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    SubjectRe: [RFC][PATCH 1/5] tracing: Make sure RCU is watching before calling a stack trace
    On Fri, 12 May 2017 13:31:45 -0700
    "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

    > On Fri, May 12, 2017 at 04:05:32PM -0400, Steven Rostedt wrote:
    > > On Fri, 12 May 2017 11:50:03 -0700
    > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
    > >
    > > > On Fri, May 12, 2017 at 02:36:19PM -0400, Steven Rostedt wrote:
    > > > > On Fri, 12 May 2017 11:25:35 -0700
    > > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
    > > > >
    > > > > > On Fri, May 12, 2017 at 01:15:45PM -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.
    > > > > > >
    > > > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    > > > > >
    > > > > > Assuming that you never get to __trace_stack() if in an NMI handler,
    > > > > > this looks good to me!
    > > > > >
    > > > > > In contrast, if if __trace_stack() ever is called from an NMI handler,
    > > > > > invoking rcu_irq_enter() can be fatal.
    > > > >
    > > > > Then someone may die.
    > > > >
    > > > > OK, what's the case of running this in nmi? How does perf do it?
    > > >
    > > > I have no idea. If it cannot happen, then it cannot happen and all
    > > > is well, RCU is happy, and I am happy. ;-)
    > > >
    > > > > Do we just skip the check if it is in an nmi?
    > > > >
    > > > > if (!in_nmi()) {
    > > > > if (unlikely(rcu_irq_enter_disabled()))
    > > > > return;
    > > > > rcu_irq_enter();
    > > > > }
    > > > >
    > > > > __ftrace_trace_stack();
    > > > >
    > > > > if (!in_nmi())
    > > > > rcu_irq_exit();
    > > > >
    > > > > ?
    > > >
    > > > If it -can- happen, bail out of the function without doing the
    > >
    > > Why?
    > >
    > > > __ftrace_trace_stack()? Or does that just cause other problems further
    > > > down the road? Or BUG_ON(in_nmi())?
    > >
    > > Why?
    > >
    > > > But again if it cannot happen, no problem and no need for extra code.
    > >
    > > We can't call stack trace from nmi anymore? It calls rcu_read_lock()
    > > which is why we need to make sure rcu is watching, otherwise lockdep
    > > complains.
    >
    > Ah, finally got it! If we are in_nmi(), you are relying on the
    > NMI handler's call to rcu_nmi_enter(), which works. The piece I was
    > forgetting was that you also recently said in an unrelated LKML thread
    > that all the functions called at the very beginings and ends of NMI
    > handlers (which can see !in_nmi()) are marked notrace, so that should
    > be covered as well.
    >
    > So never mind! (And thank you for the explanation.)

    Is this an Acked-by?

    -- Steve

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