Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code

[Steven Rostedt]

On Mon, 8 May 2017 15:43:33 -0500
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> Ok, so just to clarify.  Is there a bug in the ftrace stack tracer in
> the following situation?
> 
> 1. RCU isn't watching
> 2. An NMI hits
> 3. ist_enter() calls into the ftrace stack tracer, before
>    rcu_nmi_enter() is called, so RCU isn't watching yet

No, because in the ftrace stack tracer there's:

	if (in_nmi())
		return;

At the very beginning.

-- Steve

> 4. The ftrace stack tracer calls rcu_irq_enter(), which has no effect,
>    so RCU still isn't watching
> 5. Hilarity ensues in the ftrace stack tracer
>