linux-kernel - Re: [PATCH v3 1/4] cpumask: Fix invalid uniprocessor mask assumption

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Date:   Wed, 08 Jun 2022 19:59:24 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        kernel test robot <lkp@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Yury Norov <yury.norov@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marco Elver <elver@...gle.com>, kbuild-all@...ts.01.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] cpumask: Fix invalid uniprocessor mask assumption

On Mon, 2022-06-06 at 13:40 +0300, Andy Shevchenko wrote:
> On Mon, Jun 06, 2022 at 08:48:05AM +0800, kernel test robot wrote:
> > Hi Sander,
> > 
> > I love your patch! Yet something to improve:
> > 
> > [auto build test ERROR on akpm-mm/mm-everything]
> > [also build test ERROR on linus/master v5.18 next-20220603]
> > [If your patch is applied to the wrong git tree, kindly drop us a note.
> > And when submitting patch, we suggest to use '--base' as documented in
> > https://git-scm.com/docs/git-format-patch]
> > 
> > url:   
> > https://github.com/intel-lab-lkp/linux/commits/Sander-Vanheule/cpumask-Fix-invalid-uniprocessor-assumptions/20220606-004959
> > base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> > config: i386-randconfig-a009
> > (https://download.01.org/0day-ci/archive/20220606/202206060858.wA0FOzRy-lkp@intel.com/config)
> > compiler: gcc-11 (Debian 11.3.0-1) 11.3.0
> > reproduce (this is a W=1 build):
> >         # https://github.com/intel-lab-lkp/linux/commit/37b3f10c4604ea299b454f39ac5ba3cad903ae16
> >         git remote add linux-review https://github.com/intel-lab-lkp/linux
> >         git fetch --no-tags linux-review Sander-Vanheule/cpumask-Fix-invalid-uniprocessor-
> > assumptions/20220606-004959
> >         git checkout 37b3f10c4604ea299b454f39ac5ba3cad903ae16
> >         # save the config file
> >         mkdir build_dir && cp config build_dir/.config
> >         make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash
> > 
> > If you fix the issue, kindly add following tag where applicable
> > Reported-by: kernel test robot <lkp@...el.com>
> > 
> > All errors (new ones prefixed by >>):
> > 
> >    ld: arch/x86/kernel/cpu/cacheinfo.o: in function `__cache_amd_cpumap_setup':
> >    arch/x86/kernel/cpu/cacheinfo.c:890: undefined reference to `cpu_llc_shared_map'
> > > > ld: arch/x86/kernel/cpu/cacheinfo.c:895: undefined reference to `cpu_llc_shared_map'
> 
> Seems like somewhere we need stubs for UP builds for those cache related functions.
> 

I think I finally figured out what's going on here.

cpu_llc_shared_map is always declared with DECLARE_PER_CPU_READ_MOSTLY, but defined in
arch/x86/kernel/smpboot.c which only builds on CONFIG_SMP=y.

cpu_llc_shared_map is accessed in a for_each_cpu loop:
for_each_cpu(i, cpu_llc_shared_mask(cpu))

The old (wrong) UP implementation would ignore the mask, so cpu_llc_shared_map access was optimised
out and the linker would never see that symbol.

Adding a stub for the two inline functions in arch/x86/include/smp.h won't be sufficient I'm afraid.
But I think we can safely make the following assumptions (nobody complained before):
 * anything using cpumask_first_zero(), cpumask_next_zero(), and for_each_cpu_not() was expecting an
   empty mask on UP builds,
 * anything else would have expected a filled mask on UP builds.

I'll think about how to identify all the possible cases, but may not be able to spend a lot of time
on this in the two coming weeks. Any suggestions, or alternative solutions, are of course welcome.

Best,
Sander