[net-next-2.6.git] / lib / rwsem.c

/* 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;

	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
		goto readers_only;

	if (downgrading)
		goto out;

	/* There's a writer at the front of the queue - try to grant it the
	 * write lock.  However, we only wake this writer if we can transition
	 * the active part of the count from 0 -> 1
	 */
 try_again_write:
	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
						- RWSEM_ACTIVE_BIAS;
	if (oldcount & RWSEM_ACTIVE_MASK)
		/* Someone grabbed the sem already */
		goto undo_write;

	/* 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;

 readers_only:
	if (downgrading)
		goto wake_readers;

	/* 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_read:
	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
						- RWSEM_ACTIVE_BIAS;
	if (oldcount & RWSEM_ACTIVE_MASK)
		/* Someone grabbed the sem already */
		goto undo_read;

 wake_readers:
	/* 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.
	 */
	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 the active count, but check for a transition
	 * 1->0 */
 undo_write:
	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
		goto out;
	goto try_again_write;
 undo_read:
	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
		goto out;
	goto try_again_read;
}

/*
 * wait for a lock to be granted
 */
static struct rw_semaphore __sched *
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
 */
asmregparm struct rw_semaphore __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
 */
asmregparm struct rw_semaphore __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
 */
asmregparm struct rw_semaphore *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
 */
asmregparm struct rw_semaphore *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);
Commit	Line	Data
1da177e4 LT	1	/* rwsem.c: R/W semaphores: contention handling functions
	2	*
	3	* Written by David Howells (dhowells@redhat.com).
	4	* Derived from arch/i386/kernel/semaphore.c
	5	*/
	6	#include <linux/rwsem.h>
	7	#include <linux/sched.h>
	8	#include <linux/init.h>
	9	#include <linux/module.h>
	10
4ea2176d IM	11	/*
	12	* Initialize an rwsem:
	13	*/
	14	void __init_rwsem(struct rw_semaphore sem, const char name,
	15	struct lock_class_key *key)
	16	{
	17	#ifdef CONFIG_DEBUG_LOCK_ALLOC
	18	/*
	19	* Make sure we are not reinitializing a held semaphore:
	20	*/
	21	debug_check_no_locks_freed((void )sem, sizeof(sem));
4dfbb9d8	22	lockdep_init_map(&sem->dep_map, name, key, 0);
4ea2176d IM	23	#endif
	24	sem->count = RWSEM_UNLOCKED_VALUE;
	25	spin_lock_init(&sem->wait_lock);
	26	INIT_LIST_HEAD(&sem->wait_list);
	27	}
	28
	29	EXPORT_SYMBOL(__init_rwsem);
	30
1da177e4 LT	31	struct rwsem_waiter {
	32	struct list_head list;
	33	struct task_struct *task;
	34	unsigned int flags;
	35	#define RWSEM_WAITING_FOR_READ 0x00000001
	36	#define RWSEM_WAITING_FOR_WRITE 0x00000002
	37	};
	38
1da177e4 LT	39	/*
	40	* handle the lock release when processes blocked on it that can now run
	41	* - if we come here from up_xxxx(), then:
	42	* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
	43	* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
345af7bf	44	* - there must be someone on the queue
1da177e4 LT	45	* - the spinlock must be held by the caller
	46	* - woken process blocks are discarded from the list after having task zeroed
	47	* - writers are only woken if downgrading is false
	48	*/
	49	static inline struct rw_semaphore *
	50	__rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
	51	{
	52	struct rwsem_waiter *waiter;
	53	struct task_struct *tsk;
	54	struct list_head *next;
	55	signed long oldcount, woken, loop;
	56
345af7bf ML	57	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	58	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
	59	goto readers_only;
	60
1da177e4	61	if (downgrading)
345af7bf	62	goto out;
1da177e4	63
345af7bf ML	64	/* There's a writer at the front of the queue - try to grant it the
	65	* write lock. However, we only wake this writer if we can transition
	66	* the active part of the count from 0 -> 1
1da177e4	67	*/
345af7bf	68	try_again_write:
1da177e4 LT	69	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
	70	- RWSEM_ACTIVE_BIAS;
	71	if (oldcount & RWSEM_ACTIVE_MASK)
345af7bf ML	72	/* Someone grabbed the sem already */
345af7bf ML	73	goto undo_write;
1da177e4 LT	74
	75	/* We must be careful not to touch 'waiter' after we set ->task = NULL.
	76	* It is an allocated on the waiter's stack and may become invalid at
	77	* any time after that point (due to a wakeup from another source).
	78	*/
	79	list_del(&waiter->list);
	80	tsk = waiter->task;
d59dd462	81	smp_mb();
1da177e4 LT	82	waiter->task = NULL;
	83	wake_up_process(tsk);
	84	put_task_struct(tsk);
	85	goto out;
	86
345af7bf ML	87	readers_only:
	88	if (downgrading)
	89	goto wake_readers;
	90
	91	/* if we came through an up_xxxx() call, we only only wake someone up
	92	* if we can transition the active part of the count from 0 -> 1 */
	93	try_again_read:
	94	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
	95	- RWSEM_ACTIVE_BIAS;
	96	if (oldcount & RWSEM_ACTIVE_MASK)
	97	/* Someone grabbed the sem already */
	98	goto undo_read;
1da177e4	99
345af7bf ML	100	wake_readers:
	101	/* Grant an infinite number of read locks to the readers at the front
	102	* of the queue. Note we increment the 'active part' of the count by
	103	* the number of readers before waking any processes up.
1da177e4	104	*/
1da177e4 LT	105	woken = 0;
	106	do {
	107	woken++;
	108
	109	if (waiter->list.next == &sem->wait_list)
	110	break;
	111
	112	waiter = list_entry(waiter->list.next,
	113	struct rwsem_waiter, list);
	114
	115	} while (waiter->flags & RWSEM_WAITING_FOR_READ);
	116
	117	loop = woken;
	118	woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
	119	if (!downgrading)
	120	/* we'd already done one increment earlier */
	121	woken -= RWSEM_ACTIVE_BIAS;
	122
	123	rwsem_atomic_add(woken, sem);
	124
	125	next = sem->wait_list.next;
	126	for (; loop > 0; loop--) {
	127	waiter = list_entry(next, struct rwsem_waiter, list);
	128	next = waiter->list.next;
	129	tsk = waiter->task;
d59dd462	130	smp_mb();
1da177e4 LT	131	waiter->task = NULL;
	132	wake_up_process(tsk);
	133	put_task_struct(tsk);
	134	}
	135
	136	sem->wait_list.next = next;
	137	next->prev = &sem->wait_list;
	138
	139	out:
1da177e4 LT	140	return sem;
1da177e4 LT	141
91af7081 ML	142	/* undo the change to the active count, but check for a transition
91af7081 ML	143	* 1->0 */
345af7bf ML	144	undo_write:
	145	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
	146	goto out;
	147	goto try_again_write;
	148	undo_read:
91af7081	149	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
1da177e4	150	goto out;
345af7bf	151	goto try_again_read;
1da177e4 LT	152	}
	153
	154	/*
	155	* wait for a lock to be granted
	156	*/
c7af77b5	157	static struct rw_semaphore __sched *
1da177e4 LT	158	rwsem_down_failed_common(struct rw_semaphore *sem,
	159	struct rwsem_waiter *waiter, signed long adjustment)
	160	{
	161	struct task_struct *tsk = current;
	162	signed long count;
	163
	164	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	165
	166	/* set up my own style of waitqueue */
	167	spin_lock_irq(&sem->wait_lock);
	168	waiter->task = tsk;
	169	get_task_struct(tsk);
	170
	171	list_add_tail(&waiter->list, &sem->wait_list);
	172
	173	/* we're now waiting on the lock, but no longer actively read-locking */
	174	count = rwsem_atomic_update(adjustment, sem);
	175
	176	/* if there are no active locks, wake the front queued process(es) up */
	177	if (!(count & RWSEM_ACTIVE_MASK))
	178	sem = __rwsem_do_wake(sem, 0);
	179
	180	spin_unlock_irq(&sem->wait_lock);
	181
	182	/* wait to be given the lock */
	183	for (;;) {
	184	if (!waiter->task)
	185	break;
	186	schedule();
	187	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	188	}
	189
	190	tsk->state = TASK_RUNNING;
	191
	192	return sem;
	193	}
	194
	195	/*
	196	* wait for the read lock to be granted
	197	*/
d50efc6c	198	asmregparm struct rw_semaphore __sched *
1da177e4 LT	199	rwsem_down_read_failed(struct rw_semaphore *sem)
	200	{
	201	struct rwsem_waiter waiter;
	202
1da177e4 LT	203	waiter.flags = RWSEM_WAITING_FOR_READ;
	204	rwsem_down_failed_common(sem, &waiter,
	205	RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
1da177e4 LT	206	return sem;
	207	}
	208
	209	/*
	210	* wait for the write lock to be granted
	211	*/
d50efc6c	212	asmregparm struct rw_semaphore __sched *
1da177e4 LT	213	rwsem_down_write_failed(struct rw_semaphore *sem)
	214	{
	215	struct rwsem_waiter waiter;
	216
1da177e4 LT	217	waiter.flags = RWSEM_WAITING_FOR_WRITE;
	218	rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
	219
1da177e4 LT	220	return sem;
	221	}
	222
	223	/*
	224	* handle waking up a waiter on the semaphore
	225	* - up_read/up_write has decremented the active part of count if we come here
	226	*/
d50efc6c	227	asmregparm struct rw_semaphore rwsem_wake(struct rw_semaphore sem)
1da177e4 LT	228	{
	229	unsigned long flags;
	230
1da177e4 LT	231	spin_lock_irqsave(&sem->wait_lock, flags);
	232
	233	/* do nothing if list empty */
	234	if (!list_empty(&sem->wait_list))
	235	sem = __rwsem_do_wake(sem, 0);
	236
	237	spin_unlock_irqrestore(&sem->wait_lock, flags);
	238
1da177e4 LT	239	return sem;
	240	}
	241
	242	/*
	243	* downgrade a write lock into a read lock
	244	* - caller incremented waiting part of count and discovered it still negative
	245	* - just wake up any readers at the front of the queue
	246	*/
d50efc6c	247	asmregparm struct rw_semaphore rwsem_downgrade_wake(struct rw_semaphore sem)
1da177e4 LT	248	{
	249	unsigned long flags;
	250
1da177e4 LT	251	spin_lock_irqsave(&sem->wait_lock, flags);
	252
	253	/* do nothing if list empty */
	254	if (!list_empty(&sem->wait_list))
	255	sem = __rwsem_do_wake(sem, 1);
	256
	257	spin_unlock_irqrestore(&sem->wait_lock, flags);
	258
1da177e4 LT	259	return sem;
	260	}
	261
	262	EXPORT_SYMBOL(rwsem_down_read_failed);
	263	EXPORT_SYMBOL(rwsem_down_write_failed);
	264	EXPORT_SYMBOL(rwsem_wake);
	265	EXPORT_SYMBOL(rwsem_downgrade_wake);