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

Paul E. McKenney
2020-02-18T21:39:26Z

On Tue, Feb 18, 2020 at 09:40:21PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 18, 2020 at 12:17:28PM -0800, Paul E. McKenney wrote:
> > 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.
>
> I'm currently trying to twist my brain around all this, because I
> suspect it's all completely broken one way or another.
>
> But yes, we definitely need to fix this before your patch goes in.

OK. I will drop it later today, but leave tag in_nmi.2020.02.18a
pointing at it for future reference.

Thanx, Paul