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

From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org, Boqun Feng <boqun.feng@gmail.com>,
	Byungchul Park <byungchul.park@lge.com>,
	Ingo Molnar <mingo@redhat.com>, Julia Cartwright <julia@ni.com>,
	linux-kselftest@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Thomas Glexiner <tglx@linutronix.de>,
	Tom Zanussi <tom.zanussi@linux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU
Date: Wed, 11 Jul 2018 09:00:03 -0400	[thread overview]
Message-ID: <20180711090003.42596c2b@gandalf.local.home> (raw)
In-Reply-To: <20180711124954.GE2476@hirez.programming.kicks-ass.net>

On Wed, 11 Jul 2018 14:49:54 +0200
Peter Zijlstra <peterz@infradead.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);	\
> > +		}							\  