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 -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
