powerpc/ftrace: Do not trace restore_interrupts()

As I was adding code that affects all archs, I started testing function
tracer against PPC64 and found that it currently locks up with 3.4
kernel. I figured it was due to tracing a function that shouldn't be, so
I went through the following process to bisect to find the culprit:

 cat /debug/tracing/available_filter_functions > t
 num=`wc -l t`
 sed -ne "1,${num}p" t > t1
 let num=num+1
 sed -ne "${num},$p" t > t2
 cat t1 > /debug/tracing/set_ftrace_filter
 echo function /debug/tracing/current_tracer
 <failed? bisect t1, if not bisect t2>

It finally came down to this function: restore_interrupts()

I'm not sure why this locks up the system. It just seems to prevent
scheduling from occurring. Interrupts seem to still work, as I can ping
the box. But all user processes freeze.

When restore_interrupts() is not traced, function tracing works fine.

Cc: stable@kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

On Mon, 2012-06-04 at 22:27 -0400, Steven Rostedt wrote:
> As I was adding code that affects all archs, I started testing function
> tracer against PPC64 and found that it currently locks up with 3.4
> kernel. I figured it was due to tracing a function that shouldn't be, so
> I went through the following process to bisect to find the culprit:
> 
>  cat /debug/tracing/available_filter_functions > t
>  num=`wc -l t`
>  sed -ne "1,${num}p" t > t1
>  let num=num+1
>  sed -ne "${num},$p" t > t2
>  cat t1 > /debug/tracing/set_ftrace_filter
>  echo function /debug/tracing/current_tracer
>  <failed? bisect t1, if not bisect t2>
> 
> It finally came down to this function: restore_interrupts()
> 
> I'm not sure why this locks up the system. It just seems to prevent
> scheduling from occurring. Interrupts seem to still work, as I can ping
> the box. But all user processes freeze.
> 
> When restore_interrupts() is not traced, function tracing works fine.
> 
> Cc: stable@kernel.org

This is what I get for cut-and-pasting from a random git commit. That
should be:

Cc: stable@vger.kernel.org

-- Steve

> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> 
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index 641da9e..64eec59 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -277,7 +277,7 @@ EXPORT_SYMBOL(arch_local_irq_restore);
>   * NOTE: This is called with interrupts hard disabled but not marked
>   * as such in paca->irq_happened, so we need to resync this.
>   */
> -void restore_interrupts(void)
> +void notrace restore_interrupts(void)
>  {
>  	if (irqs_disabled()) {
>  		local_paca->irq_happened |= PACA_IRQ_HARD_DIS;
>