Re: [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference_sched() is open coded

From: Joel Fernandes
Date: Wed Feb 05 2020 - 16:59:21 EST


On Wed, Feb 05, 2020 at 06:33:49AM -0500, Steven Rostedt wrote:
>
> Paul and Joel,
>
> Care to ack this patch (or complain about it ;-) ?
>
> -- Steve
>
>
> On Wed, 05 Feb 2020 05:49:33 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> >
> > Because the function graph tracer can execute in sections where RCU is not
> > "watching", the rcu_dereference_sched() for the has needs to be open coded.
> > This is fine because the RCU "flavor" of the ftrace hash is protected by
> > its own RCU handling (it does its own little synchronization on every CPU
> > and does not rely on RCU sched).
> >
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

- Joel


> > ---
> > kernel/trace/trace.h | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> > index 022def96d307..8c52f5de9384 100644
> > --- a/kernel/trace/trace.h
> > +++ b/kernel/trace/trace.h
> > @@ -975,6 +975,11 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
> >
> > preempt_disable_notrace();
> >
> > + /*
> > + * Have to open code "rcu_dereference_sched()" because the
> > + * function graph tracer can be called when RCU is not
> > + * "watching".
> > + */
> > hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());
> >
> > if (ftrace_hash_empty(hash)) {
> > @@ -1022,6 +1027,11 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
> >
> > preempt_disable_notrace();
> >
> > + /*
> > + * Have to open code "rcu_dereference_sched()" because the
> > + * function graph tracer can be called when RCU is not
> > + * "watching".
> > + */
> > notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
> > !preemptible());
> >
>