's legitimate interruptions of NMIs
that doesn't mean "please reboot as soon as possible"?

-- Steve