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

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Message-Id: <20180427170517.GA799@linux.vnet.ibm.com>
Date:   Fri, 27 Apr 2018 10:05:17 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        LKML <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>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing
 with preempt on

On Fri, Apr 27, 2018 at 10:00:48AM -0700, Paul E. McKenney wrote:
> On Fri, Apr 27, 2018 at 12:46:41PM -0400, Steven Rostedt wrote:
> > On Fri, 27 Apr 2018 09:45:54 -0700
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > > > That shouldn't be needed. For the rcu_read_lock_sched case, there is a
> > > > > preempt_disable which needs to be a notrace, but for the srcu one,
> > > > > since we don't do that, I think it should be fine.  
> > > > 
> > > > Actually, I think I may agree here too. Because the _notrace is for
> > > > function tracing, and it shouldn't affect it. If people don't want it
> > > > traced, they could add those functions to the list in the notrace file.  
> > > 
> > > OK, feel free to ignore my notrace srcu_read_lock() patch, then.  ;-)
> > 
> > Of course I wasn't thinking about the lockdep tracepoints that Joel
> > mentioned, which happens to be the reason for all this discussion in
> > the first place :-)  Now I think we do need it. (OK, I can keep
> > changing my mind, can't I?).
> 
> You can, but at some point I start applying heavy-duty hysteresis.  ;-)
> 
> So the current thought (as of your having sent the above email) is that
> we need notrace versions of srcu_read_lock() and srcu_read_unlock(),
> but not for __srcu_read_lock() and __srcu_read_unlock(), correct?

And Joel noted offline that I messed up srcu_read_unlock_notrace(),
so here is an updated patch with that fixed.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 91494d7e8e41..3e72a291c401 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -195,6 +195,16 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
 	return retval;
 }
 
+/* Used by tracing, cannot be traced and cannot invoke lockdep. */
+static inline notrace int
+srcu_read_lock_notrace(struct srcu_struct *sp) __acquires(sp)
+{
+	int retval;
+
+	retval = __srcu_read_lock(sp);
+	return retval;
+}
+
 /**
  * srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
  * @sp: srcu_struct in which to unregister the old reader.
@@ -209,6 +219,13 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
 	__srcu_read_unlock(sp, idx);
 }
 
+/* Used by tracing, cannot be traced and cannot call lockdep. */
+static inline notrace void
+srcu_read_unlock_notrace(struct srcu_struct *sp, int idx) __releases(sp)
+{
+	__srcu_read_unlock(sp, idx);
+}
+
 /**
  * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
  *