linux-kernel - Re: [PATCH] mm-page_alloc-use-static-global-work

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Date:   Tue, 7 Feb 2017 14:14:20 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     kbuild test robot <lkp@...el.com>
Cc:     Michal Hocko <mhocko@...nel.org>, kbuild-all@...org,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH]
 mm-page_alloc-use-static-global-work_struct-for-draining-per-cpu-pages-fix

On Wed, 8 Feb 2017 05:54:56 +0800 kbuild test robot <lkp@...el.com> wrote:

> Hi Michal,
> 
> [auto build test ERROR on mmotm/master]
> [also build test ERROR on v4.10-rc7 next-20170207]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
> 
> url:    https://github.com/0day-ci/linux/commits/Michal-Hocko/mm-page_alloc-use-static-global-work_struct-for-draining-per-cpu-pages-fix/20170208-050036
> base:   git://git.cmpxchg.org/linux-mmotm.git master
> config: i386-randconfig-x001-201706 (attached as .config)
> compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
> reproduce:
>         # save the attached .config to linux build tree
>         make ARCH=i386 
> 
> All error/warnings (new ones prefixed by >>):
> 
>    In file included from include/asm-generic/percpu.h:6:0,
>                     from arch/x86/include/asm/percpu.h:542,
>                     from arch/x86/include/asm/preempt.h:5,
>                     from include/linux/preempt.h:59,
>                     from include/linux/spinlock.h:50,
>                     from include/linux/mmzone.h:7,
>                     from include/linux/gfp.h:5,
>                     from include/linux/mm.h:9,
>                     from mm/page_alloc.c:18:
>    mm/page_alloc.c: In function 'drain_all_pages':
> >> include/linux/percpu-defs.h:91:33: error: section attribute cannot be specified for local variables
>      extern __PCPU_DUMMY_ATTRS char __pcpu_unique_##name;  \
>                                     ^

huh, yes.  The DEFINE_PER_CPU() macro is broken.

If you do

foo()
{
	static DEFINE_PER_CPU(int, bar);
}

then it won't compile, as described here.  It should.

And if you do

static DEFINE_PER_CPU(int, bar);

then you still get global symbols (__pcpu_unique_bar).

The kernel does the above thing in, umm, 466 places and afaict they're
all broken.  If two code sites ever use the same identifier, they'll
get linkage errors.

huh.  Seems hard to fix.