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

From: Joel Fernandes <joelaf@google.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zilstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Tom Zanussi <tom.zanussi@linux.intel.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Thomas Glexiner <tglx@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Fenguang Wu <fengguang.wu@intel.com>,
	Baohong Liu <baohong.liu@intel.com>,
	Vedang Patel <vedang.patel@intel.com>,
	"Cc: Android Kernel" <kernel-team@android.com>
Subject: Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on
Date: Fri, 27 Apr 2018 09:22:04 -0700	[thread overview]
Message-ID: <CAJWu+oqcbdvnN=wdas9CYykCmqUKChnWWk6HScb=6VJO03eG4w@mail.gmail.com> (raw)
In-Reply-To: <20180427121330.40b7ef15@gandalf.local.home>

On Fri, Apr 27, 2018 at 9:13 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Fri, 27 Apr 2018 08:57:01 -0700
> "Paul E. McKenney" <paulmck@linux.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;

Cool, I'll do that.

>>
>> > +                   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.

Oh yes, since otherwise we call into lockdep.

Paul, then I think that's true we'd need srcu _notrace variants that
don't do the rcu_lock_acquire. Sorry for my earlier email saying its
not needed. Thanks,

- Joel