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Date:   Tue, 18 Feb 2020 21:40:21 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        rostedt@goodmis.org, mingo@kernel.org, gregkh@linuxfoundation.org,
        gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
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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.