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Date:   Tue, 18 Feb 2020 12:17:28 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.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 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