Messages in this thread | | | Date | Wed, 5 Feb 2020 09:19:15 -0500 | From | Joel Fernandes <> | Subject | Re: [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference_sched() is open coded |
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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@goodmis.org> wrote: > > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > > > 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@goodmis.org> > > --- > > 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()); > >
Could you paste the stack here when RCU is not watching? In trace event code IIRC we call rcu_enter_irqs_on() to have RCU temporarily watch, since that code can be called from idle loop. Should we doing the same here as well?
thanks,
- Joel
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