Subject: Re: linux-next: build failure after merge of the printk tree

From: Dave Young <[email-masked]>
Date: Mon, 5 Mar 2018 11:20:05 +0800

On 03/03/18 at 11:47pm, Sergey Senozhatsky wrote:
> Cc-ing Tejun
> 
> On (03/02/18 16:54), Petr Mladek wrote:
> [..]
> > > (Though it is not immediately obvious why.)
> > 
> > It is a mistery to me. The error appears when I move any of
> > dump_stack_print_info() or show_regs_print_info() function
> > definitions from kernel/printk/printk.c to lib/dump_stack.c.
> > All the other changes seems unrelated.
> > 
> > The thing is that we basically do not touch dump_stack() definition
> > by that patch.
> 
> Apparently dump_stack_print_info() was in lib/dump_stack.c a long
> time ago, but it was deliberately moved to printk.c, when kernel gained
> a "generic" (dummy) dump_stack() fallback. Some archs, like blackfin,
> define their own dump_stack() symbol and make it global via EXPORT_SYMBOL.
> 
> In case of blackfin that arch-specific dump_stack() symbol invokes a
> global dump_stack_print_info(). If we move dump_stack_print_info() back
> to lib/dump_stack.c then we link both with arch/blackfin/dumpstack.o
> and lib/dump_stack.o, which results in multiple definitions error.
> If we move dump_stack_print_info() out on libdump_stack.o, then we
> never link with lib/dump_stack.o
> 
> ... so what are we going to do with that.
> 
> a) we can drop the patch and cherry pick only the kexec part
> 
> b) we can try to mark dummy lib/dump_stack() as __weak
>    EXPORT_SYMBOL and remove EXPORT_SYMBOL from arch-specific
>    definitions.
> 
>    So we will end up with EXPORT_SYMBOL dump_stack() and archs
>    may re-define it. If some arch will accidentally mark its
>    own dump_stack() as EXPORT_SYMBOL then there should be a
>    linkage warning - a symbol is exported twice.
> 
> 
> Something like below.
> 
> Opinions? Will this work?

I would think b) is better, thanks for the fix!

> 
> 
> ========= 8< =========
> 
> From: Sergey Senozhatsky <[email-masked]>
> Subject: [PATCH] dump_stack: mark dummy dump_stack() as weak
> 
> ---
>  arch/blackfin/kernel/dumpstack.c | 1 -
>  arch/nds32/kernel/traps.c        | 2 --
>  lib/dump_stack.c                 | 4 ++--
>  3 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/blackfin/kernel/dumpstack.c b/arch/blackfin/kernel/dumpstack.c
> index 3c992c1f8ef2..61af017130cd 100644
> --- a/arch/blackfin/kernel/dumpstack.c
> +++ b/arch/blackfin/kernel/dumpstack.c
> @@ -174,4 +174,3 @@ void dump_stack(void)
>  	show_stack(current, &stack);
>  	trace_buffer_restore(tflags);
>  }
> -EXPORT_SYMBOL(dump_stack);
> diff --git a/arch/nds32/kernel/traps.c b/arch/nds32/kernel/traps.c
> index 8828b4aeb72b..455bb0787367 100644
> --- a/arch/nds32/kernel/traps.c
> +++ b/arch/nds32/kernel/traps.c
> @@ -166,8 +166,6 @@ void dump_stack(void)
>  	__dump(NULL, base_reg);
>  }
>  
> -EXPORT_SYMBOL(dump_stack);
> -
>  void show_stack(struct task_struct *tsk, unsigned long *sp)
>  {
>  	unsigned long *base_reg;
> diff --git a/lib/dump_stack.c b/lib/dump_stack.c
> index 5cff72f18c4a..9cf4465dbffa 100644
> --- a/lib/dump_stack.c
> +++ b/lib/dump_stack.c
> @@ -85,7 +85,7 @@ static void __dump_stack(void)
>  #ifdef CONFIG_SMP
>  static atomic_t dump_lock = ATOMIC_INIT(-1);
>  
> -asmlinkage __visible void dump_stack(void)
> +asmlinkage __weak __visible void dump_stack(void)
>  {
>  	unsigned long flags;
>  	int was_locked;
> @@ -118,7 +118,7 @@ asmlinkage __visible void dump_stack(void)
>  	local_irq_restore(flags);
>  }
>  #else
> -asmlinkage __visible void dump_stack(void)
> +asmlinkage __weak __visible void dump_stack(void)
>  {
>  	__dump_stack();
>  }
> -- 
> 2.16.2
> 

Thanks
Dave