MCS spinlock: Use smp_cond_load_acquire()

Hi Jason,

[auto build test ERROR on v4.6-rc3]
[also build test ERROR on next-20160412]
[cannot apply to tip/core/locking]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Jason-Low/MCS-spinlock-Use-smp_cond_load_acquire/20160413-053726
config: i386-randconfig-s0-201615 (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All error/warnings (new ones prefixed by >>):

In file included from kernel/locking/qspinlock.c:70:0:
kernel/locking/mcs_spinlock.h: In function 'mcs_spin_lock':

>> kernel/locking/mcs_spinlock.h:31:2: error: implicit declaration of function 'smp_cond_load_acquire' [-Werror=implicit-function-declaration]
     smp_cond_load_acquire(&l, VAL);     \
     ^
>> kernel/locking/mcs_spinlock.h:91:2: note: in expansion of macro 'arch_mcs_spin_lock_contended'
     arch_mcs_spin_lock_contended(&node->locked);
     ^
>> kernel/locking/mcs_spinlock.h:31:24: error: lvalue required as unary '&' operand
     smp_cond_load_acquire(&l, VAL);     \
                           ^
>> kernel/locking/mcs_spinlock.h:91:2: note: in expansion of macro 'arch_mcs_spin_lock_contended'
     arch_mcs_spin_lock_contended(&node->locked);
     ^
>> kernel/locking/mcs_spinlock.h:31:28: error: 'VAL' undeclared (first use in this function)
     smp_cond_load_acquire(&l, VAL);     \
                               ^
>> kernel/locking/mcs_spinlock.h:91:2: note: in expansion of macro 'arch_mcs_spin_lock_contended'
     arch_mcs_spin_lock_contended(&node->locked);
     ^
   kernel/locking/mcs_spinlock.h:31:28: note: each undeclared identifier is reported only once for each function it appears in
     smp_cond_load_acquire(&l, VAL);     \
                               ^
>> kernel/locking/mcs_spinlock.h:91:2: note: in expansion of macro 'arch_mcs_spin_lock_contended'
     arch_mcs_spin_lock_contended(&node->locked);
     ^
   kernel/locking/qspinlock.c: In function 'native_queued_spin_lock_slowpath':
>> kernel/locking/mcs_spinlock.h:31:24: error: lvalue required as unary '&' operand
     smp_cond_load_acquire(&l, VAL);     \
                           ^
>> kernel/locking/qspinlock.c:411:3: note: in expansion of macro 'arch_mcs_spin_lock_contended'
      arch_mcs_spin_lock_contended(&node->locked);
      ^
>> kernel/locking/mcs_spinlock.h:31:28: error: 'VAL' undeclared (first use in this function)
     smp_cond_load_acquire(&l, VAL);     \
                               ^
>> kernel/locking/qspinlock.c:411:3: note: in expansion of macro 'arch_mcs_spin_lock_contended'
      arch_mcs_spin_lock_contended(&node->locked);
      ^
   kernel/locking/qspinlock.c: In function '__pv_queued_spin_lock_slowpath':
>> kernel/locking/mcs_spinlock.h:31:24: error: lvalue required as unary '&' operand
     smp_cond_load_acquire(&l, VAL);     \
                           ^
>> kernel/locking/qspinlock.c:411:3: note: in expansion of macro 'arch_mcs_spin_lock_contended'
      arch_mcs_spin_lock_contended(&node->locked);
      ^
>> kernel/locking/mcs_spinlock.h:31:28: error: 'VAL' undeclared (first use in this function)
     smp_cond_load_acquire(&l, VAL);     \
                               ^
>> kernel/locking/qspinlock.c:411:3: note: in expansion of macro 'arch_mcs_spin_lock_contended'
      arch_mcs_spin_lock_contended(&node->locked);
      ^

cc1: some warnings being treated as errors

vim +/smp_cond_load_acquire +31 kernel/locking/mcs_spinlock.h

25 * Using smp_cond_load_acquire() provides the acquire semantics
26 * required so that subsequent operations happen after the
27 * lock is acquired.
28 */
29 #define arch_mcs_spin_lock_contended(l) \
30 do { \

  > 31		smp_cond_load_acquire(&l, VAL);					\
    32	} while (0)
    33	#endif
    34	
    35	#ifndef arch_mcs_spin_unlock_contended
    36	/*
    37	 * smp_store_release() provides a memory barrier to ensure all
    38	 * operations in the critical section has been completed before
    39	 * unlocking.
    40	 */
    41	#define arch_mcs_spin_unlock_contended(l)				\
    42		smp_store_release((l), 1)
    43	#endif
    44	
    45	/*
    46	 * Note: the smp_load_acquire/smp_store_release pair is not
    47	 * sufficient to form a full memory barrier across
    48	 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
    49	 * For applications that need a full barrier across multiple cpus
    50	 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
    51	 * used after mcs_lock.
    52	 */
    53	
    54	/*
    55	 * In order to acquire the lock, the caller should declare a local node and
    56	 * pass a reference of the node to this function in addition to the lock.
    57	 * If the lock has already been acquired, then this will proceed to spin
    58	 * on this node->locked until the previous lock holder sets the node->locked
    59	 * in mcs_spin_unlock().
    60	 */
    61	static inline
    62	void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
    63	{
    64		struct mcs_spinlock *prev;
    65	
    66		/* Init node */
    67		node->locked = 0;
    68		node->next   = NULL;
    69	
    70		/*
    71		 * We rely on the full barrier with global transitivity implied by the
    72		 * below xchg() to order the initialization stores above against any
    73		 * observation of @node. And to provide the ACQUIRE ordering associated
    74		 * with a LOCK primitive.
    75		 */
    76		prev = xchg(lock, node);
    77		if (likely(prev == NULL)) {
    78			/*
    79			 * Lock acquired, don't need to set node->locked to 1. Threads
    80			 * only spin on its own node->locked value for lock acquisition.
    81			 * However, since this thread can immediately acquire the lock
    82			 * and does not proceed to spin on its own node->locked, this
    83			 * value won't be used. If a debug mode is needed to
    84			 * audit lock status, then set node->locked value here.
    85			 */
    86			return;
    87		}
    88		WRITE_ONCE(prev->next, node);
    89	
    90		/* Wait until the lock holder passes the lock down. */
  > 91		arch_mcs_spin_lock_contended(&node->locked);
    92	}
    93	
    94	/*

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