text: /*
   * MCS lock defines
   *
   * This file contains the main data structure and API definitions of MCS lock.
   *
   * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
   * with the desirable properties of being fair, and with each cpu trying
   * to acquire the lock spinning on a local variable.
   * It avoids expensive cache bouncings that common test-and-set spin-lock
   * implementations incur.
   */
  #ifndef __LINUX_MCS_SPINLOCK_H
  #define __LINUX_MCS_SPINLOCK_H
  
  #include <asm/mcs_spinlock.h>
  
  struct mcs_spinlock {
  	struct mcs_spinlock *next;
  	int locked; /* 1 if lock acquired */
  };
  
  #ifndef arch_mcs_spin_lock_contended
  /*
   * Using smp_load_acquire() provides a memory barrier that ensures
   * subsequent operations happen after the lock is acquired.
   */
  #define arch_mcs_spin_lock_contended(l)					\
  do {									\
  	while (!(smp_load_acquire(l)))					\
  		cpu_relax_lowlatency();					\
  } while (0)
  #endif
  
  #ifndef arch_mcs_spin_unlock_contended
  /*
   * smp_store_release() provides a memory barrier to ensure all
   * operations in the critical section has been completed before
   * unlocking.
   */
  #define arch_mcs_spin_unlock_contended(l)				\
  	smp_store_release((l), 1)
  #endif
  
  /*
   * Note: the smp_load_acquire/smp_store_release pair is not
   * sufficient to form a full memory barrier across
   * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
   * For applications that need a full barrier across multiple cpus
   * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
   * used after mcs_lock.
   */
  
  /*
   * In order to acquire the lock, the caller should declare a local node and
   * pass a reference of the node to this function in addition to the lock.
   * If the lock has already been acquired, then this will proceed to spin
   * on this node->locked until the previous lock holder sets the node->locked
   * in mcs_spin_unlock().
   */
  static inline
  void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  {
  	struct mcs_spinlock *prev;
  
  	/* Init node */
  	node->locked = 0;
  	node->next   = NULL;
  
  	prev = xchg(lock, node);
  	if (likely(prev == NULL)) {
  		/*
  		 * Lock acquired, don't need to set node->locked to 1. Threads
  		 * only spin on its own node->locked value for lock acquisition.
  		 * However, since this thread can immediately acquire the lock
  		 * and does not proceed to spin on its own node->locked, this
  		 * value won't be used. If a debug mode is needed to
  		 * audit lock status, then set node->locked value here.
  		 */
  		return;
  	}
  	ACCESS_ONCE(prev->next) = node;
  
  	/* Wait until the lock holder passes the lock down. */
  	arch_mcs_spin_lock_contended(&node->locked);
  }
  
  /*
   * Releases the lock. The caller should pass in the corresponding node that
   * was used to acquire the lock.
   */
  static inline
  void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  {
  	struct mcs_spinlock *next = ACCESS_ONCE(node->next);
  
  	if (likely(!next)) {
  		/*
  		 * Release the lock by setting it to NULL
  		 */
  		if (likely(cmpxchg(lock, node, NULL) == node))
  			return;
  		/* Wait until the next pointer is set */
  		while (!(next = ACCESS_ONCE(node->next)))
  			cpu_relax_lowlatency();
  	}
  
  	/* Pass lock to next waiter. */
  	arch_mcs_spin_unlock_contended(&next->locked);
  }
  
  #endif /* __LINUX_MCS_SPINLOCK_H */
  