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

From: Steven Rostedt
Date: Thu May 18 2017 - 12:39:25 EST


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 :-(

-- 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
> >