Hi Nicholas,

I love your patch! Yet something to improve:

[auto build test ERROR on linus/master]
[also build test ERROR on asm-generic/master sparc-next/master xtensa/for_next v5.9-rc2 next-20200826]
[cannot apply to sparc/master]
[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/0day-ci/linux/commits/Nicholas-Piggin/Use-asm-generic-for-mmu_context-no-op-functions/20200826-225632
base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 2ac69819ba9e3d8d550bb5d2d2df74848e556812
config: c6x-allyesconfig (attached as .config)
compiler: c6x-elf-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=c6x 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   In file included from ./arch/c6x/include/generated/asm/mmu_context.h:1,
                    from kernel/fork.c:101:
   include/asm-generic/mmu_context.h: In function 'activate_mm':
>> include/asm-generic/mmu_context.h:59:2: error: implicit declaration of function 'switch_mm' [-Werror=implicit-function-declaration]
      59 |  switch_mm(prev_mm, next_mm, current);
         |  ^~~~~~~~~
   kernel/fork.c: At top level:
   kernel/fork.c:160:13: warning: no previous prototype for 'arch_release_task_struct' [-Wmissing-prototypes]
     160 | void __weak arch_release_task_struct(struct task_struct *tsk)
         |             ^~~~~~~~~~~~~~~~~~~~~~~~
   kernel/fork.c:743:20: warning: no previous prototype for 'arch_task_cache_init' [-Wmissing-prototypes]
     743 | void __init __weak arch_task_cache_init(void) { }
         |                    ^~~~~~~~~~~~~~~~~~~~
   kernel/fork.c:834:12: warning: no previous prototype for 'arch_dup_task_struct' [-Wmissing-prototypes]
     834 | int __weak arch_dup_task_struct(struct task_struct *dst,
         |            ^~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors
--
   In file included from ./arch/c6x/include/generated/asm/mmu_context.h:1,
                    from kernel/exit.c:69:
   include/asm-generic/mmu_context.h: In function 'activate_mm':
>> include/asm-generic/mmu_context.h:59:2: error: implicit declaration of function 'switch_mm' [-Werror=implicit-function-declaration]
      59 |  switch_mm(prev_mm, next_mm, current);
         |  ^~~~~~~~~
   kernel/exit.c: At top level:
   kernel/exit.c:1754:13: warning: no previous prototype for 'abort' [-Wmissing-prototypes]
    1754 | __weak void abort(void)
         |             ^~~~~
   cc1: some warnings being treated as errors
--
   In file included from ./arch/c6x/include/generated/asm/mmu_context.h:1,
                    from include/linux/mmu_context.h:5,
                    from kernel/kthread.c:12:
   include/asm-generic/mmu_context.h: In function 'activate_mm':
>> include/asm-generic/mmu_context.h:59:2: error: implicit declaration of function 'switch_mm' [-Werror=implicit-function-declaration]
      59 |  switch_mm(prev_mm, next_mm, current);
         |  ^~~~~~~~~
   cc1: some warnings being treated as errors
--
   In file included from ./arch/c6x/include/generated/asm/mmu_context.h:1,
                    from include/linux/mmu_context.h:5,
                    from kernel/sched/sched.h:54,
                    from kernel/sched/rt.c:6:
   include/asm-generic/mmu_context.h: In function 'activate_mm':
>> include/asm-generic/mmu_context.h:59:2: error: implicit declaration of function 'switch_mm' [-Werror=implicit-function-declaration]
      59 |  switch_mm(prev_mm, next_mm, current);
         |  ^~~~~~~~~
   kernel/sched/rt.c: At top level:
   kernel/sched/rt.c:668:6: warning: no previous prototype for 'sched_rt_bandwidth_account' [-Wmissing-prototypes]
     668 | bool sched_rt_bandwidth_account(struct rt_rq *rt_rq)
         |      ^~~~~~~~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors

# https://github.com/0day-ci/linux/commit/96f6fa7969b31878ea85427182e4d9201ab3f592
git remote add linux-review https://github.com/0day-ci/linux
git fetch --no-tags linux-review Nicholas-Piggin/Use-asm-generic-for-mmu_context-no-op-functions/20200826-225632
git checkout 96f6fa7969b31878ea85427182e4d9201ab3f592
vim +/switch_mm +59 include/asm-generic/mmu_context.h

    49	
    50	/**
    51	 * activate_mm - called after exec switches the current task to a new mm, to switch to it
    52	 * @prev_mm: previous mm of this task
    53	 * @next_mm: new mm
    54	 */
    55	#ifndef activate_mm
    56	static inline void activate_mm(struct mm_struct *prev_mm,
    57				       struct mm_struct *next_mm)
    58	{
  > 59		switch_mm(prev_mm, next_mm, current);
    60	}
    61	#endif
    62	

---
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