On Mon, May 08, 2017 at 03:43:33PM -0500, Josh Poimboeuf wrote: > 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 This would be a problem if step 2's NMI hit rcu_irq_enter(), rcu_irq_exit(), and friends in just the wrong place. I would suggest that ftrace() do something like this... if (in_nmi()) rcu_nmi_enter(); else rcu_irq_enter(); Except that, as Steven will quickly point out, this won't work at the very edges of the NMI, when NMI_MASK won't be set in preempt_count(). Other thoughts? Thanx, Paul -- 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
