Messages in this thread | | | Date | Tue, 11 Feb 2020 11:35:05 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook |
| |
On Tue, 11 Feb 2020 11:27:36 -0500 (EST) Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> ----- On Feb 11, 2020, at 11:18 AM, rostedt rostedt@goodmis.org wrote: > > > On Tue, 11 Feb 2020 16:34:52 +0100 > > Peter Zijlstra <peterz@infradead.org> wrote: > > > >> > + if (unlikely(in_nmi())) > >> > + goto out; > >> > >> unless I'm mistaken, we can simply do rcu_nmi_enter() in this case, and > >> rcu_nmi_exit() on the other end. > >> > >> > + rcu_irq_enter_irqson(); > > > > The thing is, I don't think this can ever happen. We've had in the > > tracepoint.h: > > > > /* srcu can't be used from NMI */ \ > > WARN_ON_ONCE(rcuidle && in_nmi()); \ > > > > And this has yet to trigger. > > But that "rcuidle" state is defined on a per-tracepoint basis, whereas > "!rcu_is_watching()" is a state which depends on the current execution > context. I don't follow how the fact that this WARN_ON_ONCE() never > triggered allows us to infer anything about (!rcu_is_watching() && in_nmi()). >
The "_rcuidle()" version of the tracepoint was to be used in places that RCU may not be watching, otherwise you would get a lockdep splat.
As that "rcuidle" variable is a hardcoded constant, it would be compiled out when rcuidle is zero. But, in all purposes, rcuidle is basically equivalent to rcu_is_watching(), because if it wasn't you would have lockdep splats.
-- Steve
| |