linux-kernel - Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with _

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Date:   Tue, 4 Feb 2020 20:01:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Amol Grover <frextrite@...il.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu

On Mon, 3 Feb 2020 11:33:01 -0500
Joel Fernandes <joel@...lfernandes.org> 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.

-- Steve