linux-kernel - Re: profile_pc() bogus since <= 2.6.19 (x86 at least)?

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Date:	Wed, 5 Mar 2008 16:37:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: profile_pc() bogus since <= 2.6.19 (x86 at least)?


* Jan Beulich <jbeulich@...ell.com> wrote:

> Ingo,
> 
> while the comment at the top of kernel/spinlock.c states so:
> 
>  * Note that some architectures have special knowledge about the
>  * stack frames of these functions in their profile_pc. If you
>  * change anything significant here that could change the stack
>  * frame contact the architecture maintainers.
> 
> the actual code doesn't seem to match this anymore. With all (and even 
> before that, many) functions being written in C, there cannot be 
> validly made assumptions about the stack frame layout. Indeed, if I 
> check the disassembly framed by __lock_text_{start,end} on x86, there 
> are a number of functions that push one or two registers (in 
> lock_kernel() even stack variables are being allocated), which clearly 
> breaks profile_pc()'s assumptions.
> 
> Since it's been this way for so long, I wonder how frequently this 
> code is actually being exercised...

yeah - i guess it's not really relevant anymore now that lockdep saves 
full stack traces. I doubt anyone bothers to look at wchan anymore. We 
might even remove all the __lock and __sched sections and annotations?

	Ingo
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