Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 11 Feb 2020 07:06:15 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook
Message-ID: <20200211150615.GK2935@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <20200210170643.3544795d@gandalf.local.home>
 <20200211114954.GK14914@hirez.programming.kicks-ass.net>
 <20200211090503.68c0d70f@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200211090503.68c0d70f@gandalf.local.home>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Feb 11, 2020 at 09:05:03AM -0500, Steven Rostedt wrote:
> On Tue, 11 Feb 2020 12:49:54 +0100
> Peter Zijlstra <peterz@infradead.org> 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?
> 
> The stack tracer connects to the function tracer and is called at all
> the places that function tracing can be called from. As I like being
> able to trace RCU internal functions (especially as they are complex),
> I don't want to set them all to notrace. But, for callbacks that
> require RCU to be watching, we need this check, because there's some
> internal state that we can be in an NMI and RCU is not watching (as
> there's some places in nmi_enter that can be traced!).
> 
> And if we are tracing preempt_enable and preempt_disable (as Joel added
> trace events there), it may be the case for trace events too.

Ah, thank you for the reminder!

Should Documentation/RCU/Design/Requirements/Requirements.rst be
updated to include this?

And I have to ask...  What happens if we are very early in from-idle
NMI entry (or very late in NMI exit), such that both in_nmi() and
rcu_is_watching() are returning false?  Or did I miss a turn somewhere?

							Thanx, Paul