Messages in this thread | | | Date | Thu, 18 May 2017 12:50:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace |
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On Thu, May 18, 2017 at 03:30:26PM -0400, Steven Rostedt wrote: > On Thu, 18 May 2017 11:45:28 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > 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@linux.vnet.ibm.com> wrote: > > > > > > > On Thu, May 18, 2017 at 09:38:09AM -0400, Steven Rostedt wrote: > > > > > > > > > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > > > > > > > > > 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@goodmis.org > > > > > > > > > > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> > > > > > --- > > > > > > > > > > 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. > > > > Hmm, so I can just replace the entire if with: > > if (rcu_is_watching()) > > because it should be watching in NMI context, correct?
Yes, and that would be even better! ;-)
However, this assumes that there is no possibility of this being called during the time that in_nmi() returns true but RCU is not aware of the NMI. (Yes, we did go through this a bit earlier, just my usual paranoia showing...)
Thanx, Paul
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