From: Amol Grover <frextrite@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
linux-kernel@vger.kernel.org,
Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
Joel Fernandes <joel@joelfernandes.org>,
linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu
Date: Wed, 5 Feb 2020 10:56:32 +0530 [thread overview]
Message-ID: <20200205052632.GB1540@workstation-portable> (raw)
In-Reply-To: <20200204200116.479f0c60@oasis.local.home>
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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next prev parent reply other threads:[~2020-02-05 5:26 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-01 7:27 [Linux-kernel-mentees] [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu Amol Grover
2020-02-03 16:33 ` Joel Fernandes
2020-02-04 10:02 ` Steven Rostedt
2020-02-05 1:01 ` Steven Rostedt
2020-02-05 5:26 ` Amol Grover [this message]
2020-02-05 13:11 ` Paul E. McKenney
2020-02-05 13:14 ` Steven Rostedt
2020-02-05 13:19 ` Steven Rostedt
2020-02-05 13:29 ` Paul E. McKenney
2020-02-05 22:06 ` Joel Fernandes
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