kernel/sched/wait.c - arm/linux - Git at Google

 /*
  * Generic waiting primitives.
  *
  * (C) 2004 Nadia Yvette Chambers, Oracle
  */
 #include <linux/init.h>
 #include <linux/export.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
 #include <linux/wait.h>
 #include <linux/hash.h>

 void __init_waitqueue_head(wait_queue_head_t *q, const char *name, struct lock_class_key *key)
 {
 	spin_lock_init(&q->lock);
 	lockdep_set_class_and_name(&q->lock, key, name);
 	INIT_LIST_HEAD(&q->task_list);
 }

 EXPORT_SYMBOL(__init_waitqueue_head);

 void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
 {
 	unsigned long flags;

 	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&q->lock, flags);
 	__add_wait_queue(q, wait);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(add_wait_queue);

 void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait)
 {
 	unsigned long flags;

 	wait->flags |= WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&q->lock, flags);
 	__add_wait_queue_tail(q, wait);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(add_wait_queue_exclusive);

 void remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&q->lock, flags);
 	__remove_wait_queue(q, wait);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(remove_wait_queue);


 /*
  * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
  * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
  * number) then we wake all the non-exclusive tasks and one exclusive task.
  *
  * There are circumstances in which we can try to wake a task which has already
  * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
  * zero in this (rare) case, and we handle it by continuing to scan the queue.
  */
 static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
 			int nr_exclusive, int wake_flags, void *key)
 {
 	wait_queue_t *curr, *next;

 	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
 		unsigned flags = curr->flags;

 		if (curr->func(curr, mode, wake_flags, key) &&
 				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
 			break;
 	}
 }

 /**
  * __wake_up - wake up threads blocked on a waitqueue.
  * @q: the waitqueue
  * @mode: which threads
  * @nr_exclusive: how many wake-one or wake-many threads to wake up
  * @key: is directly passed to the wakeup function
  *
  * It may be assumed that this function implies a write memory barrier before
  * changing the task state if and only if any tasks are woken up.
  */
 void __wake_up(wait_queue_head_t *q, unsigned int mode,
 			int nr_exclusive, void *key)
 {
 	unsigned long flags;

 	spin_lock_irqsave(&q->lock, flags);
 	__wake_up_common(q, mode, nr_exclusive, 0, key);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(__wake_up);

 /*
  * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
  */
 void __wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr)
 {
 	__wake_up_common(q, mode, nr, 0, NULL);
 }
 EXPORT_SYMBOL_GPL(__wake_up_locked);

 void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key)
 {
 	__wake_up_common(q, mode, 1, 0, key);
 }
 EXPORT_SYMBOL_GPL(__wake_up_locked_key);

 /**
  * __wake_up_sync_key - wake up threads blocked on a waitqueue.
  * @q: the waitqueue
  * @mode: which threads
  * @nr_exclusive: how many wake-one or wake-many threads to wake up
  * @key: opaque value to be passed to wakeup targets
  *
  * The sync wakeup differs that the waker knows that it will schedule
  * away soon, so while the target thread will be woken up, it will not
  * be migrated to another CPU - ie. the two threads are 'synchronized'
  * with each other. This can prevent needless bouncing between CPUs.
  *
  * On UP it can prevent extra preemption.
  *
  * It may be assumed that this function implies a write memory barrier before
  * changing the task state if and only if any tasks are woken up.
  */
 void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
 			int nr_exclusive, void *key)
 {
 	unsigned long flags;
 	int wake_flags = 1; /* XXX WF_SYNC */

 	if (unlikely(!q))
 		return;

 	if (unlikely(nr_exclusive != 1))
 		wake_flags = 0;

 	spin_lock_irqsave(&q->lock, flags);
 	__wake_up_common(q, mode, nr_exclusive, wake_flags, key);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL_GPL(__wake_up_sync_key);

 /*
  * __wake_up_sync - see __wake_up_sync_key()
  */
 void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr_exclusive)
 {
 	__wake_up_sync_key(q, mode, nr_exclusive, NULL);
 }
 EXPORT_SYMBOL_GPL(__wake_up_sync);	/* For internal use only */

 /*
  * Note: we use "set_current_state()" _after_ the wait-queue add,
  * because we need a memory barrier there on SMP, so that any
  * wake-function that tests for the wait-queue being active
  * will be guaranteed to see waitqueue addition _or_ subsequent
  * tests in this thread will see the wakeup having taken place.
  *
  * The spin_unlock() itself is semi-permeable and only protects
  * one way (it only protects stuff inside the critical region and
  * stops them from bleeding out - it would still allow subsequent
  * loads to move into the critical region).
  */
 void
 prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait, int state)
 {
 	unsigned long flags;

 	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&q->lock, flags);
 	if (list_empty(&wait->task_list))
 		__add_wait_queue(q, wait);
 	set_current_state(state);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(prepare_to_wait);

 void
 prepare_to_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait, int state)
 {
 	unsigned long flags;

 	wait->flags |= WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&q->lock, flags);
 	if (list_empty(&wait->task_list))
 		__add_wait_queue_tail(q, wait);
 	set_current_state(state);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(prepare_to_wait_exclusive);

 long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state)
 {
 	unsigned long flags;

 	if (signal_pending_state(state, current))
 		return -ERESTARTSYS;

 	wait->private = current;
 	wait->func = autoremove_wake_function;

 	spin_lock_irqsave(&q->lock, flags);
 	if (list_empty(&wait->task_list)) {
 		if (wait->flags & WQ_FLAG_EXCLUSIVE)
 			__add_wait_queue_tail(q, wait);
 		else
 			__add_wait_queue(q, wait);
 	}
 	set_current_state(state);
 	spin_unlock_irqrestore(&q->lock, flags);

 	return 0;
 }
 EXPORT_SYMBOL(prepare_to_wait_event);

 /**
  * finish_wait - clean up after waiting in a queue
  * @q: waitqueue waited on
  * @wait: wait descriptor
  *
  * Sets current thread back to running state and removes
  * the wait descriptor from the given waitqueue if still
  * queued.
  */
 void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
 {
 	unsigned long flags;

 	__set_current_state(TASK_RUNNING);
 	/*
 	 * We can check for list emptiness outside the lock
 	 * IFF:
 	 *  - we use the "careful" check that verifies both
 	 *    the next and prev pointers, so that there cannot
 	 *    be any half-pending updates in progress on other
 	 *    CPU's that we haven't seen yet (and that might
 	 *    still change the stack area.
 	 * and
 	 *  - all other users take the lock (ie we can only
 	 *    have _one_ other CPU that looks at or modifies
 	 *    the list).
 	 */
 	if (!list_empty_careful(&wait->task_list)) {
 		spin_lock_irqsave(&q->lock, flags);
 		list_del_init(&wait->task_list);
 		spin_unlock_irqrestore(&q->lock, flags);
 	}
 }
 EXPORT_SYMBOL(finish_wait);

 /**
  * abort_exclusive_wait - abort exclusive waiting in a queue
  * @q: waitqueue waited on
  * @wait: wait descriptor
  * @mode: runstate of the waiter to be woken
  * @key: key to identify a wait bit queue or %NULL
  *
  * Sets current thread back to running state and removes
  * the wait descriptor from the given waitqueue if still
  * queued.
  *
  * Wakes up the next waiter if the caller is concurrently
  * woken up through the queue.
  *
  * This prevents waiter starvation where an exclusive waiter
  * aborts and is woken up concurrently and no one wakes up
  * the next waiter.
  */
 void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait,
 			unsigned int mode, void *key)
 {
 	unsigned long flags;

 	__set_current_state(TASK_RUNNING);
 	spin_lock_irqsave(&q->lock, flags);
 	if (!list_empty(&wait->task_list))
 		list_del_init(&wait->task_list);
 	else if (waitqueue_active(q))
 		__wake_up_locked_key(q, mode, key);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 EXPORT_SYMBOL(abort_exclusive_wait);

 int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
 {
 	int ret = default_wake_function(wait, mode, sync, key);

 	if (ret)
 		list_del_init(&wait->task_list);
 	return ret;
 }
 EXPORT_SYMBOL(autoremove_wake_function);

 int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg)
 {
 	struct wait_bit_key *key = arg;
 	struct wait_bit_queue *wait_bit
 		= container_of(wait, struct wait_bit_queue, wait);

 	if (wait_bit->key.flags != key->flags ||
 			wait_bit->key.bit_nr != key->bit_nr ||
 			test_bit(key->bit_nr, key->flags))
 		return 0;
 	else
 		return autoremove_wake_function(wait, mode, sync, key);
 }
 EXPORT_SYMBOL(wake_bit_function);

 /*
  * To allow interruptible waiting and asynchronous (i.e. nonblocking)
  * waiting, the actions of __wait_on_bit() and __wait_on_bit_lock() are
  * permitted return codes. Nonzero return codes halt waiting and return.
  */
 int __sched
 __wait_on_bit(wait_queue_head_t *wq, struct wait_bit_queue *q,
 	      wait_bit_action_f *action, unsigned mode)
 {
 	int ret = 0;

 	do {
 		prepare_to_wait(wq, &q->wait, mode);
 		if (test_bit(q->key.bit_nr, q->key.flags))
 			ret = (*action)(&q->key);
 	} while (test_bit(q->key.bit_nr, q->key.flags) && !ret);
 	finish_wait(wq, &q->wait);
 	return ret;
 }
 EXPORT_SYMBOL(__wait_on_bit);

 int __sched out_of_line_wait_on_bit(void *word, int bit,
 				    wait_bit_action_f *action, unsigned mode)
 {
 	wait_queue_head_t *wq = bit_waitqueue(word, bit);
 	DEFINE_WAIT_BIT(wait, word, bit);

 	return __wait_on_bit(wq, &wait, action, mode);
 }
 EXPORT_SYMBOL(out_of_line_wait_on_bit);

 int __sched out_of_line_wait_on_bit_timeout(
 	void *word, int bit, wait_bit_action_f *action,
 	unsigned mode, unsigned long timeout)
 {
 	wait_queue_head_t *wq = bit_waitqueue(word, bit);
 	DEFINE_WAIT_BIT(wait, word, bit);

 	wait.key.timeout = jiffies + timeout;
 	return __wait_on_bit(wq, &wait, action, mode);
 }
 EXPORT_SYMBOL_GPL(out_of_line_wait_on_bit_timeout);

 int __sched
 __wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
 			wait_bit_action_f *action, unsigned mode)
 {
 	do {
 		int ret;

 		prepare_to_wait_exclusive(wq, &q->wait, mode);
 		if (!test_bit(q->key.bit_nr, q->key.flags))
 			continue;
 		ret = action(&q->key);
 		if (!ret)
 			continue;
 		abort_exclusive_wait(wq, &q->wait, mode, &q->key);
 		return ret;
 	} while (test_and_set_bit(q->key.bit_nr, q->key.flags));
 	finish_wait(wq, &q->wait);
 	return 0;
 }
 EXPORT_SYMBOL(__wait_on_bit_lock);

 int __sched out_of_line_wait_on_bit_lock(void *word, int bit,
 					 wait_bit_action_f *action, unsigned mode)
 {
 	wait_queue_head_t *wq = bit_waitqueue(word, bit);
 	DEFINE_WAIT_BIT(wait, word, bit);

 	return __wait_on_bit_lock(wq, &wait, action, mode);
 }
 EXPORT_SYMBOL(out_of_line_wait_on_bit_lock);

 void __wake_up_bit(wait_queue_head_t *wq, void *word, int bit)
 {
 	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(word, bit);
 	if (waitqueue_active(wq))
 		__wake_up(wq, TASK_NORMAL, 1, &key);
 }
 EXPORT_SYMBOL(__wake_up_bit);

 /**
  * wake_up_bit - wake up a waiter on a bit
  * @word: the word being waited on, a kernel virtual address
  * @bit: the bit of the word being waited on
  *
  * There is a standard hashed waitqueue table for generic use. This
  * is the part of the hashtable's accessor API that wakes up waiters
  * on a bit. For instance, if one were to have waiters on a bitflag,
  * one would call wake_up_bit() after clearing the bit.
  *
  * In order for this to function properly, as it uses waitqueue_active()
  * internally, some kind of memory barrier must be done prior to calling
  * this. Typically, this will be smp_mb__after_atomic(), but in some
  * cases where bitflags are manipulated non-atomically under a lock, one
  * may need to use a less regular barrier, such fs/inode.c's smp_mb(),
  * because spin_unlock() does not guarantee a memory barrier.
  */
 void wake_up_bit(void *word, int bit)
 {
 	__wake_up_bit(bit_waitqueue(word, bit), word, bit);
 }
 EXPORT_SYMBOL(wake_up_bit);

 wait_queue_head_t *bit_waitqueue(void *word, int bit)
 {
 	const int shift = BITS_PER_LONG == 32 ? 5 : 6;
 	const struct zone *zone = page_zone(virt_to_page(word));
 	unsigned long val = (unsigned long)word << shift | bit;

 	return &zone->wait_table[hash_long(val, zone->wait_table_bits)];
 }
 EXPORT_SYMBOL(bit_waitqueue);

 /*
  * Manipulate the atomic_t address to produce a better bit waitqueue table hash
  * index (we're keying off bit -1, but that would produce a horrible hash
  * value).
  */
 static inline wait_queue_head_t *atomic_t_waitqueue(atomic_t *p)
 {
 	if (BITS_PER_LONG == 64) {
 		unsigned long q = (unsigned long)p;
 		return bit_waitqueue((void *)(q & ~1), q & 1);
 	}
 	return bit_waitqueue(p, 0);
 }

 static int wake_atomic_t_function(wait_queue_t *wait, unsigned mode, int sync,
 				  void *arg)
 {
 	struct wait_bit_key *key = arg;
 	struct wait_bit_queue *wait_bit
 		= container_of(wait, struct wait_bit_queue, wait);
 	atomic_t *val = key->flags;

 	if (wait_bit->key.flags != key->flags ||
 	    wait_bit->key.bit_nr != key->bit_nr ||
 	    atomic_read(val) != 0)
 		return 0;
 	return autoremove_wake_function(wait, mode, sync, key);
 }

 /*
  * To allow interruptible waiting and asynchronous (i.e. nonblocking) waiting,
  * the actions of __wait_on_atomic_t() are permitted return codes.  Nonzero
  * return codes halt waiting and return.
  */
 static __sched
 int __wait_on_atomic_t(wait_queue_head_t *wq, struct wait_bit_queue *q,
 		       int (*action)(atomic_t *), unsigned mode)
 {
 	atomic_t *val;
 	int ret = 0;

 	do {
 		prepare_to_wait(wq, &q->wait, mode);
 		val = q->key.flags;
 		if (atomic_read(val) == 0)
 			break;
 		ret = (*action)(val);
 	} while (!ret && atomic_read(val) != 0);
 	finish_wait(wq, &q->wait);
 	return ret;
 }

 #define DEFINE_WAIT_ATOMIC_T(name, p)					\
 	struct wait_bit_queue name = {					\
 		.key = __WAIT_ATOMIC_T_KEY_INITIALIZER(p),		\
 		.wait	= {						\
 			.private	= current,			\
 			.func		= wake_atomic_t_function,	\
 			.task_list	=				\
 				LIST_HEAD_INIT((name).wait.task_list),	\
 		},							\
 	}

 __sched int out_of_line_wait_on_atomic_t(atomic_t *p, int (*action)(atomic_t *),
 					 unsigned mode)
 {
 	wait_queue_head_t *wq = atomic_t_waitqueue(p);
 	DEFINE_WAIT_ATOMIC_T(wait, p);

 	return __wait_on_atomic_t(wq, &wait, action, mode);
 }
 EXPORT_SYMBOL(out_of_line_wait_on_atomic_t);

 /**
  * wake_up_atomic_t - Wake up a waiter on a atomic_t
  * @p: The atomic_t being waited on, a kernel virtual address
  *
  * Wake up anyone waiting for the atomic_t to go to zero.
  *
  * Abuse the bit-waker function and its waitqueue hash table set (the atomic_t
  * check is done by the waiter's wake function, not the by the waker itself).
  */
 void wake_up_atomic_t(atomic_t *p)
 {
 	__wake_up_bit(atomic_t_waitqueue(p), p, WAIT_ATOMIC_T_BIT_NR);
 }
 EXPORT_SYMBOL(wake_up_atomic_t);

 __sched int bit_wait(struct wait_bit_key *word)
 {
 	if (signal_pending_state(current->state, current))
 		return 1;
 	schedule();
 	return 0;
 }
 EXPORT_SYMBOL(bit_wait);

 __sched int bit_wait_io(struct wait_bit_key *word)
 {
 	if (signal_pending_state(current->state, current))
 		return 1;
 	io_schedule();
 	return 0;
 }
 EXPORT_SYMBOL(bit_wait_io);

 __sched int bit_wait_timeout(struct wait_bit_key *word)
 {
 	unsigned long now = ACCESS_ONCE(jiffies);
 	if (signal_pending_state(current->state, current))
 		return 1;
 	if (time_after_eq(now, word->timeout))
 		return -EAGAIN;
 	schedule_timeout(word->timeout - now);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(bit_wait_timeout);

 __sched int bit_wait_io_timeout(struct wait_bit_key *word)
 {
 	unsigned long now = ACCESS_ONCE(jiffies);
 	if (signal_pending_state(current->state, current))
 		return 1;
 	if (time_after_eq(now, word->timeout))
 		return -EAGAIN;
 	io_schedule_timeout(word->timeout - now);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(bit_wait_io_timeout);
	/*
	* Generic waiting primitives.
	*
	* (C) 2004 Nadia Yvette Chambers, Oracle
	*/
	#include <linux/init.h>
	#include <linux/export.h>
	#include <linux/sched.h>
	#include <linux/mm.h>
	#include <linux/wait.h>
	#include <linux/hash.h>

	void __init_waitqueue_head(wait_queue_head_t q, const char name, struct lock_class_key *key)
	{
	spin_lock_init(&q->lock);
	lockdep_set_class_and_name(&q->lock, key, name);
	INIT_LIST_HEAD(&q->task_list);
	}

	EXPORT_SYMBOL(__init_waitqueue_head);

	void add_wait_queue(wait_queue_head_t q, wait_queue_t wait)
	{
	unsigned long flags;

	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&q->lock, flags);
	__add_wait_queue(q, wait);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(add_wait_queue);

	void add_wait_queue_exclusive(wait_queue_head_t q, wait_queue_t wait)
	{
	unsigned long flags;

	wait->flags \|= WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&q->lock, flags);
	__add_wait_queue_tail(q, wait);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(add_wait_queue_exclusive);

	void remove_wait_queue(wait_queue_head_t q, wait_queue_t wait)
	{
	unsigned long flags;

	spin_lock_irqsave(&q->lock, flags);
	__remove_wait_queue(q, wait);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(remove_wait_queue);


	/*
	* The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
	* wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
	* number) then we wake all the non-exclusive tasks and one exclusive task.
	*
	* There are circumstances in which we can try to wake a task which has already
	* started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
	* zero in this (rare) case, and we handle it by continuing to scan the queue.
	*/
	static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
	int nr_exclusive, int wake_flags, void *key)
	{
	wait_queue_t curr, next;

	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
	unsigned flags = curr->flags;

	if (curr->func(curr, mode, wake_flags, key) &&
	(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
	break;
	}
	}

	/**
	* __wake_up - wake up threads blocked on a waitqueue.
	* @q: the waitqueue
	* @mode: which threads
	* @nr_exclusive: how many wake-one or wake-many threads to wake up
	* @key: is directly passed to the wakeup function
	*
	* It may be assumed that this function implies a write memory barrier before
	* changing the task state if and only if any tasks are woken up.
	*/
	void __wake_up(wait_queue_head_t *q, unsigned int mode,
	int nr_exclusive, void *key)
	{
	unsigned long flags;

	spin_lock_irqsave(&q->lock, flags);
	__wake_up_common(q, mode, nr_exclusive, 0, key);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(__wake_up);

	/*
	* Same as __wake_up but called with the spinlock in wait_queue_head_t held.
	*/
	void __wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr)
	{
	__wake_up_common(q, mode, nr, 0, NULL);
	}
	EXPORT_SYMBOL_GPL(__wake_up_locked);

	void __wake_up_locked_key(wait_queue_head_t q, unsigned int mode, void key)
	{
	__wake_up_common(q, mode, 1, 0, key);
	}
	EXPORT_SYMBOL_GPL(__wake_up_locked_key);

	/**
	* __wake_up_sync_key - wake up threads blocked on a waitqueue.
	* @q: the waitqueue
	* @mode: which threads
	* @nr_exclusive: how many wake-one or wake-many threads to wake up
	* @key: opaque value to be passed to wakeup targets
	*
	* The sync wakeup differs that the waker knows that it will schedule
	* away soon, so while the target thread will be woken up, it will not
	* be migrated to another CPU - ie. the two threads are 'synchronized'
	* with each other. This can prevent needless bouncing between CPUs.
	*
	* On UP it can prevent extra preemption.
	*
	* It may be assumed that this function implies a write memory barrier before
	* changing the task state if and only if any tasks are woken up.
	*/
	void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
	int nr_exclusive, void *key)
	{
	unsigned long flags;
	int wake_flags = 1; /* XXX WF_SYNC */

	if (unlikely(!q))
	return;

	if (unlikely(nr_exclusive != 1))
	wake_flags = 0;

	spin_lock_irqsave(&q->lock, flags);
	__wake_up_common(q, mode, nr_exclusive, wake_flags, key);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL_GPL(__wake_up_sync_key);

	/*
	* __wake_up_sync - see __wake_up_sync_key()
	*/
	void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr_exclusive)
	{
	__wake_up_sync_key(q, mode, nr_exclusive, NULL);
	}
	EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */

	/*
	* Note: we use "set_current_state()" _after_ the wait-queue add,
	* because we need a memory barrier there on SMP, so that any
	* wake-function that tests for the wait-queue being active
	* will be guaranteed to see waitqueue addition _or_ subsequent
	* tests in this thread will see the wakeup having taken place.
	*
	* The spin_unlock() itself is semi-permeable and only protects
	* one way (it only protects stuff inside the critical region and
	* stops them from bleeding out - it would still allow subsequent
	* loads to move into the critical region).
	*/
	void
	prepare_to_wait(wait_queue_head_t q, wait_queue_t wait, int state)
	{
	unsigned long flags;

	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&q->lock, flags);
	if (list_empty(&wait->task_list))
	__add_wait_queue(q, wait);
	set_current_state(state);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(prepare_to_wait);

	void
	prepare_to_wait_exclusive(wait_queue_head_t q, wait_queue_t wait, int state)
	{
	unsigned long flags;

	wait->flags \|= WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&q->lock, flags);
	if (list_empty(&wait->task_list))
	__add_wait_queue_tail(q, wait);
	set_current_state(state);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(prepare_to_wait_exclusive);

	long prepare_to_wait_event(wait_queue_head_t q, wait_queue_t wait, int state)
	{
	unsigned long flags;

	if (signal_pending_state(state, current))
	return -ERESTARTSYS;

	wait->private = current;
	wait->func = autoremove_wake_function;

	spin_lock_irqsave(&q->lock, flags);
	if (list_empty(&wait->task_list)) {
	if (wait->flags & WQ_FLAG_EXCLUSIVE)
	__add_wait_queue_tail(q, wait);
	else
	__add_wait_queue(q, wait);
	}
	set_current_state(state);
	spin_unlock_irqrestore(&q->lock, flags);

	return 0;
	}
	EXPORT_SYMBOL(prepare_to_wait_event);

	/**
	* finish_wait - clean up after waiting in a queue
	* @q: waitqueue waited on
	* @wait: wait descriptor
	*
	* Sets current thread back to running state and removes
	* the wait descriptor from the given waitqueue if still
	* queued.
	*/
	void finish_wait(wait_queue_head_t q, wait_queue_t wait)
	{
	unsigned long flags;

	__set_current_state(TASK_RUNNING);
	/*
	* We can check for list emptiness outside the lock
	* IFF:
	* - we use the "careful" check that verifies both
	* the next and prev pointers, so that there cannot
	* be any half-pending updates in progress on other
	* CPU's that we haven't seen yet (and that might
	* still change the stack area.
	* and
	* - all other users take the lock (ie we can only
	* have _one_ other CPU that looks at or modifies
	* the list).
	*/
	if (!list_empty_careful(&wait->task_list)) {
	spin_lock_irqsave(&q->lock, flags);
	list_del_init(&wait->task_list);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	}
	EXPORT_SYMBOL(finish_wait);

	/**
	* abort_exclusive_wait - abort exclusive waiting in a queue
	* @q: waitqueue waited on
	* @wait: wait descriptor
	* @mode: runstate of the waiter to be woken
	* @key: key to identify a wait bit queue or %NULL
	*
	* Sets current thread back to running state and removes
	* the wait descriptor from the given waitqueue if still
	* queued.
	*
	* Wakes up the next waiter if the caller is concurrently
	* woken up through the queue.
	*
	* This prevents waiter starvation where an exclusive waiter
	* aborts and is woken up concurrently and no one wakes up
	* the next waiter.
	*/
	void abort_exclusive_wait(wait_queue_head_t q, wait_queue_t wait,
	unsigned int mode, void *key)
	{
	unsigned long flags;

	__set_current_state(TASK_RUNNING);
	spin_lock_irqsave(&q->lock, flags);
	if (!list_empty(&wait->task_list))
	list_del_init(&wait->task_list);
	else if (waitqueue_active(q))
	__wake_up_locked_key(q, mode, key);
	spin_unlock_irqrestore(&q->lock, flags);
	}
	EXPORT_SYMBOL(abort_exclusive_wait);

	int autoremove_wake_function(wait_queue_t wait, unsigned mode, int sync, void key)
	{
	int ret = default_wake_function(wait, mode, sync, key);

	if (ret)
	list_del_init(&wait->task_list);
	return ret;
	}
	EXPORT_SYMBOL(autoremove_wake_function);

	int wake_bit_function(wait_queue_t wait, unsigned mode, int sync, void arg)
	{
	struct wait_bit_key *key = arg;
	struct wait_bit_queue *wait_bit
	= container_of(wait, struct wait_bit_queue, wait);

	if (wait_bit->key.flags != key->flags \|\|
	wait_bit->key.bit_nr != key->bit_nr \|\|
	test_bit(key->bit_nr, key->flags))
	return 0;
	else
	return autoremove_wake_function(wait, mode, sync, key);
	}
	EXPORT_SYMBOL(wake_bit_function);

	/*
	* To allow interruptible waiting and asynchronous (i.e. nonblocking)
	* waiting, the actions of __wait_on_bit() and __wait_on_bit_lock() are
	* permitted return codes. Nonzero return codes halt waiting and return.
	*/
	int __sched
	__wait_on_bit(wait_queue_head_t wq, struct wait_bit_queue q,
	wait_bit_action_f *action, unsigned mode)
	{
	int ret = 0;

	do {
	prepare_to_wait(wq, &q->wait, mode);
	if (test_bit(q->key.bit_nr, q->key.flags))
	ret = (*action)(&q->key);
	} while (test_bit(q->key.bit_nr, q->key.flags) && !ret);
	finish_wait(wq, &q->wait);
	return ret;
	}
	EXPORT_SYMBOL(__wait_on_bit);

	int __sched out_of_line_wait_on_bit(void *word, int bit,
	wait_bit_action_f *action, unsigned mode)
	{
	wait_queue_head_t *wq = bit_waitqueue(word, bit);
	DEFINE_WAIT_BIT(wait, word, bit);

	return __wait_on_bit(wq, &wait, action, mode);
	}
	EXPORT_SYMBOL(out_of_line_wait_on_bit);

	int __sched out_of_line_wait_on_bit_timeout(
	void word, int bit, wait_bit_action_f action,
	unsigned mode, unsigned long timeout)
	{
	wait_queue_head_t *wq = bit_waitqueue(word, bit);
	DEFINE_WAIT_BIT(wait, word, bit);

	wait.key.timeout = jiffies + timeout;
	return __wait_on_bit(wq, &wait, action, mode);
	}
	EXPORT_SYMBOL_GPL(out_of_line_wait_on_bit_timeout);

	int __sched
	__wait_on_bit_lock(wait_queue_head_t wq, struct wait_bit_queue q,
	wait_bit_action_f *action, unsigned mode)
	{
	do {
	int ret;

	prepare_to_wait_exclusive(wq, &q->wait, mode);
	if (!test_bit(q->key.bit_nr, q->key.flags))
	continue;
	ret = action(&q->key);
	if (!ret)
	continue;
	abort_exclusive_wait(wq, &q->wait, mode, &q->key);
	return ret;
	} while (test_and_set_bit(q->key.bit_nr, q->key.flags));
	finish_wait(wq, &q->wait);
	return 0;
	}
	EXPORT_SYMBOL(__wait_on_bit_lock);

	int __sched out_of_line_wait_on_bit_lock(void *word, int bit,
	wait_bit_action_f *action, unsigned mode)
	{
	wait_queue_head_t *wq = bit_waitqueue(word, bit);
	DEFINE_WAIT_BIT(wait, word, bit);

	return __wait_on_bit_lock(wq, &wait, action, mode);
	}
	EXPORT_SYMBOL(out_of_line_wait_on_bit_lock);

	void __wake_up_bit(wait_queue_head_t wq, void word, int bit)
	{
	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(word, bit);
	if (waitqueue_active(wq))
	__wake_up(wq, TASK_NORMAL, 1, &key);
	}
	EXPORT_SYMBOL(__wake_up_bit);

	/**
	* wake_up_bit - wake up a waiter on a bit
	* @word: the word being waited on, a kernel virtual address
	* @bit: the bit of the word being waited on
	*
	* There is a standard hashed waitqueue table for generic use. This
	* is the part of the hashtable's accessor API that wakes up waiters
	* on a bit. For instance, if one were to have waiters on a bitflag,
	* one would call wake_up_bit() after clearing the bit.
	*
	* In order for this to function properly, as it uses waitqueue_active()
	* internally, some kind of memory barrier must be done prior to calling
	* this. Typically, this will be smp_mb__after_atomic(), but in some
	* cases where bitflags are manipulated non-atomically under a lock, one
	* may need to use a less regular barrier, such fs/inode.c's smp_mb(),
	* because spin_unlock() does not guarantee a memory barrier.
	*/
	void wake_up_bit(void *word, int bit)
	{
	__wake_up_bit(bit_waitqueue(word, bit), word, bit);
	}
	EXPORT_SYMBOL(wake_up_bit);

	wait_queue_head_t bit_waitqueue(void word, int bit)
	{
	const int shift = BITS_PER_LONG == 32 ? 5 : 6;
	const struct zone *zone = page_zone(virt_to_page(word));
	unsigned long val = (unsigned long)word << shift \| bit;

	return &zone->wait_table[hash_long(val, zone->wait_table_bits)];
	}
	EXPORT_SYMBOL(bit_waitqueue);

	/*
	* Manipulate the atomic_t address to produce a better bit waitqueue table hash
	* index (we're keying off bit -1, but that would produce a horrible hash
	* value).
	*/
	static inline wait_queue_head_t atomic_t_waitqueue(atomic_t p)
	{
	if (BITS_PER_LONG == 64) {
	unsigned long q = (unsigned long)p;
	return bit_waitqueue((void *)(q & ~1), q & 1);
	}
	return bit_waitqueue(p, 0);
	}

	static int wake_atomic_t_function(wait_queue_t *wait, unsigned mode, int sync,
	void *arg)
	{
	struct wait_bit_key *key = arg;
	struct wait_bit_queue *wait_bit
	= container_of(wait, struct wait_bit_queue, wait);
	atomic_t *val = key->flags;

	if (wait_bit->key.flags != key->flags \|\|
	wait_bit->key.bit_nr != key->bit_nr \|\|
	atomic_read(val) != 0)
	return 0;
	return autoremove_wake_function(wait, mode, sync, key);
	}

	/*
	* To allow interruptible waiting and asynchronous (i.e. nonblocking) waiting,
	* the actions of __wait_on_atomic_t() are permitted return codes. Nonzero
	* return codes halt waiting and return.
	*/
	static __sched
	int __wait_on_atomic_t(wait_queue_head_t wq, struct wait_bit_queue q,
	int (action)(atomic_t ), unsigned mode)
	{
	atomic_t *val;
	int ret = 0;

	do {
	prepare_to_wait(wq, &q->wait, mode);
	val = q->key.flags;
	if (atomic_read(val) == 0)
	break;
	ret = (*action)(val);
	} while (!ret && atomic_read(val) != 0);
	finish_wait(wq, &q->wait);
	return ret;
	}

	#define DEFINE_WAIT_ATOMIC_T(name, p) \
	struct wait_bit_queue name = { \
	.key = __WAIT_ATOMIC_T_KEY_INITIALIZER(p), \
	.wait = { \
	.private = current, \
	.func = wake_atomic_t_function, \
	.task_list = \
	LIST_HEAD_INIT((name).wait.task_list), \
	}, \
	}

	__sched int out_of_line_wait_on_atomic_t(atomic_t p, int (action)(atomic_t *),
	unsigned mode)
	{
	wait_queue_head_t *wq = atomic_t_waitqueue(p);
	DEFINE_WAIT_ATOMIC_T(wait, p);

	return __wait_on_atomic_t(wq, &wait, action, mode);
	}
	EXPORT_SYMBOL(out_of_line_wait_on_atomic_t);

	/**
	* wake_up_atomic_t - Wake up a waiter on a atomic_t
	* @p: The atomic_t being waited on, a kernel virtual address
	*
	* Wake up anyone waiting for the atomic_t to go to zero.
	*
	* Abuse the bit-waker function and its waitqueue hash table set (the atomic_t
	* check is done by the waiter's wake function, not the by the waker itself).
	*/
	void wake_up_atomic_t(atomic_t *p)
	{
	__wake_up_bit(atomic_t_waitqueue(p), p, WAIT_ATOMIC_T_BIT_NR);
	}
	EXPORT_SYMBOL(wake_up_atomic_t);

	__sched int bit_wait(struct wait_bit_key *word)
	{
	if (signal_pending_state(current->state, current))
	return 1;
	schedule();
	return 0;
	}
	EXPORT_SYMBOL(bit_wait);

	__sched int bit_wait_io(struct wait_bit_key *word)
	{
	if (signal_pending_state(current->state, current))
	return 1;
	io_schedule();
	return 0;
	}
	EXPORT_SYMBOL(bit_wait_io);

	__sched int bit_wait_timeout(struct wait_bit_key *word)
	{
	unsigned long now = ACCESS_ONCE(jiffies);
	if (signal_pending_state(current->state, current))
	return 1;
	if (time_after_eq(now, word->timeout))
	return -EAGAIN;
	schedule_timeout(word->timeout - now);
	return 0;
	}
	EXPORT_SYMBOL_GPL(bit_wait_timeout);

	__sched int bit_wait_io_timeout(struct wait_bit_key *word)
	{
	unsigned long now = ACCESS_ONCE(jiffies);
	if (signal_pending_state(current->state, current))
	return 1;
	if (time_after_eq(now, word->timeout))
	return -EAGAIN;
	io_schedule_timeout(word->timeout - now);
	return 0;
	}
	EXPORT_SYMBOL_GPL(bit_wait_io_timeout);