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

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Date:	Wed, 31 Oct 2012 22:56:24 +0100
From:	"Arend van Spriel" <arend@...adcom.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org
Subject: Re: x86_64: undefined symbol 'mcount' in 3.7-rc1

On 10/31/2012 07:44 PM, Steven Rostedt wrote:
> 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

Thanks Steve(n)

I just upgraded the test machine to have the same gcc as the machine 
building the kernels.

Gr. AvS

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