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

[Paul E. McKenney]

On Wed, Feb 19, 2020 at 06:03:04PM +0100, Peter Zijlstra wrote:
> 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.

Looks good to me!

							Thanx, Paul

> ---
> Subject: rcu: Provide comment for NOKPROBE() on rcu_nmi_enter()
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  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);
>  
>  /**