/* SPDX-License-Identifier: GPL-2.0 */

  /*
   * 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 */

  	int count;  /* nesting count, see qspinlock.c */

  };

  #ifndef arch_mcs_spin_lock_contended
  /*

   * Using smp_cond_load_acquire() provides the acquire semantics
   * required so that subsequent operations happen after the
   * lock is acquired. Additionally, some architectures such as
   * ARM64 would like to do spin-waiting instead of purely
   * spinning, and smp_cond_load_acquire() provides that behavior.

   */
  #define arch_mcs_spin_lock_contended(l)					\
  do {									\

  	smp_cond_load_acquire(l, VAL);					\

  } 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;

  	/*
  	 * We rely on the full barrier with global transitivity implied by the
  	 * below xchg() to order the initialization stores above against any
  	 * observation of @node. And to provide the ACQUIRE ordering associated
  	 * with a LOCK primitive.
  	 */
  	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;
  	}

  	WRITE_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 = READ_ONCE(node->next);

  
  	if (likely(!next)) {
  		/*
  		 * Release the lock by setting it to NULL
  		 */

  		if (likely(cmpxchg_release(lock, node, NULL) == node))

  			return;
  		/* Wait until the next pointer is set */

  		while (!(next = READ_ONCE(node->next)))

  			cpu_relax();

  	}

  
  	/* Pass lock to next waiter. */
  	arch_mcs_spin_unlock_contended(&next->locked);

  }
  
  #endif /* __LINUX_MCS_SPINLOCK_H */