linux-kernel - Re: ftrace scripts and make V=1

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Message-ID: <20090806034325.GA8861@elte.hu>
Date:	Thu, 6 Aug 2009 05:43:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Dave Airlie <airlied@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org
Subject: Re: ftrace scripts and make V=1


* Steven Rostedt <rostedt@...dmis.org> wrote:

> Well we tracked it down and it is powerpc64 specific.
> 
> Seems that in drivers/hwmon/lm93.c there's a function called:
> 
>    LM93_IN_FROM_REG()
> 
> But PPC64 has function descriptors and the real function names (the ones 
> you see in objdump) start with a '.'. Thus this in objdump you have:
> 
>  Disassembly of section .text:
> 
>  0000000000000000 <.LM93_IN_FROM_REG>:
>        0:       7c 08 02 a6     mflr    r0
>        4:       fb 81 ff e0     std     r28,-32(r1)
> 
> 
> The function name used is .LM93_IN_FROM_REG. But gcc considers 
> symbols that start with ".L" as a special symbol that is used 
> inside the assembly stage.
> 
> The nm passed into recordmcount uses the --synthetic option which 
> shows the ".L" symbols (my runs outside of the build did not 
> include the --synthetic option, so my older patch worked). We see 
> the function as a local.
> 
> Now to capture all the locations that use "mcount" we need to have 
> a reference to link into the object file a list of mcount callers. 
> We need a reference that will not disappear. We try to use a 
> global function and if that does not work, we use a local function 
> as a reference. But to relink the section back into the object, we 
> need to make it global. In this case, we run objcopy using 
> --globalize-symbol and --localize-symbol to convert the symbol 
> into a global symbol, link the mcount list, then convert it back 
> to a local symbol.
> 
> This works great except for this case. .L* symbols can not be 
> converted into a global symbol, and the mcount section referencing 
> it will remain unresolved.
> 
> Try this patch and see if it fixes your issue.
> 
> Thanks!
> 
> -- Steve
> 
> diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
> index d29baa2..4889c44 100755
> --- a/scripts/recordmcount.pl
> +++ b/scripts/recordmcount.pl
> @@ -414,7 +414,10 @@ while (<IN>) {
>  	    $offset = hex $1;
>  	} else {
>  	    # if we already have a function, and this is weak, skip it
> -	    if (!defined($ref_func) && !defined($weak{$text})) {
> +	    if (!defined($ref_func) && !defined($weak{$text}) &&
> +		 # PPC64 can have symbols that start with .L and
> +		 # gcc considers these special. Don't use them!
> +		 $text !~ /^\.L/) {
>  		$ref_func = $text;
>  		$offset = hex $1;
>  	    }

Ah, indeed. I'm wondering whether also emitting a build warning 
would be useful - just in the (admittedly unlikely) case of someone 
wondering about why LM93_IN_FROM_REG does not show up in function 
traces.

	Ingo
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