/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
   * generic spinlock implementation
   *
   * Copyright (c) 2001   David Howells (dhowells@redhat.com).
   * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
   * - Derived also from comments by Linus
   */
  #include <linux/rwsem.h>
  #include <linux/sched.h>
  #include <linux/module.h>
  
  struct rwsem_waiter {
  	struct list_head list;
  	struct task_struct *task;
  	unsigned int flags;
  #define RWSEM_WAITING_FOR_READ	0x00000001
  #define RWSEM_WAITING_FOR_WRITE	0x00000002
  };

  int rwsem_is_locked(struct rw_semaphore *sem)
  {
  	int ret = 1;
  	unsigned long flags;
  
  	if (spin_trylock_irqsave(&sem->wait_lock, flags)) {
  		ret = (sem->activity != 0);
  		spin_unlock_irqrestore(&sem->wait_lock, flags);
  	}
  	return ret;
  }
  EXPORT_SYMBOL(rwsem_is_locked);

  /*
   * initialise the semaphore
   */

  void __init_rwsem(struct rw_semaphore *sem, const char *name,
  		  struct lock_class_key *key)

  {

  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	/*
  	 * Make sure we are not reinitializing a held semaphore:
  	 */
  	debug_check_no_locks_freed((void *)sem, sizeof(*sem));

  	lockdep_init_map(&sem->dep_map, name, key, 0);

  #endif

  	sem->activity = 0;
  	spin_lock_init(&sem->wait_lock);
  	INIT_LIST_HEAD(&sem->wait_list);

  }

  EXPORT_SYMBOL(__init_rwsem);

  
  /*
   * handle the lock release when processes blocked on it that can now run
   * - if we come here, then:
   *   - the 'active count' _reached_ zero
   *   - the 'waiting count' is non-zero
   * - the spinlock must be held by the caller
   * - woken process blocks are discarded from the list after having task zeroed
   * - writers are only woken if wakewrite is non-zero
   */
  static inline struct rw_semaphore *
  __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
  {
  	struct rwsem_waiter *waiter;
  	struct task_struct *tsk;
  	int woken;

  	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  
  	if (!wakewrite) {
  		if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
  			goto out;
  		goto dont_wake_writers;
  	}
  
  	/* if we are allowed to wake writers try to grant a single write lock
  	 * if there's a writer at the front of the queue
  	 * - we leave the 'waiting count' incremented to signify potential
  	 *   contention
  	 */
  	if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
  		sem->activity = -1;
  		list_del(&waiter->list);
  		tsk = waiter->task;
  		/* Don't touch waiter after ->task has been NULLed */

  		smp_mb();

  		waiter->task = NULL;
  		wake_up_process(tsk);
  		put_task_struct(tsk);
  		goto out;
  	}
  
  	/* grant an infinite number of read locks to the front of the queue */
   dont_wake_writers:
  	woken = 0;
  	while (waiter->flags & RWSEM_WAITING_FOR_READ) {
  		struct list_head *next = waiter->list.next;
  
  		list_del(&waiter->list);
  		tsk = waiter->task;

  		smp_mb();

  		waiter->task = NULL;
  		wake_up_process(tsk);
  		put_task_struct(tsk);
  		woken++;
  		if (list_empty(&sem->wait_list))
  			break;
  		waiter = list_entry(next, struct rwsem_waiter, list);
  	}
  
  	sem->activity += woken;
  
   out:

  	return sem;
  }
  
  /*
   * wake a single writer
   */
  static inline struct rw_semaphore *
  __rwsem_wake_one_writer(struct rw_semaphore *sem)
  {
  	struct rwsem_waiter *waiter;
  	struct task_struct *tsk;
  
  	sem->activity = -1;
  
  	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  	list_del(&waiter->list);
  
  	tsk = waiter->task;

  	smp_mb();

  	waiter->task = NULL;
  	wake_up_process(tsk);
  	put_task_struct(tsk);
  	return sem;
  }
  
  /*
   * get a read lock on the semaphore
   */

  void __sched __down_read(struct rw_semaphore *sem)

  {
  	struct rwsem_waiter waiter;
  	struct task_struct *tsk;

  	unsigned long flags;

  	spin_lock_irqsave(&sem->wait_lock, flags);

  
  	if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
  		/* granted */
  		sem->activity++;

  		spin_unlock_irqrestore(&sem->wait_lock, flags);

  		goto out;
  	}
  
  	tsk = current;
  	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  
  	/* set up my own style of waitqueue */
  	waiter.task = tsk;
  	waiter.flags = RWSEM_WAITING_FOR_READ;
  	get_task_struct(tsk);
  
  	list_add_tail(&waiter.list, &sem->wait_list);
  
  	/* we don't need to touch the semaphore struct anymore */

  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  
  	/* wait to be given the lock */
  	for (;;) {
  		if (!waiter.task)
  			break;
  		schedule();
  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  	}
  
  	tsk->state = TASK_RUNNING;

   out:

  	;

  }
  
  /*
   * trylock for reading -- returns 1 if successful, 0 if contention
   */

  int __down_read_trylock(struct rw_semaphore *sem)

  {
  	unsigned long flags;
  	int ret = 0;

  
  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
  		/* granted */
  		sem->activity++;
  		ret = 1;
  	}
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  	return ret;
  }
  
  /*
   * get a write lock on the semaphore
   * - we increment the waiting count anyway to indicate an exclusive lock
   */

  void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)

  {
  	struct rwsem_waiter waiter;
  	struct task_struct *tsk;

  	unsigned long flags;

  	spin_lock_irqsave(&sem->wait_lock, flags);

  
  	if (sem->activity == 0 && list_empty(&sem->wait_list)) {
  		/* granted */
  		sem->activity = -1;

  		spin_unlock_irqrestore(&sem->wait_lock, flags);

  		goto out;
  	}
  
  	tsk = current;
  	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  
  	/* set up my own style of waitqueue */
  	waiter.task = tsk;
  	waiter.flags = RWSEM_WAITING_FOR_WRITE;
  	get_task_struct(tsk);
  
  	list_add_tail(&waiter.list, &sem->wait_list);
  
  	/* we don't need to touch the semaphore struct anymore */

  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  
  	/* wait to be given the lock */
  	for (;;) {
  		if (!waiter.task)
  			break;
  		schedule();
  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  	}
  
  	tsk->state = TASK_RUNNING;

   out:

  	;

  }

  void __sched __down_write(struct rw_semaphore *sem)

  {
  	__down_write_nested(sem, 0);
  }

  /*
   * trylock for writing -- returns 1 if successful, 0 if contention
   */

  int __down_write_trylock(struct rw_semaphore *sem)

  {
  	unsigned long flags;
  	int ret = 0;

  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	if (sem->activity == 0 && list_empty(&sem->wait_list)) {
  		/* granted */
  		sem->activity = -1;
  		ret = 1;
  	}
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  	return ret;
  }
  
  /*
   * release a read lock on the semaphore
   */

  void __up_read(struct rw_semaphore *sem)

  {
  	unsigned long flags;

  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	if (--sem->activity == 0 && !list_empty(&sem->wait_list))
  		sem = __rwsem_wake_one_writer(sem);
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  }
  
  /*
   * release a write lock on the semaphore
   */

  void __up_write(struct rw_semaphore *sem)

  {
  	unsigned long flags;

  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	sem->activity = 0;
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, 1);
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  }
  
  /*
   * downgrade a write lock into a read lock
   * - just wake up any readers at the front of the queue
   */

  void __downgrade_write(struct rw_semaphore *sem)

  {
  	unsigned long flags;

  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	sem->activity = 1;
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, 0);
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);

  }