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

[Paul E. McKenney]

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

This would be a problem if step 2's NMI hit rcu_irq_enter(),
rcu_irq_exit(), and friends in just the wrong place.

I would suggest that ftrace() do something like this...

	if (in_nmi())
		rcu_nmi_enter();
	else
		rcu_irq_enter();

Except that, as Steven will quickly point out, this won't work at the
very edges of the NMI, when NMI_MASK won't be set in preempt_count().

Other thoughts?

							Thanx, Paul