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

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Message-ID: <alpine.DEB.2.00.0908052011590.5010@gandalf.stny.rr.com>
Date:	Wed, 5 Aug 2009 22:00:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
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

On Wed, 5 Aug 2009, Ingo Molnar wrote:

> 
> * Dave Airlie <airlied@...il.com> wrote:
> 
> > Hey,
> > 
> > So I spent 3-4 hrs today (I'm stupid yes) tracking down a .o 
> > breakage by blaming rawhide gcc/binutils as I was using make 
> > V=1and seeing only the compiler chain running,
> 
> Hm, is this that powerpc related build bug you just reported?

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;
 	    }
--
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