Virtualization - Jan 2023 - [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

All RCU disabled code should be noinstr and hence we should never get
here -- when we do, WARN about it and make sure to not actually do
tracing.

Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
---
 arch/x86/kernel/ftrace.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return;
 
+	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n", (void
*)ip))
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, *parent);
 	if (bit < 0)
 		return;

On Mon, 23 Jan 2023 21:50:12 +0100
Peter Zijlstra <peterz at infradead.org> wrote:
> All RCU disabled code should be noinstr and hence we should never get
> here -- when we do, WARN about it and make sure to not actually do
> tracing.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
> ---
>  arch/x86/kernel/ftrace.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long
>  	if (unlikely(atomic_read(&current->tracing_graph_pause)))
>  		return;
>  
> +	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n",
(void *)ip))
> +		return;
> +
Please add this to after recursion trylock below. Although WARN_ONCE()
should not not have recursion issues, as function tracing can do weird
things, I rather be safe than sorry, and not have the system triple boot
due to some path that might get added in the future.

If rcu_is_watching() is false, it will still get by the below recursion
check and warn. That is, the below check should be done before this
function calls any other function.
>  	bit = ftrace_test_recursion_trylock(ip, *parent);
>  	if (bit < 0)
>  		return;
> 
-- Steve