Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

Paul E. McKenney
2020-02-18T20:17:29Z

On Tue, Feb 18, 2020 at 08:58:31PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote:
>
> > > > That _should_ already be the case today. That is, if we end up in a
> > > > tracer and in_nmi() is unreliable we're already screwed anyway.
>
> > I removed the static from rcu_nmi_enter()/exit() as it is called from
> > outside, that makes it build now. Updated below is Paul's diff. I also added
> > NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed
> > asymmetric.
>
> > +__always_inline void rcu_nmi_exit(void)
> > {
> > struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> >
> > @@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> > trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
> > WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> >
> > - if (irq)
> > + if (!in_nmi())
> > rcu_prepare_for_idle();
> >
> > rcu_dynticks_eqs_enter();
> >
> > - if (irq)
> > + if (!in_nmi())
> > rcu_dynticks_task_enter();
> > }
>
> Boris and me have been going over the #MC code (and finding loads of
> 'interesting' code) and ran into ist_enter(), whish has the following
> code:
>
> /*
> * We might have interrupted pretty much anything. In
> * fact, if we're a machine check, we can even interrupt
> * NMI processing. We don't want in_nmi() to return true,
> * but we need to notify RCU.
> */
> rcu_nmi_enter();
>
>
> Which, to me, sounds all sorts of broken. The IST (be it #DB or #MC) can
> happen while we're holding all sorts of locks. This must be an NMI-like
> context.

Ouch! Looks like I need to hold off on getting rid of the "irq"
parameters if in_nmi() isn't going to be accurate.

Thanx, Paul