Subject: Re: BUG: dead loop in PowerPC hcall tracepoint (Was: [LTP] [PATCH v2] Add ftrace-stress-test to LTP)

From: Li Zefan <[email-masked]>
Date: Tue, 19 Oct 2010 08:49:33 +0800

Steven Rostedt wrote:
> On Mon, 2010-10-18 at 11:19 +0800, Li Zefan wrote:
> 
>> This is a dead loop:
>>
>> trace_hcall_entry() -> trace_clock_global() -> trace_hcall_entry() ..
>>
>> And this is a PPC specific bug. Hope some ppc guys will fix it?
>> Or we kill trace_clock_global() if no one actually uses it..
> 
> trace_clock_global() is used by many. I use it (and recommend using it)
> on boxes where the TSC is horribly out of sync, and the trace needs
> synchronization between CPUs.
> 
> The trace_hcall_entry and exit has wrappers already. Just add recursion
> protection there.
> 

Right, I thought of this. But as I have no machine to test, I'll leave
this to others.

> Perhaps something like this:
> 
> (Not compiled nor ran)
> 
> +static DEFINE_PER_CPU(hcall_trace_disable);
> +
>  void hcall_tracepoint_regfunc(void)
>  {
>  	hcall_tracepoint_refcount++;
>  }
> 
>  void hcall_tracepoint_unregfunc(void)
>  {
>  	hcall_tracepoint_refcount--;
>  }
> 
> +int __trace_disable_check(void)
> +{
> +	if (!hcall_tracepoint_refcount)
> +		return 1;
> +
> +	if (get_cpu_var(hcall_trace_disable)) {
> +		put_cpu_var(hcall_trace_disable);
> +		return 1;
> +	}
> +
> +	__get_cpu_var(hcall_trace_disable)++;
> +
> +	return 0;
> +}
> +
> +void __trace_disable_put(void)
> +{
> +	__get_cpu_var(hcall_trace_disable)--;
> +	put_cpu_var(hcall_trace_disable);
> +}
> +
>  void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
>  {
> +	int trace_disable;
> +
> +	if (__trace_disable_check())
> +		return;
> +
>  	trace_hcall_entry(opcode, args);
> +	__trace_disable_put();
>  }
> 
>  void __trace_hcall_exit(long opcode, unsigned long retval,
>  			unsigned long *retbuf)
>  {
> +	if (__trace_disable_check())
> +		return;
> +
>  	trace_hcall_exit(opcode, retval, retbuf);
> +	__trace_disable_put();
>  }
> 
> -- Steve
> 
> 
> 
>