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

[Arend van Spriel]

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@xxxxxxxxxx>
> > 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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