Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code

Paul E. McKenney
2017-05-08T21:08:43Z

On Mon, May 08, 2017 at 03:51:43PM -0500, Josh Poimboeuf wrote:
> On Mon, May 08, 2017 at 03:43:33PM -0500, Josh Poimboeuf wrote:
> > On Mon, May 08, 2017 at 01:15:58PM -0700, Paul E. McKenney wrote:
> > > On Mon, May 08, 2017 at 02:47:29PM -0500, Josh Poimboeuf wrote:
> > > > On Mon, May 08, 2017 at 03:13:22PM -0400, Steven Rostedt wrote:
> > >
> > > [ . . . ]
> > >
> > > > > If rcu is not watching, calling rcu_enter_irq() will have it watch
> > > > > again. Even in NMI context I believe.
> > > >
> > > > What if you get an NMI while running in rcu_dynticks_eqs_enter() before
> > > > it increments rdtp->dynticks? Will rcu_enter_irq() still work from the
> > > rcu_irq_enter()
> > > > NMI?
> > >
> > > The rcu_nmi_enter() function willl notice that RCU is not watching, and
> > > will therefore atomically increment RCU's dynticks-idle counter, which
> > > will be atomically incremented again upon return. Since the bottom bit
> > > of this counter controls whether or not RCU is watching, RCU will be
> > > watching during the NMI, will stop watching upon return from the NMI,
> > > which restores state so as to allow rcu_irq_enter() to cause RCU to once
> > > again watch. (NMI algorithm due to Andy Lutomirski.)
> > >
> > > > I'm just trying to understand what are the cases where rcu_enter_irq()
> > > > *doesn't* work from an ftrace handler.
> > >
> > > It doesn't work from an NMI handler. Aside from possible architecture
> > > specific special cases, it should work everywhere else.
> >
> > Ok, so just to clarify. Is there a bug in the ftrace stack tracer in
> > the following situation?
> >
> > 1. RCU isn't watching
> > 2. An NMI hits
> > 3. ist_enter() calls into the ftrace stack tracer, before
> > rcu_nmi_enter() is called, so RCU isn't watching yet
> > 4. The ftrace stack tracer calls rcu_irq_enter(), which has no effect,
> > so RCU still isn't watching
> > 5. Hilarity ensues in the ftrace stack tracer
>
> Hm, technically, ist_enter() is for exceptions other than NMI, so the
> question itself is buggy. I suppose the scenario is still possible if
> you replace NMI with a debug exception or a double fault.

There are some exceptions on some architectures that look to RCU just
like NMIs, which is why RCU has to handle nested NMIs. ;-)

Thanx, Paul