linux-kernel - Re: [PATCH 1/2] perfcounter: Ignore the nmi call frames in the x86-64 backtraces

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Message-Id: <1246882668.8143.10.camel@twins>
Date:	Mon, 06 Jul 2009 14:17:48 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 1/2] perfcounter: Ignore the nmi call frames in the
 x86-64 backtraces

On Wed, 2009-07-01 at 21:02 +0200, Frederic Weisbecker wrote:

> +int x86_is_stack_id(int id, char *name)
> +{
> +	return x86_stack_ids[id - 1] == name;
> +}

OK, this bit would need a bit of a comment explaining why this works, as
I don't think the C language guarantees de-duplication of constant
strings.

Therefore the above only works correctly if its passed a pointer from
the x86_stack_ids[] array -- as it indeed is, since the string passed is
obtained through the below method:

> +static unsigned long *in_exception_stack(unsigned cpu, unsigned long stack,
> +					unsigned *usedp, char **idp)
> +{
>  	unsigned k;
>  
>  	/*
> @@ -61,7 +68,7 @@ static unsigned long *in_exception_stack(unsigned cpu, unsigned long stack,
>  			if (*usedp & (1U << k))
>  				break;
>  			*usedp |= 1U << k;
> -			*idp = ids[k];
> +			*idp = x86_stack_ids[k];
>  			return (unsigned long *)end;
>  		}
>  		/*


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