[rcu:rcu/next 19/20] include/linux/sched.h:1653:3: error: implicit declaration of function 'sync_core'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/next
head:   f0909808dc46d6ae79a08a42def577bd7880aa4d
commit: 0d6eb99818da24d0c55962c7c0d4a9f8dc71a6bb [19/20] membarrier: Provide register sync core cmd
config: xtensa-allyesconfig (attached as .config)
compiler: xtensa-linux-gcc (GCC) 4.9.0
reproduce:
        wget https://raw.githubusercontent.com/01org/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        git checkout 0d6eb99818da24d0c55962c7c0d4a9f8dc71a6bb
        # save the attached .config to linux build tree
        make.cross ARCH=xtensa 

All errors (new ones prefixed by >>):

   In file included from include/linux/ptrace.h:5:0,
                    from arch/xtensa/kernel/asm-offsets.c:21:
   include/linux/sched.h: In function 'membarrier_sched_out':
>> include/linux/sched.h:1653:3: error: implicit declaration of function 'sync_core' [-Werror=implicit-function-declaration]
      sync_core();
      ^
   cc1: some warnings being treated as errors
   make[2]: *** [arch/xtensa/kernel/asm-offsets.s] Error 1
   make[2]: Target '__build' not remade because of errors.
   make[1]: *** [prepare0] Error 2
   make[1]: Target 'prepare' not remade because of errors.
   make: *** [sub-make] Error 2

vim +/sync_core +1653 include/linux/sched.h

  1630	
  1631	#ifdef CONFIG_MEMBARRIER
  1632	static inline void membarrier_fork(struct task_struct *t,
  1633			unsigned long clone_flags)
  1634	{
  1635		/*
  1636		 * Coherence of membarrier_sync_core against thread fork is
  1637		 * protected by siglock. membarrier_fork is called with siglock
  1638		 * held.
  1639		 */
  1640		t->membarrier_sync_core = current->membarrier_sync_core;
  1641	}
  1642	static inline void membarrier_execve(struct task_struct *t)
  1643	{
  1644		t->membarrier_sync_core = 0;
  1645	}
  1646	static inline void membarrier_sched_out(struct task_struct *t)
  1647	{
  1648		/*
  1649		 * Core serialization is performed before the memory barrier
  1650		 * preceding the store to rq->curr.
  1651		 */
  1652		if (unlikely(READ_ONCE(t->membarrier_sync_core)))
> 1653			sync_core();
  1654	}
  1655	static inline void membarrier_sched_in(struct task_struct *t)
  1656	{
  1657		/*
  1658		 * Core serialization is performed after the memory barrier
  1659		 * following the store to rq->curr.
  1660		 */
  1661		if (unlikely(READ_ONCE(t->membarrier_sync_core)))
  1662			sync_core();
  1663	}
  1664	#else
  1665	static inline void membarrier_fork(struct task_struct *t,
  1666			unsigned long clone_flags)
  1667	{
  1668	}
  1669	static inline void membarrier_execve(struct task_struct *t)
  1670	{
  1671	}
  1672	static inline void membarrier_sched_out(struct task_struct *t)
  1673	{
  1674	}
  1675	static inline void membarrier_sched_in(struct task_struct *t)
  1676	{
  1677	}
  1678	#endif
  1679	

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