[linux.git] / lib / rwsem-spinlock.c

/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
 * generic spinlock implementation
 *
 * Copyright (c) 2001   David Howells ([email protected]).
 * - Derived partially from idea by Andrea Arcangeli <[email protected]>
 * - 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
};

/*
 * 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);
}

/*
 * 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 fastcall __sched __down_read(struct rw_semaphore *sem)
{
	struct rwsem_waiter waiter;
	struct task_struct *tsk;

	spin_lock_irq(&sem->wait_lock);

	if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
		/* granted */
		sem->activity++;
		spin_unlock_irq(&sem->wait_lock);
		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_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;
 out:
	;
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
int fastcall __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 fastcall __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
{
	struct rwsem_waiter waiter;
	struct task_struct *tsk;

	spin_lock_irq(&sem->wait_lock);

	if (sem->activity == 0 && list_empty(&sem->wait_list)) {
		/* granted */
		sem->activity = -1;
		spin_unlock_irq(&sem->wait_lock);
		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_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;
 out:
	;
}

void fastcall __sched __down_write(struct rw_semaphore *sem)
{
	__down_write_nested(sem, 0);
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
int fastcall __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 fastcall __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 fastcall __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 fastcall __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);
}

EXPORT_SYMBOL(__init_rwsem);
EXPORT_SYMBOL(__down_read);
EXPORT_SYMBOL(__down_read_trylock);
EXPORT_SYMBOL(__down_write_nested);
EXPORT_SYMBOL(__down_write);
EXPORT_SYMBOL(__down_write_trylock);
EXPORT_SYMBOL(__up_read);
EXPORT_SYMBOL(__up_write);
EXPORT_SYMBOL(__downgrade_write);
Commit	Line	Data
1da177e4 LT	1	/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
	2	* generic spinlock implementation
	3	*
	4	* Copyright (c) 2001 David Howells ([email protected]).
	5	* - Derived partially from idea by Andrea Arcangeli <[email protected]>
	6	* - Derived also from comments by Linus
	7	*/
	8	#include <linux/rwsem.h>
	9	#include <linux/sched.h>
	10	#include <linux/module.h>
	11
	12	struct rwsem_waiter {
	13	struct list_head list;
	14	struct task_struct *task;
	15	unsigned int flags;
	16	#define RWSEM_WAITING_FOR_READ 0x00000001
	17	#define RWSEM_WAITING_FOR_WRITE 0x00000002
	18	};
	19
1da177e4 LT	20	/*
	21	* initialise the semaphore
	22	*/
4ea2176d IM	23	void __init_rwsem(struct rw_semaphore sem, const char name,
4ea2176d IM	24	struct lock_class_key *key)
1da177e4	25	{
4ea2176d IM	26	#ifdef CONFIG_DEBUG_LOCK_ALLOC
	27	/*
	28	* Make sure we are not reinitializing a held semaphore:
	29	*/
	30	debug_check_no_locks_freed((void )sem, sizeof(sem));
4dfbb9d8	31	lockdep_init_map(&sem->dep_map, name, key, 0);
4ea2176d	32	#endif
1da177e4 LT	33	sem->activity = 0;
	34	spin_lock_init(&sem->wait_lock);
	35	INIT_LIST_HEAD(&sem->wait_list);
1da177e4 LT	36	}
	37
	38	/*
	39	* handle the lock release when processes blocked on it that can now run
	40	* - if we come here, then:
	41	* - the 'active count' _reached_ zero
	42	* - the 'waiting count' is non-zero
	43	* - the spinlock must be held by the caller
	44	* - woken process blocks are discarded from the list after having task zeroed
	45	* - writers are only woken if wakewrite is non-zero
	46	*/
	47	static inline struct rw_semaphore *
	48	__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
	49	{
	50	struct rwsem_waiter *waiter;
	51	struct task_struct *tsk;
	52	int woken;
	53
1da177e4 LT	54	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	55
	56	if (!wakewrite) {
	57	if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
	58	goto out;
	59	goto dont_wake_writers;
	60	}
	61
	62	/* if we are allowed to wake writers try to grant a single write lock
	63	* if there's a writer at the front of the queue
	64	* - we leave the 'waiting count' incremented to signify potential
	65	* contention
	66	*/
	67	if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
	68	sem->activity = -1;
	69	list_del(&waiter->list);
	70	tsk = waiter->task;
	71	/* Don't touch waiter after ->task has been NULLed */
d59dd462	72	smp_mb();
1da177e4 LT	73	waiter->task = NULL;
	74	wake_up_process(tsk);
	75	put_task_struct(tsk);
	76	goto out;
	77	}
	78
	79	/* grant an infinite number of read locks to the front of the queue */
	80	dont_wake_writers:
	81	woken = 0;
	82	while (waiter->flags & RWSEM_WAITING_FOR_READ) {
	83	struct list_head *next = waiter->list.next;
	84
	85	list_del(&waiter->list);
	86	tsk = waiter->task;
d59dd462	87	smp_mb();
1da177e4 LT	88	waiter->task = NULL;
	89	wake_up_process(tsk);
	90	put_task_struct(tsk);
	91	woken++;
	92	if (list_empty(&sem->wait_list))
	93	break;
	94	waiter = list_entry(next, struct rwsem_waiter, list);
	95	}
	96
	97	sem->activity += woken;
	98
	99	out:
1da177e4 LT	100	return sem;
	101	}
	102
	103	/*
	104	* wake a single writer
	105	*/
	106	static inline struct rw_semaphore *
	107	__rwsem_wake_one_writer(struct rw_semaphore *sem)
	108	{
	109	struct rwsem_waiter *waiter;
	110	struct task_struct *tsk;
	111
	112	sem->activity = -1;
	113
	114	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	115	list_del(&waiter->list);
	116
	117	tsk = waiter->task;
d59dd462	118	smp_mb();
1da177e4 LT	119	waiter->task = NULL;
	120	wake_up_process(tsk);
	121	put_task_struct(tsk);
	122	return sem;
	123	}
	124
	125	/*
	126	* get a read lock on the semaphore
	127	*/
	128	void fastcall __sched __down_read(struct rw_semaphore *sem)
	129	{
	130	struct rwsem_waiter waiter;
	131	struct task_struct *tsk;
	132
1da177e4 LT	133	spin_lock_irq(&sem->wait_lock);
	134
	135	if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
	136	/* granted */
	137	sem->activity++;
	138	spin_unlock_irq(&sem->wait_lock);
	139	goto out;
	140	}
	141
	142	tsk = current;
	143	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	144
	145	/* set up my own style of waitqueue */
	146	waiter.task = tsk;
	147	waiter.flags = RWSEM_WAITING_FOR_READ;
	148	get_task_struct(tsk);
	149
	150	list_add_tail(&waiter.list, &sem->wait_list);
	151
	152	/* we don't need to touch the semaphore struct anymore */
	153	spin_unlock_irq(&sem->wait_lock);
	154
	155	/* wait to be given the lock */
	156	for (;;) {
	157	if (!waiter.task)
	158	break;
	159	schedule();
	160	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	161	}
	162
	163	tsk->state = TASK_RUNNING;
1da177e4	164	out:
c4e05116	165	;
1da177e4 LT	166	}
	167
	168	/*
	169	* trylock for reading -- returns 1 if successful, 0 if contention
	170	*/
	171	int fastcall __down_read_trylock(struct rw_semaphore *sem)
	172	{
	173	unsigned long flags;
	174	int ret = 0;
	175
1da177e4 LT	176
	177	spin_lock_irqsave(&sem->wait_lock, flags);
	178
	179	if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
	180	/* granted */
	181	sem->activity++;
	182	ret = 1;
	183	}
	184
	185	spin_unlock_irqrestore(&sem->wait_lock, flags);
	186
1da177e4 LT	187	return ret;
	188	}
	189
	190	/*
	191	* get a write lock on the semaphore
	192	* - we increment the waiting count anyway to indicate an exclusive lock
	193	*/
4ea2176d	194	void fastcall __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
1da177e4 LT	195	{
	196	struct rwsem_waiter waiter;
	197	struct task_struct *tsk;
	198
1da177e4 LT	199	spin_lock_irq(&sem->wait_lock);
	200
	201	if (sem->activity == 0 && list_empty(&sem->wait_list)) {
	202	/* granted */
	203	sem->activity = -1;
	204	spin_unlock_irq(&sem->wait_lock);
	205	goto out;
	206	}
	207
	208	tsk = current;
	209	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	210
	211	/* set up my own style of waitqueue */
	212	waiter.task = tsk;
	213	waiter.flags = RWSEM_WAITING_FOR_WRITE;
	214	get_task_struct(tsk);
	215
	216	list_add_tail(&waiter.list, &sem->wait_list);
	217
	218	/* we don't need to touch the semaphore struct anymore */
	219	spin_unlock_irq(&sem->wait_lock);
	220
	221	/* wait to be given the lock */
	222	for (;;) {
	223	if (!waiter.task)
	224	break;
	225	schedule();
	226	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	227	}
	228
	229	tsk->state = TASK_RUNNING;
1da177e4	230	out:
c4e05116	231	;
1da177e4 LT	232	}
1da177e4 LT	233
4ea2176d IM	234	void fastcall __sched __down_write(struct rw_semaphore *sem)
	235	{
	236	__down_write_nested(sem, 0);
	237	}
	238
1da177e4 LT	239	/*
	240	* trylock for writing -- returns 1 if successful, 0 if contention
	241	*/
	242	int fastcall __down_write_trylock(struct rw_semaphore *sem)
	243	{
	244	unsigned long flags;
	245	int ret = 0;
	246
1da177e4 LT	247	spin_lock_irqsave(&sem->wait_lock, flags);
	248
	249	if (sem->activity == 0 && list_empty(&sem->wait_list)) {
	250	/* granted */
	251	sem->activity = -1;
	252	ret = 1;
	253	}
	254
	255	spin_unlock_irqrestore(&sem->wait_lock, flags);
	256
1da177e4 LT	257	return ret;
	258	}
	259
	260	/*
	261	* release a read lock on the semaphore
	262	*/
	263	void fastcall __up_read(struct rw_semaphore *sem)
	264	{
	265	unsigned long flags;
	266
1da177e4 LT	267	spin_lock_irqsave(&sem->wait_lock, flags);
	268
	269	if (--sem->activity == 0 && !list_empty(&sem->wait_list))
	270	sem = __rwsem_wake_one_writer(sem);
	271
	272	spin_unlock_irqrestore(&sem->wait_lock, flags);
1da177e4 LT	273	}
	274
	275	/*
	276	* release a write lock on the semaphore
	277	*/
	278	void fastcall __up_write(struct rw_semaphore *sem)
	279	{
	280	unsigned long flags;
	281
1da177e4 LT	282	spin_lock_irqsave(&sem->wait_lock, flags);
	283
	284	sem->activity = 0;
	285	if (!list_empty(&sem->wait_list))
	286	sem = __rwsem_do_wake(sem, 1);
	287
	288	spin_unlock_irqrestore(&sem->wait_lock, flags);
1da177e4 LT	289	}
	290
	291	/*
	292	* downgrade a write lock into a read lock
	293	* - just wake up any readers at the front of the queue
	294	*/
	295	void fastcall __downgrade_write(struct rw_semaphore *sem)
	296	{
	297	unsigned long flags;
	298
1da177e4 LT	299	spin_lock_irqsave(&sem->wait_lock, flags);
	300
	301	sem->activity = 1;
	302	if (!list_empty(&sem->wait_list))
	303	sem = __rwsem_do_wake(sem, 0);
	304
	305	spin_unlock_irqrestore(&sem->wait_lock, flags);
1da177e4 LT	306	}
1da177e4 LT	307
4ea2176d	308	EXPORT_SYMBOL(__init_rwsem);
1da177e4 LT	309	EXPORT_SYMBOL(__down_read);
1da177e4 LT	310	EXPORT_SYMBOL(__down_read_trylock);
4ea2176d	311	EXPORT_SYMBOL(__down_write_nested);
1da177e4 LT	312	EXPORT_SYMBOL(__down_write);
	313	EXPORT_SYMBOL(__down_write_trylock);
	314	EXPORT_SYMBOL(__up_read);
	315	EXPORT_SYMBOL(__up_write);
	316	EXPORT_SYMBOL(__downgrade_write);