linux-kernel - Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit

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Date:   Tue, 11 Feb 2020 04:59:29 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook

On Tue, Feb 11, 2020 at 12:49:54PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote:
> > +	if (!rcu_watching) {						\
> > +		/* Can not use RCU if rcu is not watching and in NMI */	\
> > +		if (in_nmi())						\
> > +			return;						\
> > +		rcu_irq_enter_irqson();					\
> > +	}								\
> 
> I saw the same weirdness in __trace_stack(), and I'm confused by it.
> 
> How can we ever get to: in_nmi() && !rcu_watching() ? That should be a
> BUG.  In particular, nmi_enter() has rcu_nmi_enter().
> 
> Paul, can that really happen?

Not sure what the current situation is, but if I remember correctly it
used to be possible to get to an NMI handler without RCU being informed.
If NMI handlers now unconditionally inform RCU, then like you, I don't
see that the "if (in_nmi()) return" is needed.

However, a quick grep for NMI_MASK didn't show me the NMI_MASK bit
getting set.  Help?

							Thanx, Paul