linux-kernel - Re: Solving section mismatches

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Message-ID: <20090927183904.GD20093@n2100.arm.linux.org.uk>
Date:	Sun, 27 Sep 2009 19:39:04 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Solving section mismatches

On Sun, Sep 27, 2009 at 08:27:07PM +0200, Sam Ravnborg wrote:
> On Sun, Sep 27, 2009 at 05:41:16PM +0100, Russell King - ARM Linux wrote:
> > Sam,
> > 
> > Any idea how to solve this:
> > 
> > WARNING: arch/arm/kernel/built-in.o(.text+0x1ebc): Section mismatch in reference from the function cpu_idle() to the function .cpuexit.text:cpu_die()
> > The function cpu_idle() references a function in an exit section.
> > Often the function cpu_die() has valid usage outside the exit section
> > and the fix is to remove the __cpuexit annotation of cpu_die.
> > 
> > WARNING: arch/arm/kernel/built-in.o(.cpuexit.text+0x3c): Section mismatch in reference from the function cpu_die() to the function .cpuinit.text:secondary_start_kernel()
> > The function __cpuexit cpu_die() references
> > a function __cpuinit secondary_start_kernel().
> > This is often seen when error handling in the exit function
> > uses functionality in the init path.
> > The fix is often to remove the __cpuinit annotation of
> > secondary_start_kernel() so it may be used outside an init section.
> > 
> > Logically, the annotations are correct - in the first case, cpu_die()
> > will only ever be called if hotplug CPU is enabled, since you can't
> > offline a CPU without hotplug CPU enabled.  In that case, the __cpuexit.*
> > sections are not discarded.
> 
> The annotation of cpu_die() is wrong.
> To be annotated __cpuexit the function shall:
> - be used in exit context and only in exit context with HOTPLUG_CPU=n
> - be used outside exit context with HOTPLUG_CPU=y
> 
> cpu_die() fails on the first condition because it is only used
> if HOTPLUG_CPU=y.
> The annotation is wrongly used as a replacement for an ifdef.
> As cpu_die() is already inside ifdef CONFIG_HOTPLUG_CPU is should
> be enough to just remove the annotation.
> 
> Like this (copy'n'paste so it does not apply)
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index e0d3277..de4ef1c 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -214,7 +214,7 @@ void __cpuexit __cpu_die(unsigned int cpu)
>   * of the other hotplug-cpu capable cores, so presumably coming
>   * out of idle fixes this.
>   */
> -void __cpuexit cpu_die(void)
> +void cpu_die(void)
>  {
>         unsigned int cpu = smp_processor_id();

This is wrong.  cpu_die() does not need to exist if hotplug CPU is
disabled.  In that case, it should be discarded and this is precisely
what __cpuexit does.  The annotation is, therefore, correct.
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