Re: [PATCH v3 05/22] rcu: Make RCU IRQ enter/exit functions rely on in_nmi()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 19, 2020 at 05:37:00PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 19, 2020 at 08:31:56AM -0800, Paul E. McKenney wrote:

> > Here is the latest version of that comment, posted by Steve Rostedt.
> > 
> > 							Thanx, Paul
> > 
> > /*
> >  * All functions called in the breakpoint trap handler (e.g. do_int3()
> >  * on x86), must not allow kprobes until the kprobe breakpoint handler
> >  * is called, otherwise it can cause an infinite recursion.
> >  * On some archs, rcu_nmi_enter() is called in the breakpoint handler
> >  * before the kprobe breakpoint handler is called, thus it must be
> >  * marked as NOKPROBE.
> >  */
> 
> Oh right, let me stick that in a separate patch. Best we not loose that
> I suppose ;-)

Having gone over the old thread, I ended up with the below. Anyone
holler if I got it wrong somehow.

---
Subject: rcu: Provide comment for NOKPROBE() on rcu_nmi_enter()
From: Steven Rostedt <rostedt@goodmis.org>

From: Steven Rostedt <rostedt@goodmis.org>

The rcu_nmi_enter() function was marked NOKPROBE() by commit
c13324a505c77 ("x86/kprobes: Prohibit probing on functions before
kprobe_int3_handler()") because the do_int3() call kprobe code must
not be invoked before kprobe_int3_handler() is called.  It turns out
that ist_enter() (in do_int3()) calls rcu_nmi_enter(), hence the
marking NOKPROBE() being added to rcu_nmi_enter().

This commit therefore adds a comment documenting this line of
reasoning.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/rcu/tree.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -842,6 +842,14 @@ void rcu_nmi_enter(void)
 {
 	rcu_nmi_enter_common(false);
 }
+/*
+ * All functions called in the breakpoint trap handler (e.g. do_int3()
+ * on x86), must not allow kprobes until the kprobe breakpoint handler
+ * is called, otherwise it can cause an infinite recursion.
+ * On some archs, rcu_nmi_enter() is called in the breakpoint handler
+ * before the kprobe breakpoint handler is called, thus it must be
+ * marked as NOKPROBE.
+ */
 NOKPROBE_SYMBOL(rcu_nmi_enter);
 
 /**