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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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

-- 
Josh