Re: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace

[Paul E. McKenney]

On Thu, May 18, 2017 at 12:39:14PM -0400, Steven Rostedt wrote:
> On Thu, 18 May 2017 08:47:11 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, May 18, 2017 at 09:38:09AM -0400, Steven Rostedt wrote:
> > > 
> > > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > > 
> > > As stack tracing now requires "rcu watching", force RCU to be watching when
> > > recording a stack trace.
> > > 
> > > Link: http://lkml.kernel.org/r/20170512172449.879684501@xxxxxxxxxxx
> > > 
> > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> > > ---
> > > 
> > > Changes since v1:
> > > 
> > >    My testing discovered that the stack trace can be called with
> > >    interrupts enabled, which is a no no to have when calling
> > >    rcu_irq_enter(). When interrupts are enabled, as with being in an
> > >    NMI, RCU will also be watching.
> > > 
> > > kernel/trace/trace.c | 26 +++++++++++++++++++++++++-
> > >  1 file changed, 25 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > index fcc9a2d..34a98ba 100644
> > > --- a/kernel/trace/trace.c
> > > +++ b/kernel/trace/trace.c
> > > @@ -2568,7 +2568,31 @@ static inline void ftrace_trace_stack(struct trace_array *tr,
> > >  void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
> > >  		   int pc)
> > >  {
> > > -	__ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL);
> > > +	struct ring_buffer *buffer = tr->trace_buffer.buffer;
> > > +
> > > +	/*
> > > +	 * When an NMI triggers, RCU is enabled via rcu_nmi_enter()
> > > +	 * Also, RCU is always enabled when interrupts are.
> > > +	 */
> > > +	if (!irqs_disabled() || in_nmi()) {  
> > 
> > You lost me on this one.  RCU might not be watching if irqs are
> > enabled, for example, in the idle loop.  What am I missing here?
> > 
> 
> Hmm, no, maybe I'm missing something :-/
> 
> OK, so if we trace in the idle loop, rcu may not be watching, so I may
> need to byte the bullet and use the rcu_irq_enter_irqon() instead :-(

If rcu_is_watching() returns false, yes.  Of course, if rcu_is_watching()
returns true, you are golden.

							Thanx, Paul

> -- Steve
> 
> 
> > 						Thanx, Paul
> > 
> > > +		__ftrace_trace_stack(buffer, flags, skip, pc, NULL);
> > > +		return;
> > > +	}
> > > +
> > > +	/*
> > > +	 * It is possible that a function is being traced in a
> > > +	 * location that RCU is not watching. A call to
> > > +	 * rcu_irq_enter() will make sure that it is, but there's
> > > +	 * a few internal rcu functions that could be traced
> > > +	 * where that wont work either. In those cases, we just
> > > +	 * do nothing.
> > > +	 */
> > > +	if (unlikely(rcu_irq_enter_disabled()))
> > > +		return;
> > > +
> > > +	rcu_irq_enter();
> > > +	__ftrace_trace_stack(buffer, flags, skip, pc, NULL);
> > > +	rcu_irq_exit();
> > >  }
> > > 
> > >  /**
> > > -- 
> > > 2.9.3
> > >   
>