linux-kernel - Re: [PATCH] dump_stack: convert generic dump

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Message-ID: <CAK8P3a3dyTN2ONmvSKv7tVODuwvu5JYNy8tUF2+JT194A9U_Tg@mail.gmail.com>
Date:   Wed, 7 Mar 2018 10:50:33 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Greentime Hu <green.hu@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tejun Heo <tj@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dave Young <dyoung@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Vincent Chen <deanbo422@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        adi-buildroot-devel@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] dump_stack: convert generic dump_stack into a weak symbol

On Wed, Mar 7, 2018 at 10:44 AM, Greentime Hu <green.hu@...il.com> wrote:
> 2018-03-07 17:09 GMT+08:00 Petr Mladek <pmladek@...e.com>:
>> Ah, there was a mid-air collision. Arnd already answered most of my
>> questions and even more.
>>
>> On Wed 2018-03-07 09:46:27, Arnd Bergmann wrote:
>>> On Wed, Mar 7, 2018 at 3:21 AM, Sergey Senozhatsky
>>> <sergey.senozhatsky.work@...il.com> wrote:
>>> > On (03/06/18 14:27), Arnd Bergmann wrote:
>>> >> Weak symbols are generally discouraged in the kernel. We have
>>> >> them in a couple of places, but I find them rather confusing as they
>>> >> make it harder to figure out what is actually going on.
>>> >
>>> > Honestly, I kind of find __weak less confusing than EXPORT_SYMBOL(dump_stack)
>>> > in 3 different places. __weak hints that the symbol likely will be overridden
>>> > somewhere, while EXPORT_SYMBOL() does not (at least not to me). Dunno.
>>>
>>> It's not either/or, they are both wrong ;-)
>>>
>>> The EXPORT_SYMBOL() is not the thing that makes it work. The duplicate
>>> declaration today only works because lib/dump_stack.o is listed as lib-y
>>> in the Makefile instead of obj-y. On blackfin and nds32, this causes the entire
>>> file to just not be included in the final vmlinux, because there are no
>>> references to it.
>>>
>>> With your patch, I would actually expect the lib/dump_stack.o file
>>> to still not be picked up, so now you have a missing EXPORT_SYMBOL()
>>> on the two unusual architectures until the point when you add another
>>> (referenced) symbol to it.
>>
>> Great catch! We should change it from lib-y to obj-y. Of course,
>> the best solution would be if nds32 uses the generic implementation
>> and we could avoid adding __weak.
>>
>
> I agree too.
> I think I can add a patch to remove the dump_stack() implementation in
> nds32 and use the generic one.

Looks good,

Acked-by: Arnd Bergmann <arnd@...db.de>

> diff --git a/arch/nds32/kernel/traps.c b/arch/nds32/kernel/traps.c
> index 8828b4aeb72b..6e34eb9824a4 100644
> --- a/arch/nds32/kernel/traps.c
> +++ b/arch/nds32/kernel/traps.c
> @@ -156,18 +156,6 @@ static void __dump(struct task_struct *tsk,
> unsigned long *base_reg)
>         pr_emerg("\n");
>  }
>
> -void dump_stack(void)
> -{
> -       unsigned long *base_reg;
> -       if (!IS_ENABLED(CONFIG_FRAME_POINTER))
> -               __asm__ __volatile__("\tori\t%0, $sp, #0\n":"=r"(base_reg));
> -       else
> -               __asm__ __volatile__("\tori\t%0, $fp, #0\n":"=r"(base_reg));
> -       __dump(NULL, base_reg);
> -}
> -
> -EXPORT_SYMBOL(dump_stack);
> -