// SPDX-License-Identifier: GPL-2.0

  #include <linux/percpu.h>

  #include <linux/sched.h>

  #include <linux/osq_lock.h>

  
  /*
   * An MCS like lock especially tailored for optimistic spinning for sleeping
   * lock implementations (mutex, rwsem, etc).
   *
   * Using a single mcs node per CPU is safe because sleeping locks should not be
   * called from interrupt context and we have preemption disabled while
   * spinning.
   */

  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);

  
  /*

   * We use the value 0 to represent "no CPU", thus the encoded value
   * will be the CPU number incremented by 1.
   */
  static inline int encode_cpu(int cpu_nr)
  {
  	return cpu_nr + 1;
  }

  static inline int node_cpu(struct optimistic_spin_node *node)
  {
  	return node->cpu - 1;
  }

  static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
  {
  	int cpu_nr = encoded_cpu_val - 1;
  
  	return per_cpu_ptr(&osq_node, cpu_nr);
  }
  
  /*

   * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
   * Can return NULL in case we were the last queued and we updated @lock instead.
   */

  static inline struct optimistic_spin_node *

  osq_wait_next(struct optimistic_spin_queue *lock,

  	      struct optimistic_spin_node *node,
  	      struct optimistic_spin_node *prev)

  {

  	struct optimistic_spin_node *next = NULL;

  	int curr = encode_cpu(smp_processor_id());
  	int old;
  
  	/*
  	 * If there is a prev node in queue, then the 'old' value will be
  	 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
  	 * we're currently last in queue, then the queue will then become empty.
  	 */
  	old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;

  
  	for (;;) {

  		if (atomic_read(&lock->tail) == curr &&

  		    atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {

  			/*
  			 * We were the last queued, we moved @lock back. @prev
  			 * will now observe @lock and will complete its
  			 * unlock()/unqueue().
  			 */
  			break;
  		}
  
  		/*
  		 * We must xchg() the @node->next value, because if we were to
  		 * leave it in, a concurrent unlock()/unqueue() from
  		 * @node->next might complete Step-A and think its @prev is
  		 * still valid.
  		 *
  		 * If the concurrent unlock()/unqueue() wins the race, we'll
  		 * wait for either @lock to point to us, through its Step-B, or
  		 * wait for a new @node->next from its Step-C.
  		 */
  		if (node->next) {
  			next = xchg(&node->next, NULL);
  			if (next)
  				break;
  		}

  		cpu_relax();

  	}
  
  	return next;
  }

  bool osq_lock(struct optimistic_spin_queue *lock)

  {

  	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
  	struct optimistic_spin_node *prev, *next;

  	int curr = encode_cpu(smp_processor_id());
  	int old;

  
  	node->locked = 0;
  	node->next = NULL;

  	node->cpu = curr;

  	/*

  	 * We need both ACQUIRE (pairs with corresponding RELEASE in
  	 * unlock() uncontended, or fastpath) and RELEASE (to publish
  	 * the node fields we just initialised) semantics when updating
  	 * the lock tail.

  	 */

  	old = atomic_xchg(&lock->tail, curr);

  	if (old == OSQ_UNLOCKED_VAL)

  		return true;

  	prev = decode_cpu(old);
  	node->prev = prev;

  
  	/*
  	 * osq_lock()			unqueue
  	 *
  	 * node->prev = prev		osq_wait_next()
  	 * WMB				MB
  	 * prev->next = node		next->prev = prev // unqueue-C
  	 *
  	 * Here 'node->prev' and 'next->prev' are the same variable and we need
  	 * to ensure these stores happen in-order to avoid corrupting the list.
  	 */
  	smp_wmb();

  	WRITE_ONCE(prev->next, node);

  
  	/*
  	 * Normally @prev is untouchable after the above store; because at that
  	 * moment unlock can proceed and wipe the node element from stack.
  	 *
  	 * However, since our nodes are static per-cpu storage, we're
  	 * guaranteed their existence -- this allows us to apply
  	 * cmpxchg in an attempt to undo our queueing.
  	 */

  	while (!READ_ONCE(node->locked)) {

  		/*
  		 * If we need to reschedule bail... so we can block.

  		 * Use vcpu_is_preempted() to avoid waiting for a preempted
  		 * lock holder:

  		 */

  		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))

  			goto unqueue;

  		cpu_relax();

  	}
  	return true;
  
  unqueue:
  	/*
  	 * Step - A  -- stabilize @prev
  	 *
  	 * Undo our @prev->next assignment; this will make @prev's
  	 * unlock()/unqueue() wait for a next pointer since @lock points to us
  	 * (or later).
  	 */
  
  	for (;;) {
  		if (prev->next == node &&
  		    cmpxchg(&prev->next, node, NULL) == node)
  			break;
  
  		/*
  		 * We can only fail the cmpxchg() racing against an unlock(),
  		 * in which case we should observe @node->locked becomming
  		 * true.
  		 */
  		if (smp_load_acquire(&node->locked))
  			return true;

  		cpu_relax();

  
  		/*
  		 * Or we race against a concurrent unqueue()'s step-B, in which
  		 * case its step-C will write us a new @node->prev pointer.
  		 */

  		prev = READ_ONCE(node->prev);

  	}
  
  	/*
  	 * Step - B -- stabilize @next
  	 *
  	 * Similar to unlock(), wait for @node->next or move @lock from @node
  	 * back to @prev.
  	 */
  
  	next = osq_wait_next(lock, node, prev);
  	if (!next)
  		return false;
  
  	/*
  	 * Step - C -- unlink
  	 *
  	 * @prev is stable because its still waiting for a new @prev->next
  	 * pointer, @next is stable because our @node->next pointer is NULL and
  	 * it will wait in Step-A.
  	 */

  	WRITE_ONCE(next->prev, prev);
  	WRITE_ONCE(prev->next, next);

  
  	return false;
  }

  void osq_unlock(struct optimistic_spin_queue *lock)

  {

  	struct optimistic_spin_node *node, *next;

  	int curr = encode_cpu(smp_processor_id());

  
  	/*
  	 * Fast path for the uncontended case.
  	 */

  	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
  					  OSQ_UNLOCKED_VAL) == curr))

  		return;
  
  	/*
  	 * Second most likely case.
  	 */

  	node = this_cpu_ptr(&osq_node);

  	next = xchg(&node->next, NULL);
  	if (next) {

  		WRITE_ONCE(next->locked, 1);

  		return;
  	}
  
  	next = osq_wait_next(lock, node, NULL);
  	if (next)

  		WRITE_ONCE(next->locked, 1);

  }