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

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Date:   Fri, 22 Jun 2018 15:03:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, joel@...lfernandes.org,
        max.byungchul.park@...il.com,
        Byungchul Park <byungchul.park@....com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks

On Fri, Jun 22, 2018 at 2:14 PM Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> On Fri, Jun 22, 2018 at 05:00:42PM -0400, Steven Rostedt wrote:
> > On Fri, 22 Jun 2018 13:58:13 -0700
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > > Something like this:
> > >
> > >     IRQ entered
> > >
> > > And never exited.  Ever.  I actually saw this in 2011.
> >
> > I still believe this was actually a bug. And perhaps you made the RCU
> > code robust enough to handle this bug ;-)
>
> Welcome to my world!
>
> But I recall it being used in several places, so if it was a bug, it
> was an intentional bug.  Probably the worst kind.
>
> Sort of like nested NMIs and interrupts within NMI handlers.  ;-)
>
> > > Or something like this:
> > >
> > >     IRQ exited
> > >
> > > Without a corresponding IRQ enter.
> > >
> > > The current code handles both of these situations, at least assuming
> > > that the interrupt entry/exit happens during a non-idle period.
> > >
> > > > > So why this function-call structure?  Well, you see, NMI handlers can
> > > > > take what appear to RCU to be normal interrupts...
> > > > >
> > > > > (And I just added that fun fact to Requirements.html.)
> > > >
> > > > Yes, I'll definitely go through all the interrupt requirements in the doc and
> > > > thanks for referring me to it.
> > >
> > > My concern may well be obsolete.  It would be good if it was!  ;-)
> >
> > I'd love to mandate that irq_enter() must be paired with irq_exit(). I
> > don't really see any rationale for it to be otherwise. If there is a
> > case, perhaps it needs to be fixed.
>
> Given that the usermode helpers now look to be common code using
> workqueues, kthreads, and calls to do_execve(), it might well be that
> the days of half-interrupts are behind us.
>
> But how to actually validate this?  My offer of adding a WARN_ON_ONCE()
> and waiting a few years still stands, but perhaps you have a better
> approach.

I think you should add a WARN_ON_ONCE().  Let's get the bugs fixed.

--Andy