linux-kernel - Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU

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Message-ID: <20180711090003.42596c2b@gandalf.local.home>
Date:   Wed, 11 Jul 2018 09:00:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Julia Cartwright <julia@...com>,
        linux-kselftest@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use
 SRCU

On Wed, 11 Jul 2018 14:49:54 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:
> > -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\  
> 
> I would convert to rcu_dereference_raw() to appease sparse. The fancy
> stuff below is pointless if you then turn off all checking.

The problem with doing this is if we use a trace event without the
proper _idle() or whatever, we wont get a warning that it is used
incorrectly with lockdep. Or does lockdep still check if "rcu is
watching" with rcu_dereference_raw()?

-- Steve

> 
> > +									\
> > +		/*							\
> > +		 * For rcuidle callers, use srcu since sched-rcu	\
> > +		 * doesn't work from the idle path.			\
> > +		 */							\
> > +		if (rcuidle) {						\
> > +			if (in_nmi()) {					\
> > +				WARN_ON_ONCE(1);			\
> > +				return; /* no srcu from nmi */		\
> > +			}						\
> > +									\
> > +			idx = srcu_read_lock_notrace(&tracepoint_srcu);	\
> > +			it_func_ptr =					\
> > +				srcu_dereference_notrace((tp)->funcs,	\
> > +						&tracepoint_srcu);	\
> > +			/* To keep it consistent with !rcuidle path */	\
> > +			preempt_disable_notrace();			\
> > +		} else {						\
> > +			rcu_read_lock_sched_notrace();			\
> > +			it_func_ptr =					\
> > +				rcu_dereference_sched((tp)->funcs);	\
> > +		}							\