linux-kernel - Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on

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Message-ID: <20180427121330.40b7ef15@gandalf.local.home>
Date:   Fri, 27 Apr 2018 12:13:30 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@...gle.com>, linux-kernel@...r.kernel.org,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Fenguang Wu <fengguang.wu@...el.com>,
        Baohong Liu <baohong.liu@...el.com>,
        Vedang Patel <vedang.patel@...el.com>, kernel-team@...roid.com
Subject: Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks
 executing with preempt on

On Fri, 27 Apr 2018 08:57:01 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > +		if (preempt_on) {					\
> > +			WARN_ON_ONCE(in_nmi()); /* no srcu from nmi */	\  
> 
> Very good on this check, thank you!

I think you need to return and not call the read lock.

			if (WARN_ON_ONCE(in_nmi()))
				return;

> 
> > +			idx = srcu_read_lock(&tracepoint_srcu);         \  
> 
> Hmmm...  Do I need to create a _notrace variant of srcu_read_lock()
> and srcu_read_unlock()?

I think so.

-- Steve

> 
> > +			it_func_ptr = srcu_dereference((tp)->funcs,	\
> > +						&tracepoint_srcu);	\
> > +		} else {						\
> > +			rcu_read_lock_sched_notrace();			\
> > +			it_func_ptr =					\
> > +				rcu_dereference_sched((tp)->funcs);	\
> > +		}							\
> > +									\