Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu

[Paul E. McKenney]

On Tue, Feb 04, 2020 at 08:01:16PM -0500, Steven Rostedt wrote:
> On Mon, 3 Feb 2020 11:33:01 -0500
> Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
> 
> > > --- a/kernel/trace/trace.h
> > > +++ b/kernel/trace/trace.h
> > > @@ -950,22 +950,25 @@ extern void __trace_graph_return(struct trace_array *tr,
> > >  				 unsigned long flags, int pc);
> > >  
> > >  #ifdef CONFIG_DYNAMIC_FTRACE
> > > -extern struct ftrace_hash *ftrace_graph_hash;
> > > +extern struct ftrace_hash __rcu *ftrace_graph_hash;
> > >  extern struct ftrace_hash *ftrace_graph_notrace_hash;
> > >  
> > >  static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
> > >  {
> > >  	unsigned long addr = trace->func;
> > >  	int ret = 0;
> > > +	struct ftrace_hash *hash;
> > >  
> > >  	preempt_disable_notrace();
> > >  
> > > -	if (ftrace_hash_empty(ftrace_graph_hash)) {
> > > +	hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());  
> > 
> > I think you can use rcu_dereference_sched() here? That way no need to pass
> > !preemptible.
> > 
> > A preempt-disabled section is an RCU "sched flavor" section. Flavors are
> > consolidated in the backend, but in the front end the dereference API still
> > do have flavors.
> 
> Unfortunately, doing it with rcu_dereference_sched() causes a lockdep
> splat :-P. This is because ftrace can execute when rcu is not
> "watching" and that will trigger a lockdep error. That means, this
> origin patch *is* correct. I'm re-applying this one.

I strongly recommend a comment stating why disabling preemption prevents
ftrace_graph_hash from going away.  I see the synchronize_rcu() after
the rcu_assign_pointer() in ftrace_graph_release(), but I don't see
anything that waits on CPUs that RCU is not watching.

Of course, event tracing -makes- RCU watch when needed, but if that
was set up, then lockdep would not have complained.

So what am I missing?

							Thanx, Paul