lib/rwsem.c - linux - Git at Google

 /* 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;
 	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 oldcount, 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;

 	/* 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
 	 */
 	adjustment = RWSEM_ACTIVE_WRITE_BIAS;
 	if (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)
 		/* 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 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;

 	/* undo the change to the active count, but check for a transition
 	 * 1->0 */
  undo_write:
 	if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
 		goto out;
 	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;
 		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);
	/* 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;
	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 oldcount, 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;

	/* 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
	*/
	adjustment = RWSEM_ACTIVE_WRITE_BIAS;
	if (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)
	/* 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 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;

	/* undo the change to the active count, but check for a transition
	* 1->0 */
	undo_write:
	if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
	goto out;
	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;
	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);