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

[Paul E. McKenney]

On Fri, Apr 27, 2018 at 12:13:30PM -0400, Steven Rostedt wrote:
> On Fri, 27 Apr 2018 08:57:01 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> 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.

Works for me either way, at least assuming that the splat actually gets
printed.  ;-)

> 			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.

OK, please see the (untested) patch below.  Of course,
srcu_read_lock_notrace() invokes __srcu_read_lock(), which looks as
follows:

	int __srcu_read_lock(struct srcu_struct *sp)
	{
		int idx;

		idx = READ_ONCE(sp->srcu_idx) & 0x1;
		this_cpu_inc(sp->sda->srcu_lock_count[idx]);
		smp_mb(); /* B */  /* Avoid leaking the critical section. */
		return idx;
	}

Do I also need to make a notrace version of __srcu_read_lock()?
Same question for __srcu_read_unlock(), which is similar.  If so,
assuming that there is no need for a notrace variant of this_cpu_inc()
and smp_mb(), I suppose I could simply macro-ize the internals in both
cases, but perhaps you have a better approach.

								Thanx, Paul

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

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 91494d7e8e41..e2e2cf05a6eb 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.
@@ -205,6 +215,13 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
 static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
 	__releases(sp)
 {
+	__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)
+{
 	rcu_lock_release(&(sp)->dep_map);
 	__srcu_read_unlock(sp, idx);
 }