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

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Date:   Wed, 7 Mar 2018 10:09:17 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     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>,
        Greentime Hu <green.hu@...il.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

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.

Best Regards,
Petr