text: /* rwsem.c: R/W semaphores: contention handling functions
   *
   * Written by David Howells (dhowells@redhat.com).
   * Derived from arch/i386/kernel/semaphore.c
   */
  #include <linux/rwsem.h>
  #include <linux/sched.h>
  #include <linux/init.h>
  #include <linux/module.h>
  
  /*
   * Initialize an rwsem:
   */
  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->count = RWSEM_UNLOCKED_VALUE;
  	spin_lock_init(&sem->wait_lock);
  	INIT_LIST_HEAD(&sem->wait_list);
  }
  
  EXPORT_SYMBOL(__init_rwsem);
  
  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
  };
  
  /*
   * handle the lock release when processes blocked on it that can now run
   * - if we come here from up_xxxx(), then:
   *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
   *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
   *   - there must be someone on the queue
   * - 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 downgrading is false
   */
  static inline struct rw_semaphore *
  __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
  {
  	struct rwsem_waiter *waiter;
  	struct task_struct *tsk;
  	struct list_head *next;
  	signed long oldcount, woken, loop;
  
  	if (downgrading)
  		goto dont_wake_writers;
  
  	/* if we came through an up_xxxx() call, we only only wake someone up
  	 * if we can transition the active part of the count from 0 -> 1
  	 */
   try_again:
  	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
  						- RWSEM_ACTIVE_BIAS;
  	if (oldcount & RWSEM_ACTIVE_MASK)
  		goto undo;
  
  	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  
  	/* try to grant a single write lock if there's a writer at the front
  	 * of the queue - note we leave the 'active part' of the count
  	 * incremented by 1 and the waiting part incremented by 0x00010000
  	 */
  	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
  		goto readers_only;
  
  	/* We must be careful not to touch 'waiter' after we set ->task = NULL.
  	 * It is an allocated on the waiter's stack and may become invalid at
  	 * any time after that point (due to a wakeup from another source).
  	 */
  	list_del(&waiter->list);
  	tsk = waiter->task;
  	smp_mb();
  	waiter->task = NULL;
  	wake_up_process(tsk);
  	put_task_struct(tsk);
  	goto out;
  
  	/* don't want to wake any writers */
   dont_wake_writers:
  	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  	if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
  		goto out;
  
  	/* grant an infinite number of read locks to the readers at the front
  	 * of the queue
  	 * - note we increment the 'active part' of the count by the number of
  	 *   readers before waking any processes up
  	 */
   readers_only:
  	woken = 0;
  	do {
  		woken++;
  
  		if (waiter->list.next == &sem->wait_list)
  			break;
  
  		waiter = list_entry(waiter->list.next,
  					struct rwsem_waiter, list);
  
  	} while (waiter->flags & RWSEM_WAITING_FOR_READ);
  
  	loop = woken;
  	woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
  	if (!downgrading)
  		/* we'd already done one increment earlier */
  		woken -= RWSEM_ACTIVE_BIAS;
  
  	rwsem_atomic_add(woken, sem);
  
  	next = sem->wait_list.next;
  	for (; loop > 0; loop--) {
  		waiter = list_entry(next, struct rwsem_waiter, list);
  		next = waiter->list.next;
  		tsk = waiter->task;
  		smp_mb();
  		waiter->task = NULL;
  		wake_up_process(tsk);
  		put_task_struct(tsk);
  	}
  
  	sem->wait_list.next = next;
  	next->prev = &sem->wait_list;
  
   out:
  	return sem;
  
  	/* undo the change to count, but check for a transition 1->0 */
   undo:
  	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
  		goto out;
  	goto try_again;
  }
  
  /*
   * wait for a lock to be granted
   */
  static struct rw_semaphore *
  rwsem_down_failed_common(struct rw_semaphore *sem,
  			struct rwsem_waiter *waiter, signed long adjustment)
  {
  	struct task_struct *tsk = current;
  	signed long count;
  
  	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  
  	/* set up my own style of waitqueue */
  	spin_lock_irq(&sem->wait_lock);
  	waiter->task = tsk;
  	get_task_struct(tsk);
  
  	list_add_tail(&waiter->list, &sem->wait_list);
  
  	/* we're now waiting on the lock, but no longer actively read-locking */
  	count = rwsem_atomic_update(adjustment, sem);
  
  	/* if there are no active locks, wake the front queued process(es) up */
  	if (!(count & RWSEM_ACTIVE_MASK))
  		sem = __rwsem_do_wake(sem, 0);
  
  	spin_unlock_irq(&sem->wait_lock);
  
  	/* wait to be given the lock */
  	for (;;) {
  		if (!waiter->task)
  			break;
  		schedule();
  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  	}
  
  	tsk->state = TASK_RUNNING;
  
  	return sem;
  }
  
  /*
   * wait for the read lock to be granted
   */
  struct rw_semaphore fastcall __sched *
  rwsem_down_read_failed(struct rw_semaphore *sem)
  {
  	struct rwsem_waiter waiter;
  
  	waiter.flags = RWSEM_WAITING_FOR_READ;
  	rwsem_down_failed_common(sem, &waiter,
  				RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
  	return sem;
  }
  
  /*
   * wait for the write lock to be granted
   */
  struct rw_semaphore fastcall __sched *
  rwsem_down_write_failed(struct rw_semaphore *sem)
  {
  	struct rwsem_waiter waiter;
  
  	waiter.flags = RWSEM_WAITING_FOR_WRITE;
  	rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
  
  	return sem;
  }
  
  /*
   * handle waking up a waiter on the semaphore
   * - up_read/up_write has decremented the active part of count if we come here
   */
  struct rw_semaphore fastcall *rwsem_wake(struct rw_semaphore *sem)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	/* do nothing if list empty */
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, 0);
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);
  
  	return sem;
  }
  
  /*
   * downgrade a write lock into a read lock
   * - caller incremented waiting part of count and discovered it still negative
   * - just wake up any readers at the front of the queue
   */
  struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&sem->wait_lock, flags);
  
  	/* do nothing if list empty */
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, 1);
  
  	spin_unlock_irqrestore(&sem->wait_lock, flags);
  
  	return sem;
  }
  
  EXPORT_SYMBOL(rwsem_down_read_failed);
  EXPORT_SYMBOL(rwsem_down_write_failed);
  EXPORT_SYMBOL(rwsem_wake);
  EXPORT_SYMBOL(rwsem_downgrade_wake);
  