Re: [PATCH] acpi: fix incompatibility with mcount-based function graph tracing

[Steven Rostedt]

On Fri, 24 Mar 2017 13:12:54 -0500
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:


> Instead I was able to "fix" it by ignoring ftrace calls in real mode:
> 
> -----
> index 8f3d9cf..5c0d0c6 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -983,6 +983,9 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
>  	unsigned long return_hooker = (unsigned long)
>  				&return_to_handler;
>  
> +	if (__builtin_return_address(0) < TASK_SIZE_MAX)
> +		return;
> +
>  	if (unlikely(ftrace_graph_is_dead()))
>  		return;
> ---------------
> 
> I'm not sure what the best fix should really be.  A few ideas off the
> top of my head:
> 
> - A real mode check similar to the above (except it should probably be
>   more precise)

The real mode check hack may be good enough for now. Make sure that
it's commented well.

-- Steve

> 
> - Make tracing_graph_pause a percpu variable so that it can be read from
>   prepare_ftrace_return()
> 
> - pause_graph_tracing() from ftrace_suspend_notifier_call()
> 
> Steven, thoughts?
>