[linux.git] / lib / rwsem.c

/* rwsem.c: R/W semaphores: contention handling functions
 *
 * Written by David Howells ([email protected]).
 * Derived from arch/i386/kernel/semaphore.c
 *
 * Writer lock-stealing by Alex Shi <[email protected]>
 */
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/export.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;
	raw_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
};

/* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
 * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
 * since the rwsem value was observed.
 */
#define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
#define RWSEM_WAKE_NO_ACTIVE  1 /* rwsem was observed with no active thread */
#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */

/*
 * 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 struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
{
	struct rwsem_waiter *waiter;
	struct task_struct *tsk;
	struct list_head *next;
	signed long woken, loop, adjustment;

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

	if (wake_type == RWSEM_WAKE_READ_OWNED)
		/* Another active reader was observed, so wakeup is not
		 * likely to succeed. Save the atomic op.
		 */
		goto out;

	/* Wake up the writing waiter and let the task grab the sem: */
	wake_up_process(waiter->task);
	goto out;

 readers_only:
	/* If we come here from up_xxxx(), another thread might have reached
	 * rwsem_down_failed_common() before we acquired the spinlock and
	 * woken up a waiter, making it now active.  We prefer to check for
	 * this first in order to not spend too much time with the spinlock
	 * held if we're not going to be able to wake up readers in the end.
	 *
	 * Note that we do not need to update the rwsem count: any writer
	 * trying to acquire rwsem will run rwsem_down_write_failed() due
	 * to the waiting threads and block trying to acquire the spinlock.
	 *
	 * We use a dummy atomic update in order to acquire the cache line
	 * exclusively since we expect to succeed and run the final rwsem
	 * count adjustment pretty soon.
	 */
	if (wake_type == RWSEM_WAKE_ANY &&
	    rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
		/* Someone grabbed the sem for write already */
		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.
	 */
	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);

	adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
	if (waiter->flags & RWSEM_WAITING_FOR_READ)
		/* hit end of list above */
		adjustment -= RWSEM_WAITING_BIAS;

	rwsem_atomic_add(adjustment, sem);

	next = sem->wait_list.next;
	for (loop = woken; 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;
}

/* Try to get write sem, caller holds sem->wait_lock: */
static int try_get_writer_sem(struct rw_semaphore *sem,
					struct rwsem_waiter *waiter)
{
	struct rwsem_waiter *fwaiter;
	long oldcount, adjustment;

	/* only steal when first waiter is writing */
	fwaiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	if (!(fwaiter->flags & RWSEM_WAITING_FOR_WRITE))
		return 0;

	adjustment = RWSEM_ACTIVE_WRITE_BIAS;
	/* Only one waiter in the queue: */
	if (fwaiter == waiter && waiter->list.next == &sem->wait_list)
		adjustment -= RWSEM_WAITING_BIAS;

try_again_write:
	oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
	if (!(oldcount & RWSEM_ACTIVE_MASK)) {
		/* No active lock: */
		struct task_struct *tsk = waiter->task;

		list_del(&waiter->list);
		smp_mb();
		put_task_struct(tsk);
		tsk->state = TASK_RUNNING;
		return 1;
	}
	/* some one grabbed the sem already */
	if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
		return 0;
	goto try_again_write;
}

/*
 * wait for a lock to be granted
 */
static struct rw_semaphore __sched *
rwsem_down_failed_common(struct rw_semaphore *sem,
			 unsigned int flags, signed long adjustment)
{
	struct rwsem_waiter waiter;
	struct task_struct *tsk = current;
	signed long count;

	set_task_state(tsk, TASK_UNINTERRUPTIBLE);

	/* set up my own style of waitqueue */
	raw_spin_lock_irq(&sem->wait_lock);
	waiter.task = tsk;
	waiter.flags = flags;
	get_task_struct(tsk);

	if (list_empty(&sem->wait_list))
		adjustment += RWSEM_WAITING_BIAS;
	list_add_tail(&waiter.list, &sem->wait_list);

	/* we're now waiting on the lock, but no longer actively locking */
	count = rwsem_atomic_update(adjustment, sem);

	/* If there are no active locks, wake the front queued process(es) up.
	 *
	 * Alternatively, if we're called from a failed down_write(), there
	 * were already threads queued before us and there are no active
	 * writers, the lock must be read owned; so we try to wake any read
	 * locks that were queued ahead of us. */
	if (count == RWSEM_WAITING_BIAS)
		sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
	else if (count > RWSEM_WAITING_BIAS &&
		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);

	raw_spin_unlock_irq(&sem->wait_lock);

	/* wait to be given the lock */
	for (;;) {
		if (!waiter.task)
			break;

		raw_spin_lock_irq(&sem->wait_lock);
		/* Try to get the writer sem, may steal from the head writer: */
		if (flags == RWSEM_WAITING_FOR_WRITE)
			if (try_get_writer_sem(sem, &waiter)) {
				raw_spin_unlock_irq(&sem->wait_lock);
				return sem;
			}
		raw_spin_unlock_irq(&sem->wait_lock);
		schedule();
		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	}

	tsk->state = TASK_RUNNING;

	return sem;
}

/*
 * wait for the read lock to be granted
 */
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
					-RWSEM_ACTIVE_READ_BIAS);
}

/*
 * wait for the write lock to be granted
 */
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
{
	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
					-RWSEM_ACTIVE_WRITE_BIAS);
}

/*
 * 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 *rwsem_wake(struct rw_semaphore *sem)
{
	unsigned long flags;

	raw_spin_lock_irqsave(&sem->wait_lock, flags);

	/* do nothing if list empty */
	if (!list_empty(&sem->wait_list))
		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);

	raw_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 *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
	unsigned long flags;

	raw_spin_lock_irqsave(&sem->wait_lock, flags);

	/* do nothing if list empty */
	if (!list_empty(&sem->wait_list))
		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);

	raw_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 ([email protected]).
	4	* Derived from arch/i386/kernel/semaphore.c
ce6711f3 AS	5	*
ce6711f3 AS	6	* Writer lock-stealing by Alex Shi <[email protected]>
1da177e4 LT	7	*/
	8	#include <linux/rwsem.h>
	9	#include <linux/sched.h>
	10	#include <linux/init.h>
8bc3bcc9	11	#include <linux/export.h>
1da177e4	12
4ea2176d IM	13	/*
	14	* Initialize an rwsem:
	15	*/
	16	void __init_rwsem(struct rw_semaphore sem, const char name,
	17	struct lock_class_key *key)
	18	{
	19	#ifdef CONFIG_DEBUG_LOCK_ALLOC
	20	/*
	21	* Make sure we are not reinitializing a held semaphore:
	22	*/
	23	debug_check_no_locks_freed((void )sem, sizeof(sem));
4dfbb9d8	24	lockdep_init_map(&sem->dep_map, name, key, 0);
4ea2176d IM	25	#endif
4ea2176d IM	26	sem->count = RWSEM_UNLOCKED_VALUE;
ddb6c9b5	27	raw_spin_lock_init(&sem->wait_lock);
4ea2176d IM	28	INIT_LIST_HEAD(&sem->wait_list);
	29	}
	30
	31	EXPORT_SYMBOL(__init_rwsem);
	32
1da177e4 LT	33	struct rwsem_waiter {
	34	struct list_head list;
	35	struct task_struct *task;
	36	unsigned int flags;
	37	#define RWSEM_WAITING_FOR_READ 0x00000001
	38	#define RWSEM_WAITING_FOR_WRITE 0x00000002
	39	};
	40
70bdc6e0 ML	41	/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and
	42	* RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
	43	* since the rwsem value was observed.
	44	*/
	45	#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */
	46	#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */
	47	#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
	48
1da177e4 LT	49	/*
	50	* handle the lock release when processes blocked on it that can now run
	51	* - if we come here from up_xxxx(), then:
	52	* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
	53	* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
345af7bf	54	* - there must be someone on the queue
1da177e4 LT	55	* - the spinlock must be held by the caller
	56	* - woken process blocks are discarded from the list after having task zeroed
	57	* - writers are only woken if downgrading is false
	58	*/
70bdc6e0 ML	59	static struct rw_semaphore *
70bdc6e0 ML	60	__rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
1da177e4 LT	61	{
	62	struct rwsem_waiter *waiter;
	63	struct task_struct *tsk;
	64	struct list_head *next;
ce6711f3	65	signed long woken, loop, adjustment;
1da177e4	66
345af7bf ML	67	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	68	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
	69	goto readers_only;
	70
70bdc6e0	71	if (wake_type == RWSEM_WAKE_READ_OWNED)
424acaae ML	72	/* Another active reader was observed, so wakeup is not
	73	* likely to succeed. Save the atomic op.
	74	*/
345af7bf	75	goto out;
1da177e4	76
ce6711f3 AS	77	/* Wake up the writing waiter and let the task grab the sem: */
ce6711f3 AS	78	wake_up_process(waiter->task);
1da177e4 LT	79	goto out;
1da177e4 LT	80
345af7bf	81	readers_only:
70bdc6e0 ML	82	/* If we come here from up_xxxx(), another thread might have reached
	83	* rwsem_down_failed_common() before we acquired the spinlock and
	84	* woken up a waiter, making it now active. We prefer to check for
	85	* this first in order to not spend too much time with the spinlock
	86	* held if we're not going to be able to wake up readers in the end.
	87	*
	88	* Note that we do not need to update the rwsem count: any writer
	89	* trying to acquire rwsem will run rwsem_down_write_failed() due
	90	* to the waiting threads and block trying to acquire the spinlock.
	91	*
	92	* We use a dummy atomic update in order to acquire the cache line
	93	* exclusively since we expect to succeed and run the final rwsem
	94	* count adjustment pretty soon.
	95	*/
	96	if (wake_type == RWSEM_WAKE_ANY &&
424acaae ML	97	rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
424acaae ML	98	/* Someone grabbed the sem for write already */
70bdc6e0	99	goto out;
1da177e4	100
345af7bf ML	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
fd41b334 ML	117	adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
	118	if (waiter->flags & RWSEM_WAITING_FOR_READ)
	119	/* hit end of list above */
	120	adjustment -= RWSEM_WAITING_BIAS;
1da177e4	121
fd41b334	122	rwsem_atomic_add(adjustment, sem);
1da177e4 LT	123
1da177e4 LT	124	next = sem->wait_list.next;
fd41b334	125	for (loop = woken; loop > 0; loop--) {
1da177e4 LT	126	waiter = list_entry(next, struct rwsem_waiter, list);
	127	next = waiter->list.next;
	128	tsk = waiter->task;
d59dd462	129	smp_mb();
1da177e4 LT	130	waiter->task = NULL;
	131	wake_up_process(tsk);
	132	put_task_struct(tsk);
	133	}
	134
	135	sem->wait_list.next = next;
	136	next->prev = &sem->wait_list;
	137
	138	out:
1da177e4	139	return sem;
ce6711f3 AS	140	}
	141
	142	/* Try to get write sem, caller holds sem->wait_lock: */
	143	static int try_get_writer_sem(struct rw_semaphore *sem,
	144	struct rwsem_waiter *waiter)
	145	{
	146	struct rwsem_waiter *fwaiter;
	147	long oldcount, adjustment;
1da177e4	148
ce6711f3 AS	149	/* only steal when first waiter is writing */
	150	fwaiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
	151	if (!(fwaiter->flags & RWSEM_WAITING_FOR_WRITE))
	152	return 0;
	153
	154	adjustment = RWSEM_ACTIVE_WRITE_BIAS;
	155	/* Only one waiter in the queue: */
	156	if (fwaiter == waiter && waiter->list.next == &sem->wait_list)
	157	adjustment -= RWSEM_WAITING_BIAS;
	158
	159	try_again_write:
	160	oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
	161	if (!(oldcount & RWSEM_ACTIVE_MASK)) {
	162	/* No active lock: */
	163	struct task_struct *tsk = waiter->task;
	164
	165	list_del(&waiter->list);
	166	smp_mb();
	167	put_task_struct(tsk);
	168	tsk->state = TASK_RUNNING;
	169	return 1;
	170	}
	171	/* some one grabbed the sem already */
fd41b334	172	if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
ce6711f3	173	return 0;
345af7bf	174	goto try_again_write;
1da177e4 LT	175	}
	176
	177	/*
	178	* wait for a lock to be granted
	179	*/
c7af77b5	180	static struct rw_semaphore __sched *
1da177e4	181	rwsem_down_failed_common(struct rw_semaphore *sem,
a8618a0e	182	unsigned int flags, signed long adjustment)
1da177e4	183	{
a8618a0e	184	struct rwsem_waiter waiter;
1da177e4 LT	185	struct task_struct *tsk = current;
	186	signed long count;
	187
	188	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	189
	190	/* set up my own style of waitqueue */
ddb6c9b5	191	raw_spin_lock_irq(&sem->wait_lock);
a8618a0e ML	192	waiter.task = tsk;
a8618a0e ML	193	waiter.flags = flags;
1da177e4 LT	194	get_task_struct(tsk);
1da177e4 LT	195
fd41b334 ML	196	if (list_empty(&sem->wait_list))
fd41b334 ML	197	adjustment += RWSEM_WAITING_BIAS;
a8618a0e	198	list_add_tail(&waiter.list, &sem->wait_list);
1da177e4	199
70bdc6e0	200	/* we're now waiting on the lock, but no longer actively locking */
1da177e4 LT	201	count = rwsem_atomic_update(adjustment, sem);
1da177e4 LT	202
424acaae ML	203	/* If there are no active locks, wake the front queued process(es) up.
	204	*
	205	* Alternatively, if we're called from a failed down_write(), there
	206	* were already threads queued before us and there are no active
	207	* writers, the lock must be read owned; so we try to wake any read
	208	* locks that were queued ahead of us. */
	209	if (count == RWSEM_WAITING_BIAS)
70bdc6e0	210	sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
424acaae ML	211	else if (count > RWSEM_WAITING_BIAS &&
	212	adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
	213	sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
1da177e4	214
ddb6c9b5	215	raw_spin_unlock_irq(&sem->wait_lock);
1da177e4 LT	216
	217	/* wait to be given the lock */
	218	for (;;) {
a8618a0e	219	if (!waiter.task)
1da177e4	220	break;
ce6711f3 AS	221
	222	raw_spin_lock_irq(&sem->wait_lock);
	223	/* Try to get the writer sem, may steal from the head writer: */
	224	if (flags == RWSEM_WAITING_FOR_WRITE)
	225	if (try_get_writer_sem(sem, &waiter)) {
	226	raw_spin_unlock_irq(&sem->wait_lock);
	227	return sem;
	228	}
	229	raw_spin_unlock_irq(&sem->wait_lock);
1da177e4 LT	230	schedule();
	231	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
	232	}
	233
	234	tsk->state = TASK_RUNNING;
	235
	236	return sem;
	237	}
	238
	239	/*
	240	* wait for the read lock to be granted
	241	*/
d1233754	242	struct rw_semaphore __sched rwsem_down_read_failed(struct rw_semaphore sem)
1da177e4	243	{
a8618a0e ML	244	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
a8618a0e ML	245	-RWSEM_ACTIVE_READ_BIAS);
1da177e4 LT	246	}
	247
	248	/*
	249	* wait for the write lock to be granted
	250	*/
d1233754	251	struct rw_semaphore __sched rwsem_down_write_failed(struct rw_semaphore sem)
1da177e4	252	{
a8618a0e ML	253	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
a8618a0e ML	254	-RWSEM_ACTIVE_WRITE_BIAS);
1da177e4 LT	255	}
	256
	257	/*
	258	* handle waking up a waiter on the semaphore
	259	* - up_read/up_write has decremented the active part of count if we come here
	260	*/
d1233754	261	struct rw_semaphore rwsem_wake(struct rw_semaphore sem)
1da177e4 LT	262	{
	263	unsigned long flags;
	264
ddb6c9b5	265	raw_spin_lock_irqsave(&sem->wait_lock, flags);
1da177e4 LT	266
	267	/* do nothing if list empty */
	268	if (!list_empty(&sem->wait_list))
70bdc6e0	269	sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
1da177e4	270
ddb6c9b5	271	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1da177e4	272
1da177e4 LT	273	return sem;
	274	}
	275
	276	/*
	277	* downgrade a write lock into a read lock
	278	* - caller incremented waiting part of count and discovered it still negative
	279	* - just wake up any readers at the front of the queue
	280	*/
d1233754	281	struct rw_semaphore rwsem_downgrade_wake(struct rw_semaphore sem)
1da177e4 LT	282	{
	283	unsigned long flags;
	284
ddb6c9b5	285	raw_spin_lock_irqsave(&sem->wait_lock, flags);
1da177e4 LT	286
	287	/* do nothing if list empty */
	288	if (!list_empty(&sem->wait_list))
70bdc6e0	289	sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
1da177e4	290
ddb6c9b5	291	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1da177e4	292
1da177e4 LT	293	return sem;
	294	}
	295
	296	EXPORT_SYMBOL(rwsem_down_read_failed);
	297	EXPORT_SYMBOL(rwsem_down_write_failed);
	298	EXPORT_SYMBOL(rwsem_wake);
	299	EXPORT_SYMBOL(rwsem_downgrade_wake);