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

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Date:   Mon, 5 Mar 2018 13:25:39 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Young <dyoung@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        akpm@...ux-foundation.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: linux-next: build failure after merge of the printk tree

On Sat 2018-03-03 23:47:39, 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

Ah, I have finally understood the meaning of the libs-y kbuild
variable. It is a nice source of these strange build failures.


> ... 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.

Using the weak symbol makes perfect sense. I am going to
look at the patch.

Thanks a lot everybody for help.

Best Regards,
Petr