linux-kernel - Re: x86_64: undefined symbol 'mcount' in 3.7-rc1

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Date:	Wed, 31 Oct 2012 14:44:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: x86_64: undefined symbol 'mcount' in 3.7-rc1

On Wed, 2012-10-17 at 22:23 +0200, Arend van Spriel wrote:
> Hi Steven,
> 
> I have nightly test machines upgraded to 3.7-rc1 and on the 64-bit 
> platform I get MODPOST warning on 'mcount'.
> 
> It is conditionally exported in x8664_ksyms_64.c:
> #ifdef CONFIG_FUNCTION_TRACER
> /* mcount is defined in assembly */
> EXPORT_SYMBOL(mcount);
> #endif
> 
> #ifdef CONFIG_FUNCTION_TRACER
> #ifdef CC_USING_FENTRY
> # define MCOUNT_ADDR           ((long)(__fentry__))
> #else
> # define MCOUNT_ADDR           ((long)(mcount))
> #endif
> #define MCOUNT_INSN_SIZE       5 /* sizeof mcount call */
> 
> I have built the kernel on x86 machine with gcc 4.6.3 and the modules 
> are built during the test execution on test machine which has gcc 4.4.5. 
> Reading commit below
> 
> commit d57c5d51a30152f3175d2344cb6395f08bf8ee0c
> Author: Steven Rostedt <srostedt@...hat.com>
> Date:   Wed Feb 9 13:32:18 2011 -0500
> 
>      ftrace/x86: Add support for -mfentry to x86_64
> 
> I suppose the gcc version mismatch is causing my problem. Is that a 
> correct assumption?

Yes.

gcc 4.6.0 added a new option to gcc called '-mfentry'. The kernel build
checks to see if this option is supported and if so it will use it. What
this option does (when added to -pg), will not do the mcount calling but
instead call fentry. The two have different semantics and are not
compatible. When you built your kernel, the build process detected that
-mfentry is supported and used that.

Now when you built your modules with gcc 4.4.5, -mfentry was not
supported and it used the mcount feature instead. In general, it's not a
good idea to use two different gcc's to build the kernel and modules.

But if you really need to, then you should disable function tracing of
your modules, where the -pg and -mfentry wont be used.

-- Steve


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