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SubjectRe: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu
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@joelfernandes.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.
>

Something new to learn, thank you for the information Steve!

Thanks
Amol

> -- Steve

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