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Date: Fri, 21 Jan 2011 15:05:24 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Hillf Danton <dhillf@gmail.com>
cc: linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: x86_64: could increment fp when getting wchan?
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On Fri, 21 Jan 2011, Hillf Danton wrote:

> When getting wchan (wait channel?) the stack of given task is scanned
> for IP that is not in the scheduling area. What looks odd is the IP
> could be checked as many times as allowed in the loop without changing
> the location on stack. Could the IP be checked up stair the stack?

Errm.
 
> Hillf
> ---
> 
> --- a/arch/x86/kernel/process_64.c	2011-01-05 08:50:20.000000000 +0800
> +++ b/arch/x86/kernel/process_64.c	2011-01-21 21:46:04.000000000 +0800
> @@ -543,7 +543,7 @@ unsigned long get_wchan(struct task_stru
>  		ip = *(u64 *)(fp+8);

IP (Return address) is read from stack via fp (Frame pointer)

>  		if (!in_sched_functions(ip))
>  			return ip;
> -		fp = *(u64 *)fp;

Now fp is set to the next stack frame, so the next loop will read a
different IP.

> +		fp += 8;
>  	} while (count++ < 16);
>  	return 0;
>  }

Thanks,

	tglx