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Date:   Sat, 23 Jun 2018 10:53:56 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>, joel@joelfernandes.org,
        max.byungchul.park@gmail.com,
        Byungchul Park <byungchul.park@lge.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
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks
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On Fri, Jun 22, 2018 at 03:03:35PM -0700, Andy Lutomirski wrote:
> On Fri, Jun 22, 2018 at 2:14 PM Paul E. McKenney
> <paulmck@linux.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@linux.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.

Or the obscure features identified, as the case may be.  ;-)

Either way, will do!

							Thanx, Paul